All I want for Christmas is a flat-plane crank. Everyone’s talking about them. All the cool kids have them. Ford even put one in the new 2016 Mustang GT350. It says “flat-plane crank” right there on the valve covers, and just about every blog and magazine article ever written about the GT350 can’t stop talking about it. Per Ford’s press release, “Unlike traditional V-8 engines, the all-new 5.2 liter uses a flat-plane crankshaft more typically found in a Ferrari sports car or in a racing application.” Sounds mighty impressive, doesn’t it?

But here’s the thing. My wife’s minivan has a flat-plane crank. The mail truck that delivered my delinquent HOA bill this morning has a flat-plane crank. Every ricer that ever put a fart can on his Civic has a flat-plane crank. Even my three-year-old daughter’s bicycle has a flat-plane crank. Mind you, this is a machine so fierce that only training wheels can harness its fury. How is it possible that these flat-plane-crank-equipped technical marvels somehow flew beneath the radar? That’s easy. Before “flat-plane crank” became a sexy new catchphrase, no one cared if an engine’s crank was flat, quasi-flat, semi-flat, kinda flat, or not flat at all.

If you want really want to buy a new Mustang, and it really needs to have a flat-plane crank, why not get the 2.3L Ecoboost model for half the price of a GT350 that also has, you guessed it, a flat-plane crank? I’ll gladly take a small cut of the $24,000 I just saved you. The inconvenient fact that the GT350’s flat-plane crank layout “typically found in a Ferrari” is also typically found in fire-breathing grocery-getters puts the silliness of all this flat-plane hype into perspective.

While Ford has every right to be proud of its phenomenal new 5.2L V-8—an engine that kicks out 526 horsepower and screams to 8,250 rpm—attributing so much of this technical achievement to the orientation of the crankshaft counterweights is beyond preposterous. Swap out the 5.2L’s flat-plane crank with a cross-plane crank, and it would turn just as many rpm and do so without the bolt-snapping secondary vibrations. In fact, the fastest and most powerful Ford modular V-8 on Earth—John Mihovetz’s Accufab Mustang— produces well over 3,000 horsepower and turns 10,000 rpm with a cross-plane crankshaft. For those unfamiliar with late-model Fords, Mihovetz’s mod motor shares the same basic engine architecture as the new 5.2L.

The fastest and most powerful Ford modular V-8 in the world turns 10,000 rpm, makes 3,000-plus horsepower, and runs 5.92-second quarter-mile passes with a cross-plane crank.

Interestingly, nowhere in the GT350 literature does Ford attribute the 5.2L’s lofty peak rpm or its impressive 102 hp per liter specific output to the flat-plane crankshaft. Sure it’s implied, but it’s not explicitly stated. Ford knows better than that, and the real engineers and engine builders of the world would laugh hysterically if Ford made such a ridiculous claim. However, countless members of the press have made that assumption all on their own. At this point, no one knows which media outlet first associated flat-plane cranks with the ability to turn lots of rpm, but in an era where online plagiarism has replaced real journalism, the hysteria and ignorance surrounding flat-plane cranks is the unfortunate consequence.

In reality, the orientation of the crank throws never has and never will determine how high an engine can rev. The key to the 5.2L’s impressive high-rpm prowess is an incredibly stable DOHC valvetrain, and outstanding CNC-ported cylinder heads that flow enough air to warrant turning that many rpm in the first place. By eliminating the heavy lifters and pushrods utilized in a traditional OHV V-8, Ford’s lightweight and deflection-free valvetrain offers a level of high-rpm stability and precise valve actuation that more primitive OE pushrod motors can only dream of. In fact, Ford’s roller finger follower arrangement is even more precise and stable than the direct-acting lifter buckets and overhead rockers used in lesser DOHC systems.

At 7,000 rpm, a typical pushrod engine can lose up to 15 degrees of valve duration due to valvetrain flex. Ford's incredibly stiff roller finger follower valvetrain virtually eliminates deflection, resulting in broad powerbands with a 2,500-3,500 rpm spread between peak torque and peak power. The Camaro Z28's 7.0L LS7 can only manage a 1,500-rpm spread.

