GM’s original Hydra-Matic transmission was one of the most important innovations in the history of the automobile. It wasn’t the first automatic transmission, but it was the first one that really worked and its resounding commercial success paved the for every subsequent auto-shifter. This week, we take a look at the origins of the Hydra-Matic and its originator, Earl Thompson, who also developed the first Synchro-Mesh gearbox back in the 1920s.





SHIFTING GEARS

In 2010, Ferrari raised the hackles of automotive purists with the announcement that it would shortly phase out its conventional manual transmissions in favor of F1-style sequential gearboxes. The announcement gave new fuel to an old debate: whether a conventional manual transmission and separate clutch pedal are fundamentally obsolete.

Outside of a small contingent of enthusiasts and professional drivers, the automotive world has long regarded the manual gearbox as at best a necessary evil. The multi-speed transmission, which dates back to the 1890s, evolved to compensate for the limitations of early engines, which had modest power and narrow rev bands. Getting a heavy car moving from rest required short (high numerical) gear ratios that would have the engine thrashing its guts out above about 15 mph (25 km/h), but a ratio optimized for brisk cruising — say, 40 mph (65 km/h)– wouldn’t have the torque multiplication to deal with steep hills. One of the great attractions of early electric cars, despite their severely limited range, was that they seldom required any gear changes at all, since electric motors produce their maximum torque from 0 rpm. Had early automakers devised a more efficient means of storing electricity, the evolution of passenger-car powertrains might have been very different indeed.

Actually shifting an early sliding-gear transmission was seldom a pleasant experience. Even upshifts often required careful timing and patience to avoid grinding gears while downshifts required double-clutching and rev-matching. Few people ever mastered those techniques, particularly since engine speed generally had to be judged by ear; tachometers were not common on mundane automobiles in those days. Then as now, there were a few who prided themselves on being adept with the gearbox, but they were definitely in the minority. Many drivers opted instead to shift to high as quickly as possible and then stay there for as long as possible.

This hatred of gear-shifting was not limited to the public. Henry Ford strongly preferred planetary (epicyclic) transmissions, so much so that he allegedly did not even learn to use a conventional gearbox until the development of the Model A in the mid-1920s. The Ford Model T’s pedal-operated planetary gearbox at least avoided clashing gears, although in practice it was scarcely less labor-intensive to use than a conventional gearbox.

Shortly after the Great War, a number of engineers, including Britain’s Walter Gordon Wilson and France’s Jean Cotal, developed more sophisticated “preselector” planetary transmissions. With a preselector gearbox, you chose a ratio with a selector lever and then engaged that gear by pushing the gear selector pedal, which took the place of the traditional clutch. Preselector gearboxes were easier to use than was a conventional manual transmission, but they were also less efficient, substantially costlier, and often too complex to be completely trustworthy. As a result, they never quite caught on for passenger car use, although Cotal and Wilson preselectors were used in a number of pricier British and French cars (and quite a few British buses) into the fifties.



The short-lived, front-wheel-drive Cord 810/812 was one of the few American production cars to use a preselector gearbox — a four-speed unit with Bendix “Electric Hand” controls. The electromagnetically controlled Electric Hand system, also available on some Hudsons (with a standard three-speed gearbox), was adopted because it simplified the linkage between the shifter and the front-mounted gearbox, but it was quite troublesome in service. (Photo: “1937 Cord 812 SC Cabriolet” © 2012 David Berry; used under a Creative Commons Attribution 2.0 Generic license)

The preselector transmissions were automatic in a sense, but, with a number of rare exceptions beyond the scope of this article, they did not relieve the driver of the need to select the appropriate gear ratio for any given circumstance. Although there had been experiments with autonomously self-shifting transmissions since at least 1904, the technical challenges remained substantial and none of the various attempts had been reliable enough or practical enough to have much success. A conventional dual-shaft transmission, whatever its deficiencies, was at least a known quantity.

SILENT SYNCHRO-MESH

Among the many people searching for easier ways to change gears was a young hydraulics engineer from Oregon named Earl A. Thompson. In 1918, Thompson applied for a patent on a three-speed preselector transmission that used a drum-shaped synchronizer to match the speed of each newly selected gear with the speed of the transmission output shaft prior to engaging that gear. The idea was that the driver would select the desired gear and then press the clutch pedal, which would disengage the clutch, synchronize the speeds of the selected gear and the output shaft, complete the shift, and then reengage the clutch automatically.

Thompson continued to develop and refine this idea, filing a second patent disclosure in 1923 that included a new synchronizer mechanism using cone clutches to match the speeds of gears to be meshed. He also managed to build a functional prototype of his preselector transmission, which he installed in a new Cadillac donated by his younger brother Kirk, a Portland-area Cadillac dealer.

That September, Thompson and his brother drove the car to Detroit, where Thompson hoped to sell his invention to the auto industry. Although Detroit’s usual reaction to outside inventions bordered on the categorically hostile, the Thompson brothers managed to secure an audience with Cadillac chief engineer Ernest W. Seaholm and then entered preliminary discussions with GM’s New Devices Committee about the possibility of GM’s purchasing or licensing Thompson’s patents. Those negotiations went nowhere, but Seaholm, who had found Thompson’s design crude but interesting, convinced Cadillac general manager Herbert Rice that Cadillac itself should take on the development of Thompson’s invention.

Thompson resettled in Michigan and went to work as a Cadillac consultant, developing his original ideas into production form. His preselector transmission concept was discarded — judging by the patent description, it added a lot of complexity to no obvious benefit — but Cadillac remained very interested in his gear synchronizer mechanism, which would work just as well in an otherwise conventional transmission. After exhaustive testing and more than two dozen prototypes, Cadillac finally put Thompson’s invention into production in August 1928. The new transmission, dubbed “Silent Synchro-Mesh,” debuted that fall on the 1929 Cadillac and LaSalle.

For cost reasons, the early Synchro-Mesh transmissions provided synchronization only between second and third gears, so shifting into first still generally required coming to a complete stop to avoid clashing. (The “all-synchro” transmission with synchronized low gear wouldn’t become universal until around 40 years later.) Still, the system was a considerable improvement on earlier ‘crashbox’ transmissions and made driving a good deal less painful. Synchro-Mesh quickly spread to other GM divisions and was subsequently licensed or imitated by many other automakers in the U.S. and Europe. By the mid-thirties, most passenger cars had some form of synchronized transmission. (After the war, Thompson’s gear synchronization design would eventually have a strong rival in Porsche’s patented balk ring system, but that’s another story.)

Thompson became a Cadillac employee roughly a year after the first Synchro-Mesh cars debuted. About a year after that, Seaholm and general manager Lawrence P. Fisher (who had succeeded Rice in May 1925) promoted Thompson to assistant chief engineer. He had taken much of the teeth-gnashing (both literal and figurative) out of shifting. The next step was to make the process automatic.



Both Cadillac and its cheaper companion make, LaSalle, gained Silent Synchro-Mesh for the 1929 model year. This 1929 Cadillac 841-B is powered by a 341 cu. in. (5,578 cc) V-8 with about 90 gross horsepower (67 kW); we believe this is a Fisher-bodied convertible coupe. (Photo: “1929-cadillac-archives” © 2006 www.RemarkableCars.com (photographed 2005 by Douglas Wilkinson at en.wikipedia); released for all use with proper attribution, resized 2010 by Aaron Severson)

AUTOMATIC TRANSMISSION GETS ROLLING

When talking about the origins of GM’s early automatic transmissions, it’s important to understand that for the first seven decades of its history, the corporation was not nearly as monolithic as the modern enthusiast or historian might assume. Each division operated more or less independently and was largely autonomous, responsible for its own engineering, manufacturing, and sales. There were occasional collaborative projects, but in general, if the divisions needed something engineered or manufactured by another GM division, they had to buy it like any other customer.

While each division did much of its own R&D work in those days, GM also had central Research Laboratories, headed from 1920 to 1947 by the inimitable Charles F. Kettering, famously the inventor of the automotive self-starter. The research engineers operated independently of the production divisions, conducting advanced engineering and research projects to develop technology (not necessarily automobile-related) that could be adopted by different GM divisions and/or licensed to outside companies. The Research Laboratories worked on all manner of projects, ranging from high-compression engines and leaded gasoline to hydraulic valve lifters.

In the twenties, the work of the Research Laboratories was primarily on the theoretical and experimental side. If a particular invention seemed worthwhile, one or more divisions would be drafted (not always happily) to work with the research engineers to develop the idea for production. In 1931, the corporation organized a central Engineering Staff, led by VP of engineering Ormond E. Hunt, that could serve as a bridge between the research engineers and the divisions. However, ultimate responsibility for the production version of any specific concept or invention still (usually) rested with the individual division, which sometimes led to different divisions offering several distinct variations on the same basic technological theme.

As you would expect, the Research Laboratories worked throughout the twenties to find alternatives to the dual-shaft transmission, exploring a wide variety of electromagnetic, hydraulic, and friction drive systems. These efforts took on some additional urgency after Alfred P. Sloan became president of General Motors in 1923. While he was no fan of engineering novelty for its own sake and had strongly opposed some of Kettering’s wilder ideas — such as the ill-fated “Copper Cooled” Chevrolet — Sloan was by his own admission a mediocre driver who could not use a conventional gearbox with any skill. Recognizing that there were many like him, Sloan understood that a reliable and effective self-shifting transmission would have powerful commercial potential.

Cadillac became involved in this work in the late twenties or early thirties when the division was assigned to support Buick in the development of an ambitious infinitely variable friction drive transmission that the Research Laboratories’ Dynamics unit had conceived. (It appears the friction drive unit was based on one or more outside patents that GM had either purchased or licensed, although the scant information we’ve found on the design’s origins is confusing and contradictory.) Nicknamed the “Roller,” the transmission used two sets of toroidal races, one set driven by the engine, the other set connected to the output shaft; power was transmitted between the races by a series of adjustable rollers.

The Roller’s development was protracted and difficult. The friction drive transmission was extremely smooth and potentially very efficient, but its mechanical complexity made it frighteningly expensive and its reliability remained at best suspect. Cadillac eventually withdrew from the development in favor of an entirely different project, conceived in-house by Thompson. It was just as well; Buick would never offer a production version of the Roller.

Thompson’s ideas on automatic transmission focused not on friction drive, but on planetary gearsets. Cadillac had actually used epicyclic transmissions many years earlier, although that had been well before Thompson’s time; Cadillac switched to dual-shaft transmissions back in 1908. Thompson’s direct inspiration was the 1931 Daimler Double Six, a copy of which Seaholm had purchased for evaluation in late 1930 or early 1931. The Daimler was fitted with the four-speed Wilson preselector transmission and a novel new feature: the Fluid Flywheel, the first fluid coupling ever offered in a production passenger car. (See the sidebar below.)

Even with the Fluid Flywheel, the Wilson gearbox still required manual gear selection, but Thompson recognized that the combination contained most of the ingredients for a practical fully automatic transmission.

SIDEBAR: Fluid Couplings Although the British automaker Daimler was the first to offer a fluid coupling on a production automobile, the fluid coupling (or fluid clutch) was by then more than 20 years old. It was first patented back in 1905 by Hermann Föttinger, a Bavarian engineer then working for the shipbuilder Stettiner Maschinenbau AG Vulcan, and became fairly common in marine applications after World War I. In the late twenties, the British engineer Harold Sinclair adapted the Vulcan fluid coupling for industrial and bus applications. It was the latter version, with some additional refinements by Daimler chief engineer Laurence Pomeroy, that became the Daimler “Fluid Flywheel” in 1930. As the vast number of related patents will attest, fluid clutches can have many possible variations, but at its simplest, a fluid coupling consists of two ring tori — torus-shaped discs — facing each other in a closed housing filled with fluid, usually some type of light oil. The inner surface of each torus member is covered with radial vanes that function much like the blades of a fan or a propeller. One torus member, called the impeller or pump, is driven by the engine. As the impeller spins, its vanes transmit that motion through the oil to the other torus member (called the turbine), which is geared or splined to an output shaft. The oil then returns to the impeller to begin the cycle over again. Once the force applied to the turbine blades by the moving oil is strong enough to overcome the turbine’s inertia (a point known as stall), the turbine and its output shaft begin to rotate.

