Why do some engines use balence shafts while other desighns of same one do not, what replaces it?

90 degree V-6 series II GM engines need balance shafts particularly the motors which fire two cylinders at the same instant. The exhaust sounds like a three cylinder engine.

60 degree V-6 engines don't have balance shafts because the ignition firing is 60 degrees apart.

Adding to Donpat . . . . . yes, inline 6s & 8s are inherently balanced, so are V12s and to a lesser degree V8s. . . .a V6 is sort of naturally unbalanced without an uneven firing crankshaft . . . .etc.

It's also a function of cubic inch displacement: a straight 4 over about 140 ci starts to get pretty rough: a function of fewer power impulses per rev and the strength of each one.

Hello Jay ,

Some engines are inherently unbalanced and balance shafts are needed to eliminate

vibration due to the inbalance.

Some engines are made so that they do not have the inherent inbalance and do not need

balance shafts.