When turning lots of rpm is the objective, perhaps nothing emphasizes the importance of airflow and valvetrain stability better than the 3.0L V-10s that competed in Formula One from 1995-2005. From a specific output and peak rpm standpoint, they are arguably the meanest naturally aspirated piston engines ever built. These engines produced over 930 horsepower and reached 20,000 rpm. Making these dizzying figures possible were pneumatic valvesprings that offered unbelievably stable and deflection-free valve actuation, and cylinder heads that flowed a whole lot of air. Sounds like a familiar formula, doesn’t it?

As in any engine, the role of the crankshaft, rods, and pistons in these magnificent 20,000-rpm motors were to simply hold together and not blow up. Since these V-10s utilized 72-degree crankshafts, then a 72-degree crank is clearly superior to a 180-degree flat-plane crank, right? Using the misguided “flat-plane crank logic” perpetuated by the media, where the orientation of the crank throws determines peak operating rpm, all performance engines should have 72-degree cranks. Who cares about the cylinder heads and valvetrain? Of course, the fatal flaw in these assumptions is that they connect dots that shouldn’t be connected, and are therefore absolute nonsense.

Anyone capable of performing simple fourth-grade mathematics can easily calculate the ideal orientation of the crank throws. Since a four-stroke internal combustion engine must rotate 720 degrees to complete one cycle (intake, compression, power, exhaust), dividing 720 by the number of cylinders ensures that the power strokes are evenly spaced for smooth engine operation. Simple enough, right?

Pneumatic valvesprings and great cylinder heads allowed Formula One V-10s to turn 20,000 rpm. Setting the crank throws 72-degrees apart had nothing to do with it.

That means a 90-degree crank delivers smooth, evenly spaced power strokes in an eight-cylinder engine, and a 180-degree crank does the same in a four-cylinder engine. It’s also why V-10 era Formula One engines utilized 72-degree cranks. Just as in my wife’s minivan, the 2.3L Ecoboost Mustang and a USPS mail truck, setting the crank throws 72-degrees apart in 20,000-rpm F1 V-10s had nothing to do with increasing peak engine rpm and everything to do with evening out the spacing of the power strokes throughout a single 720-degree cycle. It’s as simple as that.

While utilizing a 180-degree crankshaft in an eight-cylinder engine is certainly unusual, it offers some benefits but only in a very small subset of applications. By nature, a flat-plane V-8 has excellent primary balance. Just like in an inline-four, when two pistons are at TDC (top dead center) on one bank of cylinders, the other two pistons are at BDC (bottom dead center). Consequently, the mass of the pair of pistons and rods at TDC cancels out the mass of the pair of pistons at BDC (and vice-versa), which eliminates the need for heavy counterweights. The smaller and lighter counterweights also allow reducing the mass of the pistons and rods. In contrast, a cross-plane crank requires heavy counterweights to achieve smooth primary balance.

The resulting reduction in rotating and reciprocating weight (and inertia) enables an engine to accelerate and decelerate more quickly. Likewise, flat-plane cranks also allow alternating the firing pulses left and right between each bank of cylinders. Since this prevents two cylinders on the same bank from firing in succession, which is what happens in a cross-plane motor, this should theoretically improve exhaust scavenging.

Ford's flat-plane crank (left) utilizes center counterweights, whereas a typical cross-plane crank (right) does not. Center counterweights are great for evening out main bearing loads, but they obviously increase mass. Do the counterweights on the flat-plane crank actually look that much smaller?

But here’s the thing. This isn’t necessarily that big of a deal. GM’s LS-series small blocks, for example, utilize a 1-8-7-2-6-5-4-3 firing order. Only twice in that 720-degree sequence (2-6) do cylinders on the same bank fire in succession. Granted that this isn’t ideal in terms of exhaust scavenging, most performance exhaust systems utilize balance pipes that allow exhaust from one bank of cylinders to cross over into the opposite bank farther downstream in the exhaust tubing. This significantly minimizes the adverse effects of firing two cylinders on the same bank back-to-back.