A much-simplified diagram of a typical automotive fluid coupling (not, we should emphasize, that of Hydra-Matic!). The engine (not shown) drives the flywheel or flex plate; the flywheel drives the impeller through the front torus cover (dark red). The impeller creates motion in the oil within the torus housing, which turns the turbine; the turbine drives the output shaft (blue). (author diagram) Unlike a mechanical clutch, a fluid coupling never transfers 100% of the motion of the flywheel; some of the input torque is always lost to friction heat (slip) within the moving fluid, just as a slipping plate clutch loses energy to friction between the clutch surface and the flywheel. Fluid couplings are particularly inefficient at low speeds because any time the impeller is turning significantly faster than the turbine, oil actually leaves the turbine with a rotational velocity opposite that of the impeller. Slippage decreases as the turbine accelerates, allowing the coupling to reach an efficiency of 97% or better at cruising speed (when turbine and impeller speeds are nearly equal), but the only way the coupling will ever be 100% efficient is if the two torus members are somehow bypassed or mechanically locked together. (The torque converter, also invented by Föttinger and described in his 1905 German patent disclosure (albeit not under that name), is a fluid coupling with an additional member: a bladed disc called a stator, mounted on a fixed shaft. At low speeds, fluid leaving the turbine hits the blades of the stator, reversing the fluid’s direction before the fluid reenters the impeller. That change of direction increases the efficiency of the impeller so that at certain speeds the impeller multiplies the engine’s torque rather than simply transmitting it. Unlike a mechanical gear ratio, however, the torque multiplication quickly fades as the impeller and turbine speeds increase.) A fluid coupling or torque converter’s low-speed slippage provides two important advantages for passenger cars: First, it provides the slip necessary to start the car from rest. Second, a car with a fluid clutch will not stall the engine if the car comes to a stop while in gear as would be the case with a conventional plate clutch. Combined with a planetary gearbox, like GM’s Hydra-Matic, a fluid coupling can completely eliminate the need for a conventional clutch pedal. (Nevertheless, some semiautomatic transmissions, like Chrysler’s Fluid Drive, had both a friction clutch and a fluid coupling; manual de-clutching was necessary for stopping and starting.) The main drawback of a fluid coupling is that its slippage wastes power, potentially hurting both acceleration and fuel economy. Some bus and coach transmissions (and the postwar Packard Ultramatic and Borg-Warner DG) solved that problem by using a mechanical clutch to lock the flywheel to the transmission input shaft at cruising speeds, a feature that became virtually universal for torque converter automatics in the late 1970s. Hydra-Matic took a different approach, which we’ll discuss later in this article.

THE MILITARY TRANSMISSION PROJECT

In early 1932, Seaholm assigned engineers Ralph F. Beck and Walter B. Herndon to assist Thompson with his automatic transmission project, which was dubbed the “Military Transmission.” Although the project would eventually have military applications, the moniker was just a codename, intended — like the ominous “Keep Out” sign hung outside the door — to discourage prying eyes.

The Military Transmission project’s objective was to develop a planetary gearset that could be operated automatically by means of hydraulic servos. The first fruit of this work, on which Thompson filed a patent in March 1933 (subsequently U.S. Patent No. 2,285,760), was a conventional sliding-gear transmission augmented by a two-speed planetary gearset that provided automatic shifting between direct drive and overdrive ratios. The design was similar in broad principle to the new Reo Self-Shifter, patented two years earlier and announced in May 1933 as a production option for the 1934 Reo Royale and S-4 Flying Cloud. For Thompson, the semiautomatic transmission appears to have been mostly an early essay in speed-sensitive hydraulic governor systems, suggesting the direction of his thinking.

Around the time Thompson’s patent was filed, Seaholm expanded Thompson’s group from three engineers to five, adding William L. Carnegie and Maurice S. Rosenberger to the team. They soon began work on a more elaborate hydraulically operated planetary transmission, which had reached the prototype stage by mid-1934. Thompson applied for a patent on it that October.

Unfortunately, the entire project was rapidly becoming an unaffordable expense. For several years, Cadillac general manager Larry Fisher had been spending lavishly on new products and new technology, including double wishbone suspension, power steering (which for various reasons Cadillac wouldn’t actually offer until 1952), and of course the V-16 and V-12 engines. The results were often impressive, but with Cadillac sales slumping badly as the Depression worsened, the division’s future was very much in question. Service manager Nicholas Dreystadt, who succeeded Fisher as general manager in June 1934, convinced the corporation to give Cadillac a reprieve, but severe budget cuts left the Military Transmission project hanging by a thread.



A 3-D diagram of a planetary (a.k.a. epicyclic) gearset. Each planetary gearset can provide different ratios depending on which elements are driven and which are locked in place. A single planetary gearset can provide reduction gearing, direct drive, or overdrive in either forward or reverse, although not all gearsets are capable of all these combinations. Planetary gearsets can also be used as torque-splitting or torque-combining differentials, dividing output torque in multiple directions or applying the combined torque of several different engines or prime movers to a single driveshaft. (Illustration: “Epicyclic gear small” © 2005 Wapcaplet; used under a Creative Commons Attribution-ShareAlike 3.0 Unported license)

By this time, Buick’s friction drive transmission had been canceled, so for Thompson’s work to go the same way would have been a significant setback to an effort that still had Sloan’s strong personal interest. The eventual answer was to transfer Thompson’s project to O.E. Hunt’s corporate Engineering Staff, which had its own facilities and budget. In January 1935, Thompson and his team moved to the central offices to become a corporate product study group, later named the Transmission Development Group.

THE AUTOMATIC SAFETY TRANSMISSION

By this time, Thompson had developed a new four-speed semiautomatic transmissions, patent applications for which were filed in October 1934 and October 1935 (U.S. Patent Nos. 2,195,605 and 2,193,304 respectively). The transmission used two servo-operated planetary gearsets with an additional set of spur gears, interposed between the clutch and front gearset, for neutral and reverse. Thompson specified a conventional single-plate friction clutch, but it was strictly a stopgap. He still wanted to eventually substitute a fluid coupling, but his tiny team hadn’t yet had time to design a suitable one.

The semiautomatic transmission’s rear servo (and thus the rear gearset) was controlled by the driver using a selector lever on the steering column, but the front servo functioned automatically, shifting from direct drive to reduction and back based on throttle position and road speed; the latter was signaled by a centrifugal governor driven by the transmission output shaft.

The idea was that the driver would start in Neutral, disengage the friction clutch with the clutch pedal, and select Low for a first-gear start, followed in short order by an automatic shift to second. Obtaining third and fourth required shifting manually from Low to High (which did not require de-clutching). The transmission would then shift automatically between third and fourth gears.

Some time after Thompson’s group moved to the Engineering Staff, their project came to the attention of Oldsmobile general manager Charles L. McCuen. We don’t know if McCuen was aware of Buick’s now-canceled friction drive project, but he was in any case very keen for Oldsmobile to have an automatic transmission of its own. He ordered Oldsmobile chief engineer Harold T. Youngren to work closely with Thompson’s group to adapt their ideas into something Oldsmobile could build and sell.

Later that year, senior corporate management decided that the manufacturing portion of the equation should be handled by Buick, which had unused factory space that could be retooled for the purpose. Buick was also ordered to share the new transmission, presumably as a sort of consolation prize for the abortive Roller project. That directive sat ill in Flint; Buick engineers still had their own ideas about automatic transmission (some of which would be realized in the postwar Dynaflow) and had no love for the Automatic Safety Transmission, which they hadn’t developed and didn’t want.

Throughout this period, Thompson continued to refine the semiautomatic transmission, applying for an additional patent in March 1937 (U.S. Patent No. 2,362,418). Production began around the same time and the new transmission, which Oldsmobile dubbed the “Automatic Safety Transmission” (AST), went on sale in June as an option for the 1937 Oldsmobile Eight. List price was initially $80, rising to a hefty $100 that fall, when availability was extended to six-cylinder Oldsmobiles and the Buick Series 40 Special. Buick doesn’t appear to have bothered coming up with a specific trade name for the semiautomatic, describing it simply as a self-shifting transmission, but it was otherwise identical to the Oldsmobile unit.

SIDEBAR: Automatic Safety Transmission Operation Although the Automatic Safety Transmission is usually called a four-speed semiautomatic transmission, Earl Thompson’s patent disclosures actually describe it as essentially three separate transmissions in series: a synchronized dual-shaft reverse unit with helical gears followed by two separate two-speed planetary transmissions. In a car equipped with the Automatic Safety Transmission, the engine drove a conventional single-plate clutch, whose driven shaft drove the input shaft of the reverse unit. The reverse unit provided the driver with three choices: neutral, reverse reduction, or forward direct drive. With the reverse unit in gear, that unit’s output shaft drove the annulus (ring gear) of the front planetary transmission, which was surrounded by a brake drum connected to the planetary gearset’s central sun gear. A band brake could be engaged to lock the drum and thus the sun gear in place, putting the planetary unit in reduction. The drum also carried the driving plates of a multi-disc clutch pack, which could be engaged to lock the drum and the sun gear to the planetary unit’s planet carrier. The front unit planet carrier drove the dual sun gears of the rear planetary transmission, whose first annulus was connected to another brake drum and multi-disc clutch pack that could either hold the first annulus in place or else lock both the first planet carrier and both ring gears together to put the unit in direct drive. The second planet carrier drove the output shaft. Both planetary gearsets were controlled by a combination of spring and hydraulic pressure. A servo could be engaged to release the brake band while a control valve could apply hydraulic pressure to engage the clutch pack. The difference was that the rear unit was controlled manually by the driver moving the selector lever while the front unit was controlled automatically. These two gearsets could be compounded as shown in the table below. (In this table, “ENG” means “ENGAGED” while “REL” means “RELEASED.”) Automatic Safety Transmission Gearing Sequence Front Planetary Gearset (Automatic) Rear Planetary Gearset (Manual) Gear Servo Band Clutch Ratio Servo Band Clutch Ratio Overall Ratio * Although the front servo was released in neutral, the front band normally did not engage because no power was applied to the input shaft and front annulus. Neutral REL OFF REL 0* ENG OFF ENG 1.00 — 1st REL ON REL 1.42 REL ON REL 2.23 3.17 2nd ENG OFF ENG 1.00 REL ON REL 2.23 2.23 3rd REL ON REL 1.42 ENG ON ENG 1.00 1.42 4th ENG OFF ENG 1.00 ENG OFF ENG 1.00 1.00 The Automatic Safety Transmission did not actually give the driver direct control of gear selection except in Reverse and Neutral. Instead, the selector provided Low and High ranges, which provided provided automatic shifting between first and second and third and fourth gears respectively. Automatic upshifts were triggered mostly by road speed, which was signaled by a centrifugal governor driven by the output shaft. However, a throttle-controlled mechanical linkage could either delay the upshift or force a downshift within certain speed ranges. Oldsmobile cautiously recommended starting in Low, but, as Thompson’s patent disclosures noted, starting from rest in High would actually cause the transmission to start in first gear and then shift automatically into third. Buick sales literature presented this as a feature, but it was really more of a known bug, resulting from a quirk of the hydraulic system layout. Idling in Neutral with the clutch engaged pressurized the rear servo, putting the rear planetary in direct drive (as in High range); this was intentional to facilitate push-starting. Disengaging the clutch, however, cut off power to the transmission oil pump (again by design) while moving the shift lever to the Low position dumped pressure from the rear servo. If you then started in High, it took a moment or two for the pump to build up enough hydraulic pressure to reengage the rear servo. Once it did, the rear planetary gearset would then shift from reduction to direct drive and belatedly put the transmission in third gear. As a result, owners seldom bothered using Low, whether the owner’s manual recommended it or not. First, third, and fourth were perfectly adequate for most conditions and not having to shift from Low to High was obviously more convenient, although the practice did the transmission’s reliability no favors. Oldsmobile advertising claimed that cars with the Automatic Safety Transmission returned up to 20% better fuel economy than did cars with the standard three-speed manual transmission. That claim, which in practice was rather optimistic (Buick claimed only 8%), was not due to the efficiency of the semiautomatic transmission, which consumed more power than did a conventional gearbox, but rather to the use of a significantly taller (lower numerical) axle ratio than was specified with the standard three-speed.

