[eschaider]

There is a lot of urban legend surrounding flat plane cranks. Most of it is just that, urban legend! There are two couples or forces in two different planes that a conventional non-flat plane crank makes easier to resolve (balance out). With a flat plane crank the balance in one of those planes remains unresolved. It is typically the horizontal plane.

While you might possibly detect the out of balance condition it is more likely you will not. The fasteners on the vehicle that are not Loktited® however will and without some type of thread locker they can, over time, potentially loosen.

While the design of the crankshaft has caused some to speculate the V8 versions of a flat crank motor fire two cylinders simultaneously that is most often because the appearance of two pistons at TDC simultaneously is for some reason more visible. In fact out 90˚ degree cranks also bring two pistons to TDC simultaneously and God knows we don't light both of them at the same time! They get lit 360˚ apart from one another — same thing on a flat crank engine as someone has already pointed out.

The V8 version of the engines sound like two synchronized four cylinder engines on each side of the car because that is essentially what is happening. The staccato note in our exhausts is caused by a successive cylinder firing but not on the same side of the engine. With a flat crank there is a firing pulse form each cylinder, on each bank, every 90˚ of crankshaft rotation. Not so on a traditional 90˚ crank.

The design is so poorly thought of that you will find it in F-1 engines competing (very effectively) with other F-1 engines that use 90˚cranks! The flat crank while possible to manufacture w/o counterweights is infact, in all modern implementations, manufactured with counterweights. The flat crank design allows for a small but measurable reduction in both engine weight and rotating weights which in highly competitive classes like F-1 can potentially translate into a performance advantage.

There are those who will also argue that the more uniform bank to bank firing order of a flat crank allows for an intake manifold tuning advantage not available to the more conventional 90˚ crank designs. In supercharged applications the intake manifold design issues / considerations tend to be different than for similar n/a engines. Any resonance dynamics available to n/a engines because of the crankshaft design, for all intents and purposes, do not translate over to the supercharged versions of the engines.


Ed