Not surprisingly, the theoretical advantages of a flat-plane crank don’t always pan out in the real world. The top race teams in the country already experimented with 180-degree flat-plane cranks many years ago in every form or racing ranging from NASCAR Sprint Cup to NHRA Pro Stock, Top Fuel, Funny Car, and Comp Eliminator. Despite the fact that these V-8 engines turn between 9,000 rpm on the low side (as in Sprint Cup) and 11,000 rpm on the high side (as in Pro Stock), ultimately, any theoretical gains in performance were more than offset by the increase in highly detrimental secondary engine vibrations inherent to the flat-plane crankshaft design. In the upper echelons of racing where cost is no object, an edge as small as two horsepower over the competition is considered a big deal. Even so, at the end of the day, the top race teams in the country stuck with their 90-degree cross-plane cranks and never looked back.

Considering that the primary purpose of the flat-plane crank hype is to lend an aura of sophistication to a relatively boring piece of hardware, it’s not surprising that no one’s talking about the drawbacks of a flat-plane crank. For obvious reasons, Ford doesn’t mention any of that stuff in the press release, and you can’t realistically expect a lazy blogger to actually pick up the phone and talk to an engine builder. If they did, they might have learned that the most significant downside of a flat-plane crank is that they generate some very severe and potentially destructive secondary vibrations. By definition, these vibrations are produced twice per engine revolution opposed to a primary vibrations that occurs just once per revolution.

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Imagine drawing a line at the half-way point of travel as the pistons move from TDC to BDC. Since the wristpin is positioned slightly below the piston crown, when the crank pin rotates downward to half the length of the stroke, the piston actually travels a distance greater than half the length of the stroke. As a result, the piston accelerates away from TDC toward the halfway point more quickly than it accelerates from the halfway point toward BDC. The same applies as the piston reverses direction back up the bore. The piston’s rate of acceleration increases once it passes the halfway point on its way back up toward TDC. This disparity in piston acceleration creates an upward vibration that occurs twice per crankshaft revolution. Without balance shafts, there is no way to completely cancel out these vibrations.

In contrast, since each of the four crankpins in a cross-plane V-8 are phased 90 degrees apart, there are always pairs of pistons moving through different phases of the crankshaft rotation cycle. As the first crank pin (from the front) rotates downward from TDC to 90 degrees after TDC, the third crank pin travels from 90 degrees before TDC to TDC. Likewise, as the second crank pin rotates downward from 90 degrees after TDC to BDC, the fourth crank pin travels upward from BDC to 90 degrees before TDC. Consequently, the fast downward movement of the first crank pin cancels out the fast upward movement of the third crank pin, and the slow downward movement of the second crank pin cancels out the slow upward movement of the fourth crank pin. This effectively cancels out the secondary forces. This excellent secondary balance is why Cadillac invented the cross-plane crank in the first place in the early 1900s. Prior to that, flat-plane cranks were the norm not because of any performance advantages, but simply because they were easier to manufacture.

The secondary vibration inherent to a flat-plane crank isn’t something that should be taken lightly. Many blog entries and articles have suggested that the lighter pistons and rods used in flat-plane crank motors reduce secondary vibrations enough to where they’re no longer a concern. It’s an interesting theory, but that’s not how things pan out in real-world testing. According to Ford Group Vice President of Global Product Development Raj Nair, Ford considered scrapping the flat-plane crank concept entirely due to the vibration issues experienced by early 5.2L prototype engines. Ford’s solution was fitting the engine with a revised crank damper and a dual-mass flywheel to quell vibrations, and stiffening up the block, accessory brackets, and exhaust system to survive the vibrations. Other measures may or may not have been taken, but Ford is remaining hush-hush.

When balancing a crank, primary imbalance can be corrected, but secondary imbalance can not. In most applications, lightening up the pistons and rods simply isn't enough to attenuate secondary vibrations.

More demanding environments, such as in the 2.4L Formula One V-8s used from 2006-2013, require far more extreme measures. During the development phase of the Cosworth F1 V-8 prior to the 2006 season, the new flat-plane crank engines vibrated so severely that they broke the bolts holding the scavenge pumps to the block. Consequently, engineers fitted dampers on the back of the crank, on the front and back of all four camshafts, and throughout the valvetrain. In total, 13 dampers were required to get these vibrations under control. Considering that these vibrations increase as rpm and stroke length increase, and the 5.2L Ford V-8 turns a fraction of the rpm but also employs a much longer stroke than a 2.4L F1 motor, this is obviously an apples-to-oranges comparison. Even so, these secondary vibration issues can be a very big deal.