By the time the Automatic Safety Transmission appeared on the market, Reo’s Self-Shifter had come and gone and buyer interest in semiautomatic transmissions had proven to be limited. Not only were the transmissions complicated, expensive, and often troublesome, they were still not really automatic. While their operation was different than that of a conventional transmission, saying they were easier to use (much less safer, as Oldsmobile claimed) was arguable, particularly since they retained the clutch pedal, of which many American motorists would have been happily rid. (This lesson was apparently lost on Chrysler, which by 1937 was working on its own M3 semiautomatic transmission for a 1940 introduction.)

It certainly didn’t help that the Automatic Safety Transmission wasn’t very reliable, particularly early on. The transmission’s teething problems were extensive and Oldsmobile found that Buick engineers and production people, eager to wash their hands of the project, were not much help. The transmission was also unfamiliar territory for technicians, and Oldsmobile did not encourage tinkering, in part because it took a while for even the factory production engineers to figure out how to resolve certain common issues. As a result, dealers responded to most problems by pulling the transmission and replacing it with a new factory-refurbished unit. (Substituting a standard gearbox, which some unhappy owners would probably have preferred, was a more complicated chore that required also replacing the steering column, the entire driveshaft, and ideally the rear axle gears.)



An Oldsmobile ad for the 1938 Automatic Safety Transmission. The transmission’s supposed safety advantage was that it reduced the need for the driver to remove one hand from the steering wheel to change gears. (ad image: “1938 Oldsmobile Eight & Six Sedans” courtesy Alden Jewell; used with permission)

Buick dropped the semiautomatic transmission after only a year and never offered the unit in their bigger cars, but Oldsmobile persisted through the 1939 model year, reducing the list price by $25 for the transmission’s final season. Total production, which continued through September 1939, was limited. We have no exact figures, but we’ve seen estimates ranging from about 15,000 to as many as 40,000. Buick took only about 3,000 of those for itself, so most went into 1937–1939 Oldsmobiles. Warranty costs were high and the Automatic Safety Transmission’s suggested retail price was considerably less than Oldsmobile paid Buick for each transmission, so Oldsmobile undoubtedly lost money on the whole endeavor. However, if nothing else, the project provided plenty of real-world experience for the fully automatic transmission Thompson and McCuen still hoped to offer.

SIDEBAR: Oscar Banker’s Automatic Even before Earl Thompson’s team started work on its semiautomatic and automatic transmissions, Armenian Oscar H. Banker (born Asadoor Sarafian) was developing his own automatic transmission concepts, applying for a host of patents in that area between 1927 and 1937. Banker’s transmission-related patents covered an assortment of two-, three-, and four-speed designs, some using spur gears, some using planetary gearsets, and some with a combination of the two. Unlike Thompson’s designs (or later Oliver K. Kelly’s), Banker’s automatic transmissions were controlled mechanically by one or more centrifugal governors rather than by hydraulic servos. Also, Banker mostly eschewed brake bands in favor of one-way clutches, a feature GM would adopt for the second-generation Controlled Coupling Hydra-Matic about 20 years later. According to Banker, he demonstrated a prototype of his transmission in late 1930 to O.E. Hunt, Larry Fisher, Ernest Seaholm, Thompson, and Charles Kettering. Banker claimed that GM’s New Devices Committee was prepared to license the design, but Banker’s business partner and financial backer, Franklin Magill of the Magill-Weinsheimer Company, wanted more money than GM was willing to pay. In 1934–1935, however, GM’s Yellow Coach subsidiary (the Yellow Truck and Coach Manufacturing Company) did briefly license one of Banker’s transmission designs, which was used under the trade name Monodrive (sometimes styled “Mono-Drive”) in a modest number of buses for the Chicago Motor Coach Company. Magill later secured a separate license agreement with Borg-Warner. The author is not an engineer or a lawyer, much less a patent lawyer, so we’re not qualified to weigh the legal merits of Banker’s later allegation that Hydra-Matic infringed his patents. While we don’t doubt that Thompson’s group was aware of Banker’s work and probably would have been even if Thompson and Banker had never actually met (it’s both customary and prudent to pay close attention to rival inventors’ published work), consciously developing a competing design does not necessarily constitute an infringement. The question of whether any specific elements of Thompson and Kelley’s transmission designs infringed Banker’s patents would have had to be weighed by a federal court. Since to our knowledge neither Banker nor his licensees ever pursued formal legal action, there seems little point in speculating on the potential outcome of such a case. That said, having examined many (though not all) of the patents in question, our layman’s observation is that Thompson and Kelley’s designs appear quite a bit different from Banker’s in both principle and operation. Furthermore, many of the broad similarities that do exist, such as the use of planetary gearsets, were already established in prior art. Neither Thompson nor Banker invented the epicyclic gear train, nor for that matter the idea of an automatic transmission — brothers Thomas L. and Thomas J. Sturtevant patented the first version of their centrifugally operated automatic transmission in 1904, when Thompson and Banker were still children. A patent search on Banker’s name reveals more than a dozen patents related to automatic transmissions, but if you’re curious, the most pertinent are probably U.S. Patent Nos. 1,795,464 (assigned to Continental Illinois Bank and Trust Company, filed 21 October 1927, issued 10 March 1931); 1,795,465 (filed 26 November 1928, issued 10 March 1931); 1,843,193 (assigned to Continental Illinois Bank and Trust Company, filed 9 April 1930, issued 2 February 1932); and 2,262,747 (assigned to New Products Corporation, filed 18 September 1936, issued 31 January 1942, and reissued 18 May 1943).

SEMIAUTOMATIC NO MORE

Even before the Automatic Safety Transmission went on sale, Thompson’s group was already working to make the semiautomatic transmission obsolete. All that remained was to eliminate the clutch pedal and design a hydraulic control unit smart enough to autonomously manage all shifting in normal driving.

Thompson’s solution to the latter problem was outlined in the patent disclosure he filed in April 1938 (U.S. Patent No. 2,204,872). It again described a four-speed transmission using two planetary gearsets and an additional set of spur gears for neutral and reverse. As in the Automatic Safety Transmission, the planetary gearsets’ brake bands and clutches were operated by servos. Oil flow to each servo was controlled by a separate hydraulically operated valve, each of which was now operated by a spring-loaded control piston.

Actuating pressure for those pistons was supplied by an engine-driven centrifugal governor whose output (in this iteration of the invention) was proportional to engine speed. However, the earlier designs’ completely mechanical throttle linkage was deleted in favor of a new throttle-controlled hydraulic compensator line that served to vary the amount of hydraulic pressure necessary to operate the control valves. The wider the throttle was opened, the more governor pressure it took to move each piston rod and open the associated shift valve. In this way, each forward gear ratio was ‘mapped’ to a specific range of governor output pressure. Pressure exceeding or falling below that range would trigger an automatic shift up or down, respectively.

In essence, the hydraulic controls now functioned as an analog computer that could be programmed to shift autonomously between all available forward gears at different combinations of engine speed and throttle setting. The throttle valve gave the driver some ability to either hasten or delay gear changes, but not to override them. Therefore, you couldn’t hold a lower gear past redline, deliberately lug the engine in high gear under load, or force a downshift that would over-rev the engine. (As in the earlier semiautomatic transmission, there was still a separate Low range which could be manually selected to lock the control valve for the rear unit servo in the closed position, which limited automatic shifts to first and second gears.)

The table below summarizes the actual gearing sequence and ratios for the new transmission when it made its production debut in late 1939. (As in the previous table, “REL” means “RELEASED” and “ENG” means “ENGAGED.”)

Prewar Oldsmobile Hydra-Matic Gearing Sequence Front Planetary Rear Planetary Reverse Planetary Gear Band Clutch Ratio Band Clutch Ratio Pawl Ratio Overall Ratio * In Neutral, the rear band is applied with the engine off, but released with the engine running. † Negative signifies reverse. Neutral OFF REL — OFF REL — OFF* — — 1st ON REL 1.44 ON REL 2.53 OFF — 3.66 2nd OFF ENG 1.00 ON REL 2.53 OFF — 2.53 3rd ON REL 1.44 OFF ENG 1.00 OFF — 1.44 4th OFF ENG 1.00 OFF ENG 1.00 OFF — 1.00 Reverse ON REL 1.44 OFF REL — ON -2.99† -4.31†

The new transmission’s final major element — a fluid coupling that could replace the Automatic Safety Transmission’s plate clutch — was outlined in the patent filed in February 1937 (U.S. Patent No. 2,176,138) by Oliver K. (“O.K.”) Kelley, who had worked with Thompson at Cadillac and later joined him in the Transmission Development Group after a stint at GM’s Yellow Coach and Truck subsidiary.

Although it was actually patented more than a year before Thompson’s hydraulic controls, it seems more appropriate to discuss Kelley’s design second. While Thompson’s patent was essentially an extrapolation of the group’s previous semiautomatic transmissions, Kelley’s was a substantially new design with some significant variations. Not only did it use a fluid coupling rather than a plate clutch, there were now three planetary gearsets rather than two (although there were still only four forward speeds) and no spur gears; the third gearset was now used to provide reverse reduction. There was also an important new feature: an additional intermediate shaft, concentric with the transmission main shaft, whose leading end was permanently attached to the impeller. The intermediate shaft, which was driven by the planet carrier of the front planetary gearset, served to connect the impeller to the clutch assembly of the second planetary gearset.

The intermediate shaft served two important functions. First, it provided an indirect connection between the engine and the impeller of the fluid coupling. With most fluid clutches, the impeller is bolted or otherwise attached directly to the engine flywheel — in automotive applications, typically through the coupling’s torus housing. In Kelley’s design, the engine instead drove the annulus (ring gear) of the front planetary gearset. The annulus then drove that gearset’s planet pinions and planet carrier, which in turn drove the immediate shaft and with it the impeller. As a result, the impeller and the engine only turned at the same speed if the front clutch were engaged, which locked the front planetary gearset in direct drive (which in this case happened in second or fourth). With the front clutch released and the front brake band engaged (i.e., in first, third, or reverse), the speed of the impeller would be reduced by the ratio of the front gearset. The purpose of this unusual arrangement was to deliberately reduce the efficiency of the coupling at idle and off-idle speeds to provide smoother takeoffs and minimize ‘creep’ in first or reverse without hampering efficiency at higher speeds.

