The difference between cross-plane and flat-plane cranks – Jalopnik

Discussions about engines are not always limited to cylinder capacity, number of cylinders and horsepower. Sometimes discussions center around the crankshaft debate, specifically cross-plane versus flat-plane. Unsurprisingly, both designs have pros and cons, but let’s start with the most obvious difference: their names.

In a cross-plane arrangement, the crankpins (to which the connecting rods attach) are arranged in two planes that are perpendicular to each other, resembling a cross (or a plus sign). When talking about cross-plane V8s or inline-4s (like Yamaha’s CP4 engine), the first and second pins are 90 degrees apart, as are the third and fourth pins. However, the second and third crank pins are 180 degrees apart. On the other hand, a flat-plane arrangement has all the crankpins resting in one plane, 180 degrees apart.

Flatplane engines are commonly seen in expensive European sports cars, in addition to some American examples such as the C8 Corvette Z06, the ZR1 and the now discontinued Ford Mustang Shelby GT350. Cross-plane cranks are found in virtually every other American V8, but some foreign models have also used them. Both crank designs are easily recognized by their distinct engine sounds, with flat-plane V8s having a screaming high-pitched whine in contrast to the characteristic rumble of the crossplane.

Another thing you may have noticed is how flat-plane V8s are responsive and high-revving, while cross-plane V8s typically are not. To explain why, it is important to understand engine balance, especially with primary and secondary forces. The essence is that primary vibrations are caused by the piston moving up and down, while secondary vibrations are caused by the relative movement of the piston and connecting rod, and in most engines are much smaller in magnitude (about 25% of the primary forces).

Thanks to its inherent design, a flat V8 eliminates primary vibrations, but secondary forces remain relatively intact. The cross-plane crank can cancel out the latter better, but has poor primary balance, creating intense vibrations that are only eliminated with the help of huge counterweights. This leads to more inertia, causing the engine to run slower than a flat V8. On the plus side, a cross-plane V8 makes for much smoother operation.