Vibration issues notwithstanding, the fast-revving nature of flat-plane cranks made them the configuration of choice during F1’s most recent V-8 era. Not only did these 18,000-rpm screamers have extremely narrow powerbands, but tight tracks like Monaco require shifting over 60 times per lap. When running through the gears that many times per lap, the ability to rev through the powerband a tiny fraction of a second quicker between each shift can add up to much more substantial chunks of time throughout the course of a race. Furthermore, lighter cranks and rotating assemblies transmit less torsional load through the chassis during acceleration (upshifts) and braking (downshifts), which stabilizes the load on the tires and optimizes grip.

Nevertheless, the needs of an 18,000-rpm Formula One engine that only has to last a few hundred miles are far different than the needs of an 8,250-rpm street engine that must last hundreds of thousands of miles. Furthermore, street engines have much broader powerbands than an F1 engine, and a car like the GT350 only requires a dozen or so shifts to get around the typical 2.5- to 3-mile road course. This begs the question, how much of a performance advantage is a flat-plane crank over a cross-plane crank in a street car?

What does a GT350 cruising down the road in 6th gear have in common with an F1 car that's shifting 60 times per lap? Not much. (Photo courtesy of Ford)

Several professional race engine builders I recently spoke with regarding the pros and cons of a flat-plane crank stated that, given their vibration issues, they had no idea why Ford opted to use one in the new GT350. Others suggested that it was purely for marketing. If there were indeed marketing considerations behind the decision to go flat-plane, kudos to Ford's marketing department. Ford turned “flat-plane crank” into a sexy catch phrase that sounds really impressive to people who know absolutely nothing about engines. Everyone’s talking about the GT350’s flat-plane crank, and you got to hand it to Ford for pulling off one heck of a PR coups that transformed the entire automotive press corps into a flat-plane crank propaganda machine.

Still, I can’t help but feel bad for the engineers at Ford getting snubbed by all this flat-plane nonsense. Someone at Ford designed some badass CNC-ported cylinder heads for the new 5.2L, but no one’s talking about that. Someone at Ford designed the F1-inspired roller finger follower DOHC valvetrain that makes OE pushrod motors look stupid, but no one’s talking about that. Someone at Ford designed the camshaft profiles and a variable valve-timing strategy that—when combined with the phenomenal low-lift airflow of the 5.2L’s four-valve cylinder heads—enables it to produce 24-percent more torque per cubic inch than GM’s 7.0L LS7 (1.36 vs. 1.10), but no one’s talking about that. All of these factors play a far more substantial role in both the 5.2L’s specific output and high-rpm capability than its flat-plane crank, but no one’s talking about that.

Ford's new 5.2L sets new benchmarks in specific output and engine speed for American V-8s. Focusing so much attention on a single component (the crank), which plays a very minor role in its overall achievements, is an insult to the rest of the design work that went into creating this great new engine. (Photo courtesy of Ford)

Ultimately, when an engine builder unboxes a crank, they hope to marvel at the quality of the forging, the smooth profiles of the counterweights, the trueness of the journals, and the precision of the machining. Not a single engine builder on Earth has ever unboxed a crank and said “hot damn, look how flat this sumbitch sits on the table,” then called in the boys for an impromptu circle jerk. I’m not so sure that I want a flat-plane crank for Christmas anymore, but the marketing guys at Ford surely deserve a raise.

ABOUT THE AUTHOR

A 12-year veteran of the automotive publishing industry, Stephen Kim regularly contributes technical and feature articles for magazines such as Hot Rod, Car Craft, Chevy High Performance, Super Chevy, Race Pages, Fastest Street Car, Muscle Mustangs & Fast Fords, Mustang Monthly, and Mopar Muscle. Creating relevant technical content isn’t possible without excellent sources, and he’s grateful to all the great minds in the racing industry that are always willing to lend their expertise.