The intermediate shaft’s second purpose was to reduce slippage in the cruising gears — third and fourth — by providing a partial mechanical connection between the engine and the second planetary gearset in those gears. As we mentioned above, the intermediate shaft also drove the second unit clutch hub. Engaging the second unit clutch (as in third or fourth) caused the intermediate shaft to simultaneously drive both the fluid coupling impeller and the second unit annulus, creating a “split torque” arrangement. Some of the torque on the intermediate shaft was still applied to the impeller and thus the fluid coupling impeller, turbine, main shaft, and second unit sun gear, but most — more than 60% — was now applied directly to the second unit annulus. (The actual torque split depended on how many teeth the annulus and sun gear each had; in the earliest production Hydra-Matic, the split was 60.5% mechanical, 39.5% hydraulic.)

The second unit planet carrier then reintegrated these two torque inputs in the same manner as a marine or locomotive transmission using multiple engines to turn a common shaft. This effectively reduced coupling slip by more than 60% in both third and fourth gears. (It didn’t actually prevent the coupling from slipping, but it allowed a substantial portion of intermediate shaft torque to bypass the coupling. For a further explanation of this principle, see our article on split torque transmissions.)



Hydra-Matic was optional even on the cheapest six-cylinder Oldsmobile Series 60 in 1940, although we suspect it was more common on the larger and more expensive Series 70 and Series 90 models. The Series 60 and Series 70 both shared Oldsmobile’s 95 hp (71 kW) 230 cu. in. (3,536 cc) inline six, but the Series 70 had a taller 4.30 axle (3.63 with Hydra-Matic), compared to the Series 60’s 4.10 (3.42 with Hydra-Matic), to compensate for the extra weight of the larger body; a Series 70 four-door sedan weighed 113 lb (51 kg) more than the equivalent Series 60. (Photo: “1940 Oldsmobile Dynamic Series 70 Sedan” © 2014 Sicnag; used under a Creative Commons Attribution 2.0 Generic license)



Even Kelley recognized that this arrangement was more complicated than it probably needed to be, but the underlying principles were solid and by this time mostly well-understood. If the design was inelegant, it was at least functional and, just as important, production-feasible.

ENTER HYDRA-MATIC

Production feasibility was still a major priority for Oldsmobile, which remained actively involved in the development process. Based on the timetable, it appears that as soon as Thompson’s team came up with a viable-seeming idea, it was handed off to Oldsmobile engineers for evaluation and testing; McCuen wanted something that could replace the Automatic Safety Transmission in the near future. By early 1939, Oldsmobile engineer Harold N. Metzel was already overseeing the road-testing of some 5,000 preproduction examples of the new transmission.

Inevitably, the transmission underwent further changes before reaching production; some but not all of those changes are described in Kelley’s April 1939 patent disclosure (subsequently U.S. Patent No. 2,211,233). Thompson’s single oil pump, which as originally described was driven both by the engine and the tail shaft, was replaced with separate front and rear pumps, the shaft of the latter also driving the governor assembly. As a result, shift points of the production transmission were once again governed primarily by road speed rather than engine speed. The design and layout of the shift valves and throttle-controlled pressure lines was extensively revised (although the basic principle was still as described above). Finally, Thompson developed a new and significantly more efficient fluid coupling design, patent applications for which were filed in 1940 (U.S. Patents 2,357,295 and 2,430,258).

The other major mechanical change was a completely redesigned third (reverse) planetary gearset. In reverse, the second and third units acted together as a single compound gearset. The second-unit annulus was attached to and rotated with the third-unit sun gear while both the second- and third-unit planet carriers were splined to the output shaft. Reverse rotation was provided by releasing both the clutch and the brake band of the second unit. Rotation of the second-unit sun gear, still driven by the fluid coupling turbine through the main shaft, would then cause the second-unit annulus and third-unit sun gear to rotate backward. Engaging a toothed reverse pawl locked the third-unit annulus in place, turning the carriers and thus the output shaft backward in reduction. The reverse pawl also served to lock the driveshaft with the engine off.



A diagrammatic representation, not to scale, of the early Hydra-Matic, omitting the servos and valve body (located below the planetary gearsets) and the multitude of hydraulic lines. Each color represents components that are integral or otherwise permanently attached so that they rotate together. As you can see by studying this diagram, the rear planetary gearset provides a torque-splitting function in third and fourth gears. In those gears, the rear annulus (light blue), driven by the intermediate shaft (red), turns at the speed of the impeller while the rear sun gear (medium blue), driven by the main shaft (also medium blue), turns at turbine speed. This rotates the planet carrier and the output shaft (purple) a little slower than the impeller, but still faster than the turbine, which is subject to hydraulic slippage. (Author diagram)

The resulting transmission still bore a clear resemblance to the Automatic Safety Transmission it would shortly supersede, but it no longer required a clutch pedal or much driver intervention in most normal operation. In High, the transmission would start in first and then shift for itself through all four gears. The previous Low range was retained, allowing the driver to keep the transmission in first and second at speeds up to about 40 mph (64 km/h), but that was usually only necessary for steep hills or perhaps hauling a heavy load at low speeds.

GM christened the new transmission Hydra-Matic. Although Oldsmobile would have exclusive use of it for the first year — a corporate policy acknowledging the considerable resources the division had poured into the transmission’s development — GM had big plans for the automatic. A new Detroit Transmission Division, headed by Victor A. Olsen, was established specifically to manufacture Hydra-Matic. William Carnegie, who had been with Thompson’s group since 1933, was appointed chief engineer of the new division, which got its own assembly facilities in a former Fisher Body plant in eastern Detroit.



The shift pattern of prewar Hydra-Matics was N-Hi-Lo-R. Unlike the earlier Automatic Safety Transmission, High used all four forward speeds; postwar shift quadrants re-labeled this position “Drive,” which was more accurate and probably less confusing. (author photo)

Oldsmobile received the first production Hydra-Matic transmissions (known internally as Model 180) that October. When the 1940 Oldsmobiles debuted late that year, Hydra-Matic was optional across the line at a low introductory price of $57 — actually $19 less than the 1939 price of the Automatic Safety Transmission. As with the semiautomatic transmission, Hydra-Matic included much taller, economy-oriented 3.42 or 3.63 axle ratios (compared to 4.10 or 4.30 for manually shifted cars).

Oldsmobile advertising was predictably breathless about Hydra-Matic, extolling its ease of use and reduced fuel and oil consumption. If the latter claims were again optimistic (Oldsmobile started off claiming savings of up to 20%, soon amended to 10–15%), the other boasts were well-earned. Hydra-Matic was a genuinely paradigm-changing innovation that finally made good on the promises its predecessors hadn’t quite fulfilled.

That isn’t to say the early Hydra-Matic was flawless. Aside from its cost, it was bulky and quite heavy; we don’t have any precise weight figures for the early units, but they were probably at least 100 lb (45 kg) heavier than a conventional gearbox. Even discounting the inevitable teething problems, Hydra-Matic was also a complicated and fussy device. For it to work as intended, the various brake bands and linkages had to be kept properly adjusted, which required special tools and a certain amount of finesse. Even then, the original Hydra-Matic was never a paragon of smoothness, tending to shift with a distinct thump.

However, the more important thing to most potential buyers was that Hydra-Matic worked, offering the painless two-pedal driving for which so many people had been longing. If the automatic transmission’s commendable efficiency didn’t quite match that of a manual gearbox, that seemed like a small price to pay for the permanent banishment of a generally hated chore. Hydra-Matic also made driving accessible to a whole range of disabled people for whom manual shifting was difficult or impossible. After the war, Oldsmobile would capitalize on that potential by offering “Valiant” models equipped with Hydra-Matic and special driver controls, intended for use by disabled veterans.



In addition to its obvious convenience, Oldsmobile claimed that Hydra-Matic returned up to 20% better fuel economy than did a standard three-speed transmission. As with the earlier Automatic Safety Transmission, the claimed edge in fuel economy was due less to the transmission and more to the 3.42 or 3.63 axle ratios included with it, which was almost 20% taller (lower numerically) than the 4.10 or 4.30 axles standard with manual shift. Hydra-Matic cars were fairly thirsty in stop-and-go driving, but gentle highway cruising could return up to 19 mpg (12.4 L/100 km), which was excellent for a big American car of this era. (author photo)

The upshot of all this was that Oldsmobile sold about 60,000 Hydra-Matic transmissions for 1940, substantially better than the Automatic Safety Transmission had done in two and a half years. Olds would nearly double those sales for 1941 despite raising the transmission’s price from $57 to a more realistic $100.

Oldsmobile’s exclusivity period ended with the 1940 model year, so for 1941, the automatic transmission became available to other GM divisions. Buick still wanted nothing to do with Hydra-Matic, which Buick chief engineer Charles A. Chayne nicknamed “Hydra-Jerk,” nor did conservative Pontiac chief engineer Benjamin H. Anibal, but Cadillac adopted a new heavier-duty Model 250 Hydra-Matic. The Model 250, most of the particulars of which are described in Kelley’s December 1941 patent disclosure (U.S. Patent No. 2,377,696), functioned much like the Oldsmobile unit, but had greater torque capacity and different first and second gear ratios, courtesy of a new second planetary unit with compounded gears (two distinct but interconnected planetary gear trains) like those of the Automatic Safety Transmission. The second gearset’s rearmost planet carrier handled the job of reintegrating the torque split in third and fourth gears.



Another late addition: a diagram (not to scale) of the 1941–1945 Model 250 Hydra-Matic used in prewar and wartime Cadillacs and a variety of military vehicles. Note that there are two rear sun gears rather than one, both driven at the same speed by the main shaft. The rear gearset also has two ring gears and two sets of planet gears on separate planet carriers; the first carrier also forms the second annulus. Cadillac reverted to a simpler non-compounded rear planetary gearset (like that of the Oldsmobile unit) for 1946. (Author diagram)

About 30% of Cadillac buyers ordered the automatic transmission despite its $125 price tag — almost 10% of the list price of a Series 61 club coupe, the division’s cheapest and most popular 1941 model. The option continued for the 1942 model year, which was cut short by the War Production Board in February 1942 so that manufacturing resources could be redirected to the production of war matériel. By then, the Detroit Transmission Division had delivered almost 215,000 Hydra-Matic transmissions.



Big chrome badges on either side of the hood of this 1942 Oldsmobile proudly proclaim the presence of Hydra-Matic transmission; by this time, about 45% of new Oldsmobiles were so equipped. Note the dull finish of the grille: Like many 1942 American cars, Oldsmobile’s trim reflected an industry-wide program to reduce the use of aluminum, zinc, chrome, and other strategic materials. (author photo)

BATTLE TESTED

Unlike Oldsmobile and Cadillac passenger cars, Hydra-Matic would not cease production during the war. Instead, the automatic transmission would find a whole new application.

When America entered the war in late 1941, the principal U.S. light tank was the M3 Stuart, manufactured by the American Car & Foundry Co. In its initial production form, the M3 weighed 14 tons (12.7 metric tons) and was armed with a 37mm cannon and four 0.30-caliber (7.62mm) machine guns. It was powered by a seven-cylinder Continental W-670 radial engine with 262 gross horsepower (195 kW), providing a reasonably sprightly top speed of 36 mph (58 km/h). Even before the U.S. declaration of war, the M3 was already seeing active duty with the British Army, which nicknamed the tank “Honey” and made extensive use of it in North Africa. By 1942, the M3 would also be in widespread service with the U.S. Army and the United States Marine Corps.

With demand for the Continental radial engine already outpacing supply by mid-1941, some M3s were built with the less-powerful nine-cylinder Guiberson diesel. As an alternative, Cadillac proposed a new M3 variant that would trade the nine-cylinder radial engine for two of the division’s passenger car V-8s. That project, supervised by Cadillac engineer Edward N. Cole (later to become chief engineer of Cadillac and Chevrolet and eventually president of General Motors), used two more-or-less stock 346 cu. in. (5,676 cc) Cadillac L-head V-8 engines, each rated at 148 gross horsepower (110 kW) and each driving one tread via a beefed-up Hydra-Matic transmission.



