Piston Slap: Flat Plane Crankshaft Design?

Sajeev Mehta
by Sajeev Mehta

My 98 booming with a trunk of funk, Don’t Believe the Hype. (photo courtesy: Ford)

GCH writes:


Could you/somebody please explain what a “flat plane crankshaft” is in the new Mustang GT350, and older Porsches and Ferraris?

I have seen numerous references to it online and in print but nobody, including Wikipedia explains it in non-calculus terms.

Sajeev answers:

Dumbing it down sadly glosses over hundreds (thousands?) of salient details in casting technology and/or computer-aided design. But I left Engineering school for a reason, so let’s simplify: rest a flat plane crankshaft on a table and it’s flat like a sheet of paper.

Ok, maybe not “paper flat” with those boomerang counterweights at the ends…but compared to the crossplane crank in most V8 passenger vehicles?

LS9 crankshaft. (Photo Courtesy: General Motors)

Crankshafts, like damn near everything else in our lives, benefits from the KISS principle. A flat plane crankshaft has the potential for significant weight savings to optimize a motor’s moment of inertia and more even firing to benefit the exhaust stroke, allowing for more revs/horsepower. And that unique sound!

But NVH control is a problem: hence widespread adoption of crossplane crankshafts.

Which means flat plane crankshaft-ed Mustangs shall be completely pointless moot when trapped, idling at a red light in American surburbia…which is precisely where 88.7% of Mustang GT350s shall live.

The stock Coyote V8 is a better option, cool/brag factor aside. Why? Because it’s got a damn good crank, and here’s 8000+ reasons why:

Sajeev Mehta
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  • Akatsuki Akatsuki on Dec 08, 2014

    I want to see an RTR vs GT 350 vs M3 face off.

  • Lou_BC Lou_BC on Dec 09, 2014

    Maybe this link will help in relation to cross plane versus flat plane cranks. http://www.ashonbikes.com/inertial_torque http://www.ashonbikes.com/cross-plane_crank Yamaha used to run a flat plane crank in the R1 sport bike. Power got to the point that tires could not find traction. Flat plane cranks were the norm for inline 4 sport bikes. The power deliver was too brutal as HP wars heated up. V4's and V twins like the Ducati had a HP disadvantage but had a traction advantage. Yamaha went to a cross plane crank to negate the negatives of the flat plane banshee power delivery without loosing the RPM range of a flat plane. Unlike a bike, a car does not have it's tire contact patch shrink in a corner. Also a car is much heavier and will not be unsettles as much as a bike with quick building RPM and more abrupt power delivery.

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