What Is a Flat-Plane Crankshaft?

You might be familiar with the term “flat-plane crankshaft”,” thanks to the C8 Chevrolet Corvette Z06 or the Ford Mustang GT350. Though it’s a relatively new thing to see in an American V8, the flat-plane crank has been a staple of European supercars for decades now. They have a signature wailing sound and typically rev much higher than a cross-plane V8. That being said, what exactly is a flat-plane crankshaft?

Flat-plane vs. cross-plane crankshafts

According to Motortrend, the difference between a flat-plane crankshaft and a cross-plane crankshaft all boils down to where the connecting rods meet the crankshaft. In a cross-lane layout, which is standard for most American V8 engines, there is a connecting rod every 90 degrees. So, if you look at the crankshaft from the front, the jutting rod-bearing journals make the shape of a cross.

On the contrary, a flat-plane crankshaft has a connecting rod every 180 degrees. Seeing a crankshaft like this is common in virtually every four-cylinder engine. In the case of a V8, though, this placement means that pistons going up and down will always alternate from one side of the engine to the other. This allows for smoother running in its natural state.

Cross-plane crankshafts typically have a slew of counterweights to allow for smooth operation. However, the addition of these weights and the inherent instability of a cross-plane crankshaft limits its upper RPM range. A flat-plane V8, though, can rev to the moon comparatively, thanks to its lightweight crankshaft and smooth operation.

Can you put a flat-plane crank in any engine?

In theory, you could have a custom flat-plane crankshaft made for any V8 engine. There are, however, more things to consider.

You have to remember that by changing the crankshaft, you’d be changing the engine timing as a whole. So, the firing order would be different, and you’d need a custom-ground camshaft to align the timing with the new piston orientation.

It’s certainly not impossible, though. In fact, companies like Stevies Crazy Garage produce flat-plane conversion kits for small-block Ford and Chevy engines that include lightweight pistons, rods, and custom camshafts.

What is the downside of a flat-plane crank?

Though they are beneficial, as with anything, flat-plane crankshafts have their issues. For example, though they are naturally more balanced than a cross-plane crankshaft V8, they are still prone to vibration due to their lack of counterbalance weights. This is especially noticeable at high RPMs.

Finally, flat-plane crankshaft V8s typically have much lower torque output than their cross-plane counterparts.