An M5A1 Stuart light tank, photographed outside the Yad La’Shiryon Museum in Latrun, Israel. (Photo: “M5A1-Stuart-latrun-2” © 2005 User:Bukvoed; resized and used under a Creative Commons Attribution 2.5 Generic license)

The twin Cadillac engines gave the redesigned tank the same top speed as the M3A1 despite a weight increase of about 5,100 lb (2,313 kg). The Hydra-Matic transmissions not only reduced driver workload (an important consideration in a combat vehicle — particularly tanks, which as a rule are cramped, deafeningly loud, and have dreadful visibility), but also allowed the installation of full dual controls so that the tank could be operated by either the driver or co-driver as needed.

The Army Ordnance Department was duly impressed, so the first production example of the redesigned tank, designated M5 Stuart (Stuart VI in British service), rolled off the Cadillac assembly lines in March 1942. In June, Cadillac created the M8 Howitzer Motor Carriage, which shared the M5’s chassis and powertrain, but had a different turret carrying a 75mm artillery piece. About 1,800 M8s and almost 9,000 M5s and improved M5A1s were built in all, some by Cadillac and some by the tractor manufacturer Massey-Harris.

Although the M5 was fast for a light tank, it soon became painfully apparent that it was too lightly armed for the European theater. Cadillac responded with an enlarged version that retained the dual-engine powertrain, but traded one of the 0.30-caliber (7.62mm) machine guns for a 0.50-caliber (12.7mm) gun and the 37mm cannon for a new 75mm cannon shared with the North American B-25H Mitchell bomber. The new tank, dubbed M24 Chaffee, began entering service in April 1944 and reached frontline units that November. More than 4,300 M24s were built by the end of the war and some remained in service until the late eighties. Late in the war, there were also a number of M24 derivatives sharing its chassis, engines, and automatic transmissions, including the M19 anti-aircraft gun carriage and the M37 and M41 self-propelled howitzers.



An M24 Chaffee tank at the Fort Lewis Military Museum. (Photo: “M-24 Chaffee Light Tank 01” © 2009 Joe Mabel; resized and used under a Creative Commons Attribution-ShareAlike 3.0 Unported license)

In 1944, the Cadillac V-8/Hydra-Matic powertrain was adapted for the LVT-3 Bushmaster tracked amphibious landing vehicle, although the LVT-3 was built by Borg-Warner and Graham-Paige rather than any GM division. Hydra-Matic transmissions, though not Cadillac engines, were also used in the Chevrolet T-17E1 and T-17E2 Staghound 4×4 armored cars, the GMC T-18 and T-18E2 Boarhound 8×8 armored cars, and the abortive Chevrolet M38 Wolfhound 6×6 armored car. (None of the armored cars saw any U.S. service except for testing and evaluation.)

In all, around 25,000 wartime Allied military vehicles used the Hydra-Matic transmission, not including passenger cars. Military duty brought only minor design changes to the civilian Hydra-Matic, but the experience pushed the Detroit Transmission Division to resolve most of the transmission’s early issues and demonstrated that Hydra-Matic was fundamentally sound and reasonably reliable despite its complexity. Postwar Cadillac and Oldsmobile advertising would proudly proclaim that Hydra-Matic had been proven in combat.

HYDRA-MATIC PROLIFERATION

Hydra-Matic really took off during the postwar boom. Buick and Chevrolet still disdained it, eventually opting to develop their own torque converter automatics based on the latest concepts from the corporate engineers, but Pontiac reluctantly adopted Hydra-Matic in 1948. The transmission was enormously popular despite its high prices, which in 1948 ran to $174.25 on a new Cadillac and $185 on an Oldsmobile or a Pontiac (the latter equivalent to more than $1,600 in 2010 dollars). That year, 73% of Pontiac buyers, 97% of Cadillac buyers, and nearly all Oldsmobile buyers opted for Hydra-Matic.



Postwar Hydra-Matics now had a N-D-L-R shift pattern and revised gear ratios. Hydra-Matic didn’t have a Park position until the second-generation Controlled Coupling Hydra-Matic arrived in 1956, but with the engine off, the reverse pawl would effectively lock the transmission output shaft, serving the same purpose. (Photo: “Hydramatic Drive” © 2010 David Cory; used under a Creative Commons Attribution 2.0 Generic license)

The Detroit Transmission Division built its 1 millionth Hydra-Matic in January 1949. By then, it was apparent that not only was automatic transmission a major marketing advantage, lacking an automatic was becoming a serious competitive handicap. Despite the scorn of critics like Mechanix Illustrated‘s Tom McCahill, American buyers were more than happy to accept the drawbacks of automatic transmission if it meant not having to shift.

Inevitably, other automakers were soon forced to follow GM’s lead. Ford and Studebaker turned to Borg-Warner to develop three-speed torque converter transmissions while Packard introduced its proprietary Ultramatic in May 1949.

With the ever-growing demand for Hydra-Matic, the Detroit Transmission Division needed more production capacity than the original factory in Detroit could accommodate. Later that year, the division relocated to a new and much bigger plant in Livonia, Michigan, allowing GM to further expand production and offer Hydra-Matic to automakers that couldn’t afford to develop their own automatics.

Remarkably, one of the first non-GM customers was Lincoln-Mercury, which added Hydra-Matic as a Lincoln option in mid-1949; Ford’s new Fordomatic/Merc-O-Matic wasn’t yet ready and in any case didn’t have the torque capacity for the big Lincoln V-8. Lincoln was followed in short order by Nash, which added Hydra-Matic for the 1950 model year, and then Hudson and Kaiser-Frazer, which introduced Hydra-Matic for 1951. Most outside customers didn’t bother to conceal the Hydra-Matic’s GM origins, happy to take advantage of its reputation and name recognition.



This 1954 Nash Ambassador Custom’s badges proudly announce that it carries the optional Hydra-Matic transmission, which Nash purchased from GM.

Detroit Transmission Division also developed several grades of extra-heavy-duty Hydra-Matic for commercial chassis and heavier vehicles. GMC Truck & Coach introduced Hydra-Matic for some models in 1949 and later extended the option to GMC trucks up to 1½ tons. (Chevrolet made Hydra-Matic optional for some trucks beginning in 1954.) There would also be various other military users, including the M59 armored personnel carrier and, in the early sixties, the M114 tracked command and reconnaissance carrier.

With so many new customers, it took GM less than two years to sell another million Hydra-Matics, making the Hydra-Matic by far the most successful automatic transmission in the world.

Along the way, Hydra-Matic’s internal ratios changed several times, as summarized on the following table. Early Cadillac and military Hydra-Matic transmissions used a compound rear planetary gearset, but to our knowledge, all other iterations used a non-compound rear planetary with a ratio of either 2.53 or 2.63:1.

Hydra-Matic Gear Ratios (Single-Coupling Transmissions) Gear Early Oldsmobile Early Cadillac/

Military Postwar Late Postwar

Civilian* Late Postwar

Military * Not all civilian users adopted these ratios; those that did (principally Cadillac and Pontiac) adopted them for the 1955 model year. 1st 3.66 3.26 3.82 4.10 3.92 2nd 2.53 2.26 2.63 2.63 2.53 3rd 1.44 1.44 1.45 1.55 1.55 4th 1.00 1.00 1.00 1.00 1.00 Reverse -4.31 -3.77 -4.30 -4.62 -4.54

DUAL-RANGE HYDRA-MATIC

The Hydra-Matic received a variety of minor changes throughout its life, including revised gearing, new clutches, and several new oil pump designs. For 1951, there was also a new reverse planetary gearset engaged via a hydraulically operated cone clutch. The previous pawl was retained, but now served only as a parking brake.

From an owner standpoint, the most significant revision was the introduction for the 1952 model year of the new Dual-Range Hydra-Matic. The production Dual-Range transmission was designed by corporate transmission engineer Kenneth W. Gage, although the basic idea had been described in Earl Thompson’s patent disclosures as far back as 1934.

As the name implied, the Dual-Range Hydra-Matic now provided two Drive ranges. For the sake of clarity, we’ll describe them as “D4” and “D3,” although depending on the make of the car, “D3” might also be called “DLeft” or “S” (for Super, not Second) while “D4” might be alternatively described as “DRight” or just “D.” In D4, the transmission would shift normally through all four gears. In D3 range, line pressure was applied to hold the 3–4 shift valve closed, just as if the throttle were floored. The other shift valves were unaffected, so the transmission could still shift normally between the three lower gears. However, it wouldn’t shift into fourth until road speed reached the maximum full-throttle upshift point, which, depending on axle ratio and tire size, was typically between 65 and 72 mph (105 and 115 km/h). If you did floor the throttle in D3, the transmission would kick down into second as long as you were below the maximum allowable downshift speed for that gear. (The normal Low range was also retained, but was revised to give second-gear starts at part throttle, a useful feature in slippery conditions where first would cause too much wheelspin.)

Since the maximum speed of the 3–4 upshift speed was the same in both Drive ranges, selecting D3 didn’t make much difference in flat-out acceleration. (You could slightly improve your times by also using Low to delay the two-three upshift.) The additional Drive range was mostly intended to provide greater flexibility in hilly terrain, although being able to force a manual downshift to third was sometimes handy for highway passing.



The shift quadrant of this 1952 Pontiac shows the dual Drive ranges, indicated by the arrows on either side of the “DR” mark. Oldsmobile labeled these positions “DR” and “S” respectively while other manufacturers typically used “D4” and “D3.” Note that there’s still no separate Park position and Reverse is still located below Low. The latter arrangement was deliberately chosen to make it easier to rock a car free of mud or snow, but some experts (including Oscar Banker) argued that the pattern made it too easy to accidentally select Reverse. The new reverse unit Hydra-Matic adopted in 1951 was supposed to prevent that by locking out Reverse except at very low speeds, but in the mid-sixties, GM finally adopted the now-standard PRNDL pattern for all the corporation’s various automatics. (author photo)

In 1952, General Motors signed a licensing agreement with Rolls-Royce that allowed the prestigious British automaker to manufacture its own version of the Dual-Range Hydra-Matic for Bentley and Rolls-Royce passenger cars. GM also continued to supply Hudson, Kaiser, Lincoln, Nash, and (from 1953 to 1955) Willys, bringing annual Hydra-Matic production to more than 700,000 units.

That production was interrupted in August 1953 by a catastrophic fire that completely destroyed the Detroit Transmission plant in Livonia. The fire was one of the worst industrial disasters of its era, causing six deaths and more than $80 million in damage. To replace the Livonia facility, GM arranged to lease Kaiser’s Willow Run factory, a former bomber plant in Ypsilanti that Kaiser-Frazer had acquired in 1945. Kaiser was happy to be rid of the plant; it had been sitting idle since June, when Kaiser transferred production to Toledo following its merger with Willys-Overland. Willow Run was converted to Hydra-Matic production by mid-September and GM bought the plant outright in November for $26 million. While Willow Run was being retooled, Hydra-Matic users briefly had to substitute other transmissions — Dynaflow for Oldsmobile and Cadillac, Powerglide for Pontiac, and Borg-Warner for most non-GM customers. Hydra-Matic deliveries resumed that November.

The original Hydra-Matic was by then nearing the end of its useful life, mostly because customers were becoming less tolerant of its occasionally harsh shifts. Back in 1952, the Transmission Development Group had started work on a thoroughly redesigned second-generation Hydra-Matic, which finally debuted for the 1956 model year. We’ll look at that transmission and its successors in our second installment.



