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My understanding is that 90 degrees is the best bank angle for a V8 while 60 degrees is best for a V6. I have studied the math that suggests that a bank angle of 90 degrees permits equating the effects of two pistons/rods sharing one crank pin to a rotating mass, which can then be counteracted with a counterweight. However, this analysis doesn't have anything to do with the total number of cylinders, just two pistons sharing one crank pin in a V configuration.
"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz
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In the early Nineties when Chrysler developed the V-10 engine for the Viper (basically a 90-degree V-8 with two additional cylinders), it didn’t use a split-journal crankshaft, and the V-10 subsequently fires unevenly, which produces the Viper’s unusual sound. Ideally, a V-10 would use a 72-degree V angle that would produce even firing without the use of a split-journal crankshaft. The Lexus LFA V-10 uses a 72-degree bank angle for that exact reason.
The first two of these forces—rotational and reciprocating—can often be balanced through engine configuration, as in, for example, a 90-degree V-2 [see “Two-Timer”].
For example, a four-cylinder would like to fire at every 180 degrees of crankshaft rotation (720/4=180). Having firing events that occur in equal increments, as in this instance, is best for balance. The flat-four fires at 180-degree intervals, and its V angle is 180 degrees, which leads to a balance of firing forces. The flat-four, in fact, balances all three of the different types of forces. here
The received wisdom on this subject is clear: Any V-8 engine is well balanced when its two cylinder banks form a 90-degree V. And V-6s tend to be best off when that V is set at 60 degrees. But the explanation of why all this is (at least conditionally) true, well, that’s a bit convoluted.
For an engine to survive all the rocking and rolling it produces, those forces need to be counterbalanced with equal—or at least nearly equal—forces. Today, most production-car engines with more than four cylinders are arranged in “V” configurations that separate the cylinders into banks. Determining the angle between the banks, i.e., the angle of the V, is crucial to the subtle yet brutal art of engine balancing.
A cross-plane, 90-degree V-8 has balanced rotational and reciprocating forces because it is a lot like four of the balanced 90-degree V-2s shown in the aforementioned illustration. To balance the firing force, a cylinder has to fire every time the crankshaft rotates 90 degrees. Since the bank angle is 90 degrees and the firing forces occur in 90-degree intervals, the cross-plane V-8 also manages to balance all three of the forces. more
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GM working back toward profitability after the bankruptcy/buyout meant reducing costs, so the pushrod V6 engines were phased out. [links]
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However. when it comes to the practice of building 90° V6 engines, while it may often look like a sloppy execution of a V8 engine with a couple cylinders lopped off. it's hard for me to ignore that some of the most impressive, and most of my own personal favourite V6 engines are of the 90° configuration. here
-The V6 is an inherently unbalanced configuration.
-You get a more compact package with far less overall width which means it can fit more easily in more applications. Especially in transverse mount scenarios. more
-You halve that 120° and with the right balance apparatus you can somewhat balance some harmonic elements of a V6 with a 60°.
1)F1 as the "pinnacle of motorsport" when forced to the V6 configuration, were pressed to the 90° angle.