It’s hard to fully comprehend the ambition of some automakers, engineers or gearheads. But a look at their failed experiments sometimes helps.

The powerplants in this list are, to put it mildly, out there, and offer us a look at what manufacturers would stuff under their vehicles’ heads if they were given free reign.

But for one reason or another, they couldn’t or can’t make these engines in large numbers, and instead had to scrap them. Still—we can’t help wondering “what if?”

Cadillac 7.4-litre V12

When you think of a V12, you probably think of high-revving low-displacement screaming monsters designed to power the highest echelon of sports car, but that was never really Cadillac’s scene.

In the 1930s, Cadillac V12s were implemented to instead create the smoothest drive possible, and in the mid-’60s the company tried to do it again with a wild overhead-cam design.

Between 1963 and 1964, Cadillac built six prototypes of the would-be V8-replacement, displacing 7.4-litres initially, but growing to 8.2 by the final version. Depending on the induction system, the V12 made between 295 and 394 horsepower, and from 418 to 506 lb.-ft. of torque.

The engine was slated to debut in the new front-wheel-drive Eldorado for 1967, although engineers raised concerns about it fitting in the original transverse layout. Once plans were changed for the engine to fit longitudinally, the engine would theoretically fit, but ultimately emissions regulations would be the death of Cadillac’s new V12.

Corvair modular air-cooled V10

The story of the Corvair is pretty tragic—almost every aspect of the car makes you think about what “could have been.” This engine is no exception, and with a modular design that allowed up to 12 cylinders, it was truly a marvel.

In 1961, the Corvair program seemed to be going strong enough that GM decided it would develop a modular engine for the next generation of the vehicle.

The modular engine used a separate individual cylinder heads for each cylinder, instead of one head per bank like the standard six. This would allow each iteration of the engine to use the same pistons and cylinders while only changing the “bottom end”, allowing for as little as two cylinders or as many as 12.

GM built a 10-cylinder version of the engine, and installed it in a 1962 Impala for testing; as well as a four-cylinder version (made by cutting two cylinders off of the six-cylinder) for small front-wheel-drive vehicles. Eight- and twelve-cylinder versions were never actually built.

The proposed engine was slated to be introduced in 1964, but in the end would prove too expensive to build, as all of the tooling would have to be updated. Furthermore, GM had begun its move toward a conventional rear-wheel-drive platform for its upcoming small-car program.

Oldsmobile DOHC 455 OW43

This engine was born out of a General Motors edict from 1967 that stated that no other product would receive multiple carburetors but the Corvette—so whatever Oldsmobile was making would have to make do with a single four-barrel Quadrajet.

It was that rule that pushed unsatisfied engineers to seek, and earn, approval to develop a new W43 V8 project. The engine started life as an overhead-valve engine but quickly moved to dual-overhead cams due to extreme pushrod angles.

This 455-cubic-inch engine featured four valves per cylinder, and a gear-drive system; the cams themselves were drilled to hold oil in their journals. The OW43 produced an amazing 12.2:1 compression ratio when dyno-tested in 1969, which helped it make 700 horsepower at 7,000 rpm measured at the crankshaft.

Unfortunately, this engine never saw production due to General Motors’ plans to reduce compression for all engines in 1971 so they could run on unleaded 91-octane gasoline.

General Motors Rotary Combustion Engine (GMRCE)

In November of 1970, GM wanted to build its own rotary engine, so it paid US$50 million to licence the Wankel designs made famous by Mazda vehicles. Instead of the tradition single rotor, though, GM’s version was a two-rotor design that displaced 206 cubic inches, or 3.38-litres.

The engine was all-aluminum instead of iron like other GM engines—this helped it cool down much easier, and had the added side effect of being much lighter. The GMRCE scored well in GM’s testing trials, where it was able to run up to 800,000 kilometres showing only minimal wear.

The final version of the engine, designated RC2-206, actually saw cold-weather testing in Canada, with 1973 Vega economy cars being the recipient of those prototype mules. The engine also most likely made its way into the XP-987 GT mid-engine Corvette concept from 1973.

The program would ultimately be a waste of time, as the engine would not meet the emissions standards of 1975, and with the fuel economy of the rotary unable to best the standard four-cylinder from the Vega, it was doomed. The looming fuel crisis also made the engine particularly unattractive.

Subaru 1235 V12 F1

In the late ’80s, inspired by other Japanese automakers entering Formula One as engine suppliers, Subaru decided to jump into the racing-motor game, contracting Motori Moderni to help them build an engine in 1989.

What they came up with was a 3.5-litre 60-valve flat-twelve engine in a boxer configuration, just like its modern road cars. It was named “1235: for its cylinder count, and its displacement.

The engine only made 559 horsepower in initial testing, which was more than 100 less than rival Honda’s RA109E V10, which produced 660. Motori Moderni’s owner Carlo Chiti was targetting a maximum output of 599 horsepower, which would have made it competitive.

The flat layout, however, was somewhat outdated at this point, with other manufacturers dropping it in favour of engines that weren’t as wide. But Chiti believed the engine would provide a lower centre of gravity, and didn’t give up on it.

The engine was 261 kg when all was said and done, a lot heavier than its competitors, which didn’t help its case. It was supposed to help drive Coloni’s F1 efforts—Subaru even purchased half of the team. Stuffed into a modified Coloni C3 called the C3B, the engine was pushed into service for the 1990 season.

During qualifying at the 1990 United States Grand Prix, the engine proved to be fragile and under-powered, and after a gear linkage failure it ended up leaving them dead last. That pretty much spelled out the story for the engine; it was never able to get the team through pre-qualifying after that, and was dropped before the 1991 season.

Jaguar V8

The success of the E-Type pressed Jaguar to draft a multitude of prototypes in its hunt for a worthy successor. The company tried out many different body styles and layouts before settling on the 5.3-litre V12-powered XJS.

In between the E-Type and XJS, though, were the XJ21 and the XJ17, which some consider more direct replacements for the E-Type.

The XJ17 was meant to use a brand-new engine from Jaguar that borrowed design traits from the 5.0-litre V12 unit found in its XJ13 race car. The result was a 3.5-litre 60-degree dual-overhead-cam V8.

The engine was essentially the same V12, but with the rear four cylinders hacked off the block. This made production simple, as the block could be machined on the same line as the V12. The engine provided 200 horsepower, and fit much better under the hood than the company’s long 6- or 12-cylinder engines.

Because it had a 60-degree angle instead of the 90-degree vee angle needed to make the engine properly balanced, the V8 suffered from secondary out-of-balance forces. Even fitting separate balance shafts couldn’t help the problem, an added complication that killed the engine.

Jaguar also tried to build a straight-six based on the new V12 design, but it was found to be insufficiently powerful.