The Dual-Range Hydra-Matic became available on GMC light trucks in 1952; GMC’s M135 and M211 series of 6×6 military trucks had adopted it in 1951, mated to a two-speed axle. Hydra-Matic became optional on some Chevrolet trucks in 1954. (author photo)

AFTERWORD

Earl Thompson left GM in May 1940 and subsequently established his own company, the Earl A. Thompson Manufacturing Company, based in Ferndale, Michigan. In 1963, Thompson received the Elmer A. Sperry Award and the other members of his group (including Beck, Carnegie, Herndon, Kelley, and Rosenberger) received Citations for their work on the original Hydra-Matic. Thompson died in April 1967 at the age of 75.

Three months after Thompson’s resignation, Charles McCuen, who as Oldsmobile general manager had played no small part in bringing the Hydra-Matic to production, was promoted to vice president of engineering, succeeding O.E. Hunt. Seven years later, McCuen became head of GM’s research division, replacing the retiring Charles Kettering. In 1955, he was badly injured in the crash of the turbine-powered Firebird I research vehicle and took early retirement. He died in 1975.

Harold Metzel, who led Oldsmobile’s development work on the production Hydra-Matic, became Oldsmobile chief engineer in January 1951 and the division’s general manager from July 1964 until his retirement in April 1969. During his later years at Oldsmobile, he would oversee the introduction of the Oldsmobile Toronado, GM’s first front-wheel-drive car.

In August 1940, O.K. Kelley became Thompson’s de facto successor as the head of a reorganized corporate transmission development unit. As we’ll see in our next installment, Kelley would play an important role in GM’s subsequent transmission development efforts, including Dynaflow and Powerglide.

Production of the original single-coupling Hydra-Matic eventually reached more than 7 million units, which does not include heavy trucks, military vehicles, or transmissions built under license by Rolls-Royce (for which we don’t have a count). Most non-military users switched to later transmissions by the late fifties, but GMC offered the older Dual-Range Hydra-Matic on some trucks into the 1962 model year. Rolls-Royce continued to use its license-built version of that transmission for the Phantom V and Phantom VI limousines through 1978.

The Hydra-Matic’s impact, however, has extended far beyond even those numbers. As we said at the beginning, the Hydra-Matic was not the first automatic transmission, but it would be hard to deny that it was the first really successful one. The precedents it set shaped powertrain development for the next 60 years.

In our next installment, we’ll take a look at some of GM’s other early automatics along with the second-generation Controlled Coupling Hydra-Matic and the third-generation Roto Hydra-Matic.

FIN

ACKNOWLEDGMENTS

Special thanks to Alden Jewell and to Lead Archivist Christo Datini of the GM Media Archive, who contacted us after the original publication of this article and provided a very useful table of original Hydra-Matic production totals.

NOTES ON SOURCES

Our sources on Earl Thompson, Charles McCuen, and the development of Synchro-Mesh and the Automatic Safety Transmission included “Almost Automatic,” Special Interest Autos #20 (January-February 1974), pp. 24-27; John Barach, “Automobile Drivetrain History,” Motor Era, 1999, www.motorera. com/ history/ hist10.htm, accessed 10 May 2010; Buick Motor Division of General Motors Sales Corporation, “Buick 1938,” [brochure, ca. October 1937]; “Deaths,” Astronautics & Aeronautics Vol. 5 (1967), p. 33; George Derby and James Terry White, eds., The National Cyclopædia of American Biography Vol. 53 (New York: J.T. White & Co., 1971), pp. 462–463; Terry B. Dunham and Lawrence R. Gustin, The Buick: A Complete History (An Automobile Quarterly Magnificent Marque Book), Third Edition (Kurtztown, PA: Automobile Quarterly, 1987); Helen Jones Earley and James R. Walkinshaw, Setting the Pace: Oldsmobile’s First 100 Years (Lansing, MI: Oldsmobile Division of General Motors Corporation, 1996); General Motors Corporation, “GM Powertrain: Past, Present, Future,” www.gm. com/experience/ technology/ gmpowertrain/ about/powertrain_history.jsp [now www.gmpowertrain. com], accessed 28 May 2010; “Generations of GM History: McCuen, Charles L.,” Generations of GM History, GM Heritage Center, history.gmheritagecenter. com, accessed 17 May 2010; Philip G. Gott, Changing Gears: The Development of the Automotive Transmission (SAE Historical Series) (Warrendale, PA: Society of American Engineers, 1991); Stan Grayson, “Retrospect: 1938 Oldsmobile LA38,” Motor Trend Vol. 32, No. 11 (November 1980): 114–121; Maurice D. Hendry, Cadillac: Standard of the World: The Complete History (Fourth Edition update by David R. Holls) (Princeton, NJ: Automobile Quarterly, 1990); “Result of Impatience,” The Montreal Gazette 7 February 1931, p. 6; Harvey S. Jacobs, Ferndale, 1918-1943: 25 Years of Progress (Ferndale, MI: Harvey S. Jacobs, 1943); the Old Car Brochures website (oldcarbrochures.org); the Old Car Manual Project (www.oldcarmanualproject. com); Scott Oldham and Michael Lamm, “Happy 100th!” Popular Mechanics Vol. 173, No. 5 (May 1996), pp. 47–59; Oldsmobile Division of General Motors Sales Corporation, “Oldsmobile Six and Eight” [brochure], ca. October 1937, and “Oldsmobile Series 60 – Series 70 – Series 80” [brochure], ca. October 1938; Alfred P. Sloan with John McDonald, My Years with General Motors (Garden City, NY: Doubleday, 1964); Earl A. Thompson, “Automatic Gear-Shifting Mechanism for Sliding Gear Transmission,” U.S. Patent No. 1,435,430, filed 9 March 1918, issued 14 November 1922, reissued 25 September 1928; assignor to General Motors Corporation, “Automatic Gear Shifting Mechanism,” U.S. Patent No. 2,101,825, filed 9 October 1923, issued 7 December 1937; “Transmission,” U.S. Patent No. 1,827,960, filed 15 April 1925, issued 20 October 1931; assignor to General Motors Corporation, “Power Transmission Mechanism,” U.S. Patent No. 1,876,098, filed 15 April 1925, issued 6 September 1932; assignor to General Motors Corporation, “Transmission,” U.S. Patent No. 1,888,640, filed 15 April 1925, issued 22 November 1932; assignor to General Motors Corporation, “Clutch Mechanism for Transmissions,” U.S. Patent No. 1,854,281, filed 2 August 1926, issued 19 April 1932; assignor to General Motors Corporation, “Change Speed Gearing with Automatic Overdrive,” U.S. Patent No. 2,285,760, filed 6 March 1933, issued 9 June 1942; assignor to General Motors Corporation, “Change-Speed Transmission and Control,” U.S. Patent No. 2,195,605, filed 8 October 1934, issued 2 April 1940; assignor to General Motors Corporation, “Change-Speed Mechanism and Control,” U.S. Patent No. 2,193,304, filed 16 October 1935, issued 12 March 1940; assignor to General Motors Corporation, “Clutch and Gearing Control,” U.S. Patent No. 2,362,418, filed 15 March 1937, divided 14 February 1940, issued 7 November 1944; and assignor to Earl A Thompson Mfg. Company, “Vehicle Visor,” U.S. Patent No. 2,492,074, filed 16 July 1946, issued 20 December 1949; and Angelo Van Bogart, Cadillac: 100 Years of Innovation (Iola, WI: Krause Publications, 2003).

