However, for cross-plane V8s, there is vibration from end to end of the engine, this is because the first piston of bank A is not in the same position as the last piston of bank A (the same goes for bank B), unlike an inline-4 engine. No problem, the 90° V8 solves this problem by introducing an extra-heavy counter weight to every cylinder. The counter weight is heavy enough to balance the weight of crank throw, con-rod and piston of that cylinder, thus resulting in lack of vibration.
Now you must be
why such counter weight is not used in other kinds of engines. It is
this counter weight must be used in 90° V-type engines which have
crank pins. It our previous study, you’ll find only V8 fulfills all
requirements. Why are there such requirements? good question. As you
all engines have counter weights just enough to balance the weight of
throws and part of the connecting rods, leaving the remaining weight of
connecting rods and the whole, all-important pistons unbalanced. This
because the rotating counter weight can only balance rotating mass.
the whole piston moves vertically rather than rotates about the
while the CG of con-rod is somewhat rotating but also somewhat going up
and down. If we insist to use heavy counter weight, it will cause side
Assume the counter weight in vertical position is heavy enough to
the crank throw, con-rod and pistons. When the crankshaft rotate
the counter weight is repositioned to the right, but the piston doesn’t
go to the left, and the con-rod just partially moves to the left. Only
the crank pin moves completely to the left. Now you can see the system
is not balanced. The counter weight will generate a net force towards
However, for 90° V8, when such a heavy counter weight moves to the right, the piston from another bank will cancel it completely, because their movement are exactly opposed at this moment. (see illustrations below) The same result can be found for the counter weight moving to the left. Therefore 90° cross-plane V8 employs full-weight counter weights can achieve near perfect smoothness.
However, the disadvantage of cross-plane V8s is also about the counter weights - not only increase the weight of engine, they also contribute to rotational inertia, thus making the engine less responsive and less revvy, dropping upper rev limit and top-end power. Moreover, the larger counter weights usually requires a larger crankcase to house them, thus raising the height (and more important, center of gravity) of the enigne. Therefore Ferrari all V8 models, TVR Cerbera AJP V8 and Lotus Esprit V8 employ flat-plane V8s instead.
Flat-plane V8 is named according to the shape of the crankshaft, which is in a flat plane. It is very much like two inline-4 engines mated together. In particular, it achieves end-to-end balance because the first piston and last piston of a bank is exactly in the same position, so are the center two pistons. This is just the same as straight-four engines, therefore the sound of flat-plane V8 is usually somewhat like a pair of four-pot engines screaming simultaneously, unlike the rumble-bumble of cross-plane V8s.
As both banks run like an inline-4 engine, there is second-order vibration. For a 90° flat-plane V8, the sum of second-order force generated in the 2 banks is - by simple vector analysis - 1.41 times (root-2) of the force generated by each of the inline-4 it consists of. And the direction of vibration is left-right instead of top-down. In other words, while displacement increases 100% compare with the inline-4, the second-order vibration increases just 41%. That makes the flat-plane V8 more refined than an inline-4 although it is not as smooth and quiet as cross-plane V8.
To exotic sports cars, less refinement is not a big problem. Especially they usually employ short stroke and light weight pistons / con-rods, the second-order vibration is greatly reduced.
Of course, the disadvantages are cost, size and weight.
There’s no structural differences in crankshaft for all V12s, no matter for luxurious cars or supercars. Most employ 60°, although Ferrari prefers 65°.