What Is A Flat-Plane Crank?
In the world of 90-degree V8s, there are two basic crankshaft designs.
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Cross-plane cranks gets their name because when viewed along its axis from either end, the four crank journals are arranged at 90-degree intervals, making a +. The tradition V8 firing order used by GM, AMC, and Mopar (and most Ford OHV V8s if the cylinders were numbered similarly) is 1-8-4-3-6-5-7-2. With this firing order, the odd-numbered cylinders are on the driver side, the even-numbered cylinders are on the passenger side. Notice that the cylinders 8 and 4 fire 90 degrees apart and are on the same bank, just as 5 and 7 (which do the same). There are alternative firing orders, including the 4/7 swap, but none of them get rid of having two pairs of adjacent cylinders firing in succession, pulling an intake charge from the same corner of the intake. Having two consecutive exhaust firings on one bank is what helps give the rumble associated with American V8s, although it makes exhaust scavenging less efficient. It also means cross-plane crankshafts have to use large counterweights to properly balance the engine, helping to keep it from rocking up and down, while keeping rotational mass high, making for a slower-revving engine. The benefit is that it is smooth and helps mitigate second order vibrations (a vibration at twice the engine rpm).
Flat-plane cranks have two pairs of journals 180 degrees apart from each other, so when viewed from either end, they look—well, flat. Flat-plane V8s, no matter the firing order, will always alternate between banks. That makes for efficient exhaust scavenging without needing to have header primaries cross over from one bank to the other, as Ford did in the GT40 for the classic "bundle of snakes" exhaust. Flat-plane cranks don't require massive counterweights and consequently rev very quickly, although they tend to have a noticeable secondary vibration. The vibration is typically not an issue, as flat-plane-crank V8s are used almost exclusively in race applications or in exotic sports cars.