Additional information on the Hydra-Matic came from Thomas Bonsall, “The Great Hydra-Matic Fire,” February 1999, www.autotran. us/ TheGreatHydraMaticFire.html, accessed 10 June 2010; Ray T. Bohacz, “Mechanical Marvels: Shiftless Driving: Oldsmobile introduces the fully automatic transmission,” Special Interest Autos #180 (November-December 2000), pp. 54-56, and “Mechanical Marvels: Shiftless Pleasure: The 1956 General Motors Hydra-Matic Transmission,” Hemmings Classic Car #35 (August 2007), pp. 68–71; Griff Borgeson, “Cadillac Motor Trial,” Motor Trend Vol. 3, No. 11 (November 1951), reprinted in Cadillac Automobiles 1949-1959, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1990), pp. 14–16; Arch Brown, “1949 Pontiac Eight: ‘The Most Beautiful Thing on Wheels,'” Special Interest Autos #111 (May-June 1989), reprinted in The Hemmings Motor News Book of Pontiacs: driveReports from Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor News, 2001), pp. 18–26; “SIA comparisonReport: 1949 Cadillac vs. 1949 Oldsmobile 98: Similar But Different,” Special Interest Autos #149 (September-October 1995), reprinted in The Hemmings Book of Oldsmobiles: driveReports from Hemmings Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor News, 2001), pp. 34–41; and “1948 Cadillac 61: The First Shall Be Last,” Special Interest Autos #171 (May-June 1999), reprinted in The Hemmings Motor News Book of Cadillacs: driveReports from Hemmings Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor News, 2000), pp. 52-59; Arch Brown, Richard M. Langworth, and the Auto Editors of Consumer Guide, Great Cars of the 20th Century (Lincolnwood, IL: Publications International, Ltd., 1998); Martin Bunn, “Gus and Joe Go to the Show,” Popular Science Vol. 137, No. 5 (November 1940), pp. 136–140, 241–242; the Cadillac & LaSalle Club Modified Chapter website (www.modifiedcadillac.org); Cadillac Motor Car Division of General Motors Corporation, Service Department, “Cadillac…From Peace to War” [brochure], 1943; Cadillac Shop Manual for 1941 (Detroit, MI: General Motors Division, 1940); Cadillac Shop Manual Supplement for 1946 (Detroit, MI: General Motors Division, 1945); Cadillac Shop Manual Supplement for 1951 (Detroit, MI: General Motors Corporation, 1951); Cadillac Shop Manual Supplement for 1955 (Detroit, MI: Cadillac Motor Car Division, 1955); Cadillac Hydra-Matic Drive Shop Manual: Shop Manual Covering Construction, Operation, Adjustment and Repair of the Hydra-Matic Drive as Used on the 1941 Cadillac Cars (Detroit, MI: General Motors Division, 1941); “Cadillac: Standard of the World 1946” [brochure], December 1945; and “Data Book” [salesman’s guide], 13 September 1940; William L. Carnegie, assignor to General Motors Corporation, “Variable Speed Control,” U.S. Patent No. 2,221,393, filed 1 July 1938, issued 12 November 1940, and “Compound Planetary Gear Train,” U.S. Patent No. 2,606,459, filed 6 December 1947, issued 12 August 1952; Alan Chanter, “Landing Vehicle Tracked,” World War II Database, ca. 2012, ww2db. com/ vehicle_spec.php?q=301, accessed 20 September 2015; Chevrolet Motor Division of General Motors Corporation, “1954 Chevrolet Advance-Design Trucks: For Loads of Value: [brochure 1,000 M], October 1953; “Continental Road Test No. 2C/50 — The Cadillac V.8 S.62,” The Motor 22 March 1950, reprinted in reprinted in Cadillac Automobiles 1949-1959, pp. 10-12; John Day, The Bosch Book of the Motor Car: Its evolution and engineering development (New York: St. Martin’s Press, 1976); Detroit Transmission Division of General Motors Corporation, “Detroit Transmission Division Welcomes You to the Home of Hydra-Matic” [flyer, ca. 1958], via GM Heritage Center; “Detroit Transmission Division, General Motors Factory,” Emporis, n.d., www.emporis. com/ buildings/ 334613/ detroit-transmission-division- general-motors-factory-detroit-mi-usa, accessed 19 September 2015; David Edwards, Antique Automatic Transmission Parts, 21 July 2002, www.autotran.us, accessed 15 May 2010; J. Kelly Flory, Jr., American Cars, 1946–1959: Every Model, Year By Year (Jefferson, NC: McFarland & Company, Inc., Publishers, 2008); GMC Truck & Coach Division, General Motors Corporation, “GMC Pickups” [brochure Adv. 430 6-61, June 1961]; the GM Heritage Archive (gmheritagecenter. com/ gm-heritage-archive/); T. Grace, Automatic Transmission Service Guide (Union, NJ: Lincoln Technical Institute, September 1966); Ken Gross, “1942 Oldsmobile B-44,” Special Interest Autos #40 (May-July 1977), reprinted in The Hemmings Book of Oldsmobiles, pp. 20–24; John Gunnell, ed., Standard Catalog of Pontiac 1926–2002 (Iola, WI: Krause Publications, 2002); Roger Huntington, “The Great Transmission Controversy: Coupling vs. Converter,” Car Life Vol. 10, No. 2 (March 1963), pp. 18-21; “History of the Hydra-Matic transmission, 1932–1967,” www.autotran. us/ hydramatic_history.html, accessed 10 October 2015; David D. Jackson, “The U.S. / American Automobile Industry in World War II / WW II: Detroit Transmission Division of General Motors Corporation in World War Two / WWII,” 19 February 2015, usautoindustryworldwartwo. com/ General%20Motors/ detroit-transmission.htm, accessed 20 September 2015; Robert Johnson, “Cadillac 62 Sedan,” Motor Life August 1954, reprinted in Cadillac Automobiles 1949-1959, pp. 39–40; Robert C. Juvinall and Kurt M. Marshek, Machine Component Interrelationships, Fifth ed. (Hoboken, NJ: John Wiley and Sons, Ltd., 2011); Oliver K. Kelley, assignor to General Motors Corporation, “Combination Fluid Turbo Clutch and Variable Speed Gearing,” U.S. Patent No. 2,176,138, applied 5 February 1937, issued 17 October 1939; “Fluid Flywheel Gearing Arrangement,” U.S. Patent No. 2,211,233, applied 10 April 1939, issued 13 August 1940; and “Transmission Drive,” U.S. Patent No. 2,377,696, filed 15 December 1941, issued 5 June 1945; Al Kidd, “’55 Olds Super 88,” Motor Trend Vol. 7, No. 5 (May 1955), reprinted in Oldsmobile Automobiles 1955-1963, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1989), pp. 5–6, 55; Ted Koopman, “Speed Age Tests the 1952 Pontiac,” Speed Age March 1952, reprinted in Pontiac Limited Edition Extra 1949-1960, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1999), pp. 14–17; Julian P. Leggett, “Testing the Cadillac Series 62,” Science and Mechanics October 1952, reprinted in Cadillac Automobiles 1949-1959, pp. 24–26; Jim Lodge, “Pontiac Star Chief,” Motor Trend Vol. 6, No. 5 (May 1954), reprinted in Pontiac Limited Edition Extra 1949-1960, pp. 32–34; Terry McGean, “GM Hydra-Matic,” Hemmings Motor News September 2006; Pete Molson and Walt Woron, “Pontiac Eight,” Motor Trend Vol. 5, No. 5 (May 1953), reprinted in Pontiac Limited Edition Extra 1949-1960, pp. 23–26; Mike Mueller, Pickup Trucks (Osceola, WI: Motorbooks, 2003); L.H. Nagler, “How Your Car Shifts for Itself,” Popular Mechanics Vol. 89, No. 5 (May 1948): 102–106, 264, 268, 272; ‘Nailhed,’ “The Night Shift,” Nailhed.com, June 2014, www.nailhed. com/ 2014/06/ the-night-shift.html, accessed 25 June 2014; Eric Nielssen, “Six Luxury Cars: a view from the Automotive Engineering Side,” Car and Driver Vol. 11, No. 1 (July 1965), 26–31, 62–65, 75; Jan P. Norbye and Jim Dunne, Pontiac 1946-1978: The Classic Postwar Years (Osceola, WI: Motorbooks International, 1979); “Oldsmobile,” Automotive Industries Vol. 83 (1 October 1940), p. 302; Oldsmobile Motor Division of General Motors Corporation, “For 1952 Oldsmobile Rockets to New Highs” [1952 model year brochure], 1952; “Hydra-Matic Drive: Designed, Developed and Introduced by Oldsmobile” [brochure, ca. Oct. 1940], “Oldsmobile Hydra-Matic Drive” [brochure, ca. 1946], and “Oldsmobile’s Exclusive Hydra-Matic Drive” [brochure, ca. October 1939], via GM Heritage Center; and “Driving Sensation of the Year! Whirlaway with Hydra-Matic Drive” [brochure], 1948; 1940 Oldsmobile Shop Manual (Lansing, MI: Oldsmobile Division of General Motors Sales Corporation, 1939); “Oldsmobile Six and Eight” [1938 model year brochure], 1937; “Oldsmobile: Styled to Lead, Built to Last” [1941 model year brochure], 1940; “New Oldsmobile” [1946 model year brochure], 1945]; “The One BIG New Engineering Feature in the 1940 Cars!” [advertisement], Popular Science Vol. 72, No. 6 (December 1939), p. 5; Terry Shea, “A Valiant Effort! 1947 Oldsmobile 66,” Hemmings Classic Car #118 (July 2014); Bill Siuru, “Hydra-Matic celebrates its 60th birthday,” Old Cars Weekly 1 November 1999; “Six Luxury Cars: A subjective, seat-of-the-pants evaluation by the editors,” Car and Driver Vol. 11, No. 1 (July 1965): 23–25, 62–65; “Sperry Award Recipients,” The Elmer A. Sperry Award website, sperryaward. org/recipients.htm, accessed 24 September 2015; “Streamliners and Chieftains by Pontiac,” The Motor 16 March 1949, reprinted in Pontiac Limited Edition Extra 1949-1960, p. 5; “Testing the New Dual-Range H-M Pontiac,” Motor Trend Vol. 4, No. 4 (April 1952), reprinted in Pontiac Limited Edition Extra 1949-1960, pp. 19–21; “The Autocar Road Tests No. 1447: Cadillac Series 62 Saloon,” The Autocar 9 November 1951, reprinted in Cadillac Automobiles 1949-1959, pp. 17–19; “The High-Compression Cadillacs,” The Motor 2 March 1949, reprinted in Cadillac Automobiles 1949-1959, pp. 5–7; Earl A. Thompson, assignor to General Motors Corporation, “Change Speed Gearing and Control,” U.S. Patent No. 2,204,872, filed 1 April 1938, issued 18 July 1940; “Fluid Coupling Rotor,” U.S. Patent No. 2,357,295, filed 5 February 1940, issued 5 September 1944; and “Rotary Hydraulic Coupling of the Turbine Type,” U.S. Patent No. 2,430,258, filed 17 February 1941, issued 4 November 1947; William K. Toboldt and Larry Johnson, Goodheart-Willcox Automotive Encyclopedia (South Holland, IL: The Goodheart-Willcox Company, Inc., 1975), pp. 558-574; Robert Temple, “Transmissions and Drive Lines (Know Your Car Part Two),” Motor Trend Vol. 15, No. 1 (January 1963), pp. 54-59; United Motors Service Division, The Hydra-Matic Transmission 1946-1955: On-the-Car Adjustment Service Manual (Detroit, MI: United Motors Service Division, 1956); Josiah Work, “This Is Not Your Grandpa’s Oldsmobile: 1949 Rocket 88,” Special Interest Autos #139 (January-February 1994), reprinted in The Hemmings Book of Oldsmobiles, pp. 26–33; Walt Woron, “Pontiac Motor Trial: 3500-Mile Motor Trial Proves Fine Family Car Tops in Town and on the Open Road,” Motor Trend Vol. 3, No. 9 (September 1951), reprinted in Pontiac Limited Edition Extra 1949-1960, pp. 10–13, and “Road Testing the 50th Anniversary Cadillac,” Motor Trend Vol. 4, No. 9 (September 1952), reprinted in Cadillac Automobiles 1949-1959, pp. 20–23, 33; and Walt Woron and Pete Molson, “Cadillac: America’s Favorite Luxury Car,” Motor Trend Vol. 5, No. 5 (May 1953), reprinted in Cadillac Automobiles 1949-1959, pp. 34–37; and emails to the author from Christine Crawford, 23–25 June 2014. Production figures for passenger car Hydra-Matics came from a table provided by Christo Datini, General Motors Media Archive, General Motors Heritage Center (email to author, 1 June 2010). Prof. John D. Kelly later helped us to sort out some technical points in emails to the author, 7 to 8 March 2017.

Additional information on Reo’s Self-Shifter came from David Traver Adolphus, “Almost Classic: 1935 Reo Royale 75,” Hemmings Classic Car #36 (September 2007), pp. 50–55; the Auto Editors of Consumer Guide, “How Reo Cars Work,” HowStuffWorks.com, 15 June 2007, auto.howstuffworks. com/ reo-cars.htm, accessed 15 May 2010; Horace T. Thomas, John Bethune, and Albert B. Hays, assignors to Reo Motor Car Co., “Transmission Mechanism,” U.S. Patent No. 1,885,156, filed 3 April 1931, issued 1 November 1932; Horace T. Thomas and John Bethune, assignors to Reo Motor Car Co., U.S. Patent No. 1,988,636, “Transmission Mechanism,” filed 12 February 1932, granted 22 January 1935; Horace T. Thomas and Albert B. Hays, assignors to Reo Motor Car Co., U.S. Patent No. 1,988,466, “Transmission Mechanism,” filed 17 October 1932, granted 22 January 1935; Horace T. Thomas, assignor to Reo Motor Car Co., U.S. Patent No. 1,950,580, “Automatic Variable Speed Transmission,” filed 16 January 1933, granted 13 March 1934, and U.S. Patent No. 1,950,581, “Variable Speed Transmission Mechanism,” filed 30 January 1933, granted 13 March 1934; Horace T. Thomas and Albert B. Hays, assignors to Reo Motor Car Co., U.S. Patent No. 2,016,350, “Transmission Mechanism,” filed 18 September 1933, granted 8 October 1935, and U.S. Patent No. 2,038,812, “Transmission Mechanism,” filed 8 June 1934, granted 28 April 1936; Reo Motor Car Co., “Gearshifting ABOLISHED by Reo–Not Merely Made Easier” [advertisement], Popular Science Vol. 125, No. 1 (July 1934), p. 11, and “This Amazing Invention Abolishes Gear-Shifting by Hand in the New 1934 REO” [advertisement], Popular Science Vol. 124, No. 4 (May 1934), p. 8.

Additional information on the Daimler Fluid Flywheel came from Kevin Bennett, “How to drive a Wilson Pre-selector,” My Daimlers and Lanchesters, n.d., daimlerandlanchester. com/ how-to-drive-a-pre-selector/, accessed 12 September 2015; Tony Cooper, “Daimler and Lanchester History and Models, 1925-1938,” n.d., www.daimler. co.uk, accessed 30 August 2015; “Daimler History,” Unique Cars and Parts, n.d., www.uniquecarsandparts.com. au, accessed 12 September 2015; Laurence H. Pomeroy, “Power Transmission Mechanism,” U.S. Patent No. 1,975,700, filed 32 May 1932, issued 2 October 1934; Laurence H. Pomeroy and Alfred Blundell, assignors to the Daimler Company Ltd., “Power Transmission Mechanism,” U.S. Patent No. 1,914,289, filed 21 April 1932, issued 13 June 1933; Harold Sinclair, “Power Transmission Mechanism and Clutch,” U.S. Patent No. 1,831,770, filed 16 March 1929, issued 10 November 1931; “Hydraulic Transmission Gear and Brake,” U.S. Patent No. 1,859,607, filed 17 June 1929, issued 24 May 1932; “Hydraulic Coupling,” U.S. Patent No. 1,937,364, filed 8 January 1931, issued 28 November 1933; “Hydraulic Coupling,” U.S. Patent No. 1,963,720, filed 8 January 1931, issued 19 June 1934; “Power Transmission Mechanism,” U.S. Patent No. 1,978,172, filed 19 September 1931, issued 23 October 1934; and “Power Transmission Mechanism,” U.S. Patent No. 2,102,755, filed 19 September 1931, issued 21 December 1937; and “Vulcan-Sinclair Fluidrive Makes headway,” The Commercial Motor 16 August 1946, p. 43, archive.commercialmotor. com, accessed 15 September 2015.

Additional information on the M-3, M5, and M24 tanks and other Hydra-Matic-equipped military vehicles came from “AUTOS: The New Generation,” TIME 5 October 1959, www.time. com, accessed 14 July 2010; Joe Baugher, “North American B-25G Mitchell,” 10 March 2000, www.joebaugher. com/ usaf_bombers/ b25_13.html, accessed 17 May 2010; “North American B-25H Mitchell,” 11 March 2000, www.joebaugher. com/ usaf_bombers/ b25_15.html, accessed 17 May 2010; Alan Chanter, “Landing Vehicle Tracked,” World War II Database, ca. 2012, ww2db. com/ vehicle_spec.php?q=301, accessed 20 September 2015; “Chevrolet Staghound T-17E2 (AA),” n.d., ramrao2.tripod. com/ staghound/, accessed 20 September 2015; “Cole, Edward N.,” Generations of GM, GM Heritage Center, n.d., history.gmheritagecenter. com/wiki/ index.php/ Cole,_Edward_N., accessed 14 July 2010; Chris Conners, The AFV Database, “Armored Car T17E1 Staghound,” 1 May 2012, afvdb.50megs. com/ usa/ t17e1.html, accessed 21 September 2015; “Armored Command and Reconnaissance Carrier M114,” afvdb.50megs. com/usa/ m114.html, accessed 21 September 2015; “Armored Personnel Carrier M59,” 3 December 2002, afvdb.50megs. com/usa/ apcm59.html, accessed 21 September 2015; “Light Armored Car M38 Wolfhound,” 2 April 2014, afvdb.50megs. com/ usa/ m38wolfhound.html, accessed 21 September 2015; “Light Tank M3 Stuart,” 22 July 2007, afvdb.50megs. com/ usa/ m3stuart.html, accessed 17 May 2010; “Light Tank M5 Stuart,” 1 June 2009, afvdb.50megs. com/ usa/ m5stuart.html, accessed 21 September 2015; “Light Tank M24 Chaffee”(29 May 2007, afvdb.50megs. com/ usa/ m24chaffee.html, accessed 21 September 2015; “105mm Howitzer Motor Carriage M37,” 8 March 2008, afvdb.50megs. com/usa/ 40mmgmcm19.html, accessed 22 September 2015; “155mm Howitzer Carriage M41,” 15 October 2014, afvdb.50megs. com/usa/ 40mmgmcm19.html, accessed 22 September 2015; and “Twin 40mm Gun Motor Carriage M19,” 24 April 2014, afvdb.50megs. com/usa/ 40mmgmcm19.html, accessed 22 September 2015; the Editors of Publications International, Ltd., “M-3 Stuart (Honey)/M-5 Light Tank,” HowStuffWorks.com, 17 November 2007, science.howstuffworks. com/ m-3-stuart-honey-m-5-light-tank.htm, accessed 17 May 2010, and “M-24 Chaffee Light Tank,” HowStuffWorks.com, 14 November 2007, science.howstuffworks. com/ m-24-chaffee-light-tank.htm, accessed 17 May 2010; Shawn A. Fisher, GURPS WWII: Dogfaces (Austin, TX: Steve Jackson Games, 2003), p. 78; Martin Foray, “M114 CRV Command and Reconnaissance Vehicle,” Military Factory, 3 August 2015, www.militaryfactory. com/ armor/ detail.asp?armor_id=108, accessed 21 September 2015; and “M-59 Armored Personnel Carrier,” and “M-135 Series 2 1/2-ton, 6×6 Trucks,” Olive-Drab.com, n.d., olive-drab.com, accessed 22 September 2015; the Historical Forces Association 9th Division Infantry website (www.easy39th.com); U.S. Department of the Army, Light Tank M24 (Department of the Army Technical Manual TM 9-729) (Washington, DC: U.S. Government Printing Office, May 1951); and Ordnance Field and Depot Maintenance: 301MG and 303M Hydra-Matic Transmissions (Department of the Army Technical Manual TM 9-8025-2) (Washington, D.C.: U.S. Government Printing Office, 18 June 1957); and U.S. War Department, Ordnance Maintenance: Hydra-Matic Transmission and Propeller Shafts for Light Tanks M5, M5A1, and 75-MM Howitzer Carriage (War Department Technical Manual TM 9-1727C (Washington, DC: U.S. Government Printing Office, 5 February 1943).

Other background information came from David Traver Adolphus, “Cast-Iron Wonder – 1931 AE Sport Coupe,” Hemmings Classic Car #21 (June 2006), pp. 28–35; John O. Almen, assignor to General Motors Corporation, “Control for Toric Friction Transmission,” U.S. Patent No. 2,045,558, filed 20 December 1934, issued 23 June 1936; John O. Almen and John Dolza, assignors to General Motors Corporation, “Hoop Governor with Double Acting Weight,” U.S. Patent No. 1,984,006, filed 11 July 1932, issued 11 December 1934; John O. Almen and Winfield D. Gove, assignors to General Motors Corporation, “Friction Transmission Control Mechanism,” U.S. Patent No. 2,073,134, filed 7 May 1936, issued 9 March 1937; John O. Almen and Harry Hawkins, assignors to General Motors Corporation, “Control for Variable Speed Power Transmissions,” U.S. Patent No. 2,131,157, filed 9 March 1931, issued 27 September 1938; William C. Anderson, “Charles A. Chayne, Buick’s Unsung Hero,” The Buick Bugle September 2003, www.buickheritagealliance. org/ pdf/ chayne.pdf, accessed 20 May 2010; “Armenia: Contributions,” www.jdemirdjian. com, 11 January 2009, accessed 3 July 2010); Oscar H. Banker, “Transmission Mechanism,” U.S. Patent No. 1,795,465, filed 26 November 1928, issued 10 March 1931; Oscar H. Banker, assignor to Continental Illinois Bank and Trust Company, “Transmission,” U.S. Patent No. 1,795,464, filed 21 October 1927, issued 10 March 1931; “Transmission,” U.S. Patent No. 2,003,963, filed 21 March 1930, issued 4 June 1935; “Automatic Transmission,” U.S. Patent No. 1,843,193, filed 9 April 1930, issued 2 February 1932; “Automatic Change Speed Transmission,” U.S. Patent No. 1,843,195, filed 12 February 1931, issued 2 February 1932; “Automatic Clutch,” U.S. Patent No. 1,851,146, filed 20 March 1930, issued 29 March 1932; “Automatic Change Speed Transmission,” U.S. Patent No. 1,943,293, filed 24 July 1931, issued 16 January 1934; Oscar H. Banker, assignor to New Products Corporation, “Variable Speed Transmission,” U.S. Patent No. 1,937,503, filed 3 September 1931, issued 5 December 1933; “Clutch Mechanism,” U.S. Patent No. 2,042,454, filed 19 March 1932, issued 2 June 1936; “Automatic Change Speed Transmission,” U.S. Patent No. 1,996,790, filed 3 November 1932, issued 9 April 1935; “Change Speed Transmission,” U.S. Patent No. 1,985,884, filed 14 December 1932, issued 1 January 1935; “Planetary Transmission Mechanism,” U.S. Patent No. 2,005,726, filed 29 June 1933, issued 25 June 1935; “Change Speed Transmission,” U.S. Patent No. 2,077,387, filed 16 July 1934, issued 20 April 1937; “Clutch Mechanism,” U.S. Patent No. 2,104,014, filed 16 July 1934, issued 4 January 1938; “Automatic Transmission,” U.S. Patent No. 2,199,095, filed 13 October 1934, issued 30 April 1940; “Change Speed Transmission,” U.S. Patent No. 2,140,502, filed 30 November 1934, issued 20 December 1938; “Automatic Transmission,” U.S. Patent No. 2,171,534, filed 29 May 1935, issued 5 September 1939; “Automatic Transmission,” U.S. Patent No. 2,262,747, filed 18 September 1936, issued 18 November 1941, reissued 18 May 1943; and “Automatic Transmission,” U.S. Patent No. 2,237,297, filed 15 September 1937, issued 8 April 1941; Oscar H. Banker (with Robert Hull), Dreams and Wars of an American Inventor: an immigrant’s romance (Bay Village, OH: Bob Hull Books & Features, 1982); John Barach’s Cadillac History site, Motor Era, June 2002, www.motorera. com/ cadillac/ index.htm, accessed 15 May 2010; Bendix Products Corporation, “The Hudson Electric Hand – Transmission Control: Technical Information 1935 – 1938,” ca. 1938; Arch Brown, “High-Fashion Hauler: 1948 Buick Roadmaster Estate Wagon,” Special Interest Autos #136 (July-August 1993): pp. 12–19, 62–63; and “Supercharged Sensation: 1937 Cord 812-SC,” Special Interest Autos #110 (April 1989): 28–35; Bill Carroll, “Inside Pontiac’s Terrific Tempest!” Sports Cars Illustrated October 1960 and “Pontiac Tempest Road Research Report,” Sports Cars Illustrated March 1961, reprinted in Car and Driver on Pontiac 1961–1975, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1986), pp. 5-16; Chevrolet Motor Division of General Motors Corporation, Chevrolet 1950–1953 Powerglide Automatic Transmission Repair Manual (Detroit, MI: General Motors Corporation, 1952);Jim Dunne and Jan P. Norbye, Buick 1946-1978: The Classic Postwar Years, 2nd. ed. (Osceola, WI: MBI, Inc./Motorbooks International, 1993); Alfred D. Flinn and Ruth Cobb, The National Research Council of the National Academy of Sciences, “Research Laboratories in Industrial Establishments of the United States,” Bulletin of the National Research Council Vol. 3, No. 16 (December 1921); “New Cars Described: Porsche Modifications,” The Autocar 12 December 1952, reprinted in Porsche 356 Ultimate Portfolio, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 2006), pp. 8–9; Hermann Föttinger, “Flüssigkeitsgetriebe mit einem oder mehreren treibenden und einem oder mehreren getriebenen Turbinenräder zur Arbeitsübertragung zwischen benachbarten Wellen,” DRP Nr. 221422, filed 24 June 1905, issued 25 April 1910; Hermann Föttinger, assignor to Stettiner Maschinenbau AG ‘Vulcan,’ “Hydraulic Device for Transmitting Power,” U.S. Patent No. 1,199,359, filed 19 June 1906, issued 26 September 1916; “Transmission Device,” U.S. Patent No. 1,199,360, filed 26 January 1910, issued 26 September 1916; and “Transmission Device,” U.S. Patent No. 1,199,361, filed 26 January 1910, issued 26 September 1916; General Motors Corporation, “Thirty-First Annual Report of General Motors Corporation: Year Ended December 31, 1939,” 30 Ap