The other thing that Buick did with the 3.8L (as I recall) is to
offset (angularly) the crank throws so that the pistons still each
reach every part of their cycle exactly sixty degrees behind the
previous piston (so the firing pulses are now regularly spaced in
time). The original 90-degree V-6 used the simple crank with throws
every 90 degrees, just like the V-8, so the torque pulses were spaced
90-90-180-90-90-180 -- just like a V-8 with the back two cylinders
misfiring -- instead of 120-120-120-120-120-120. THAT was a LUMPY
motor.
The balance shaft counteracts the fact that the pistons do not move up
and down with nice smooth sinusoidally varying velocity. The
acceleration of the moving parts is greater than normal during certain
parts of the cycle (when the piston is above halfway up the cylinder),
but less than normal during other parts of the cycle (when the piston
is below halfway up the cylinder.) Since these out-of-balance forces
occur at twice the engine rotation rate, the engineers put in an extra
out-of-balance shaft spinning twice as fast whose imbalance nearly
cancels the imbalance of the primary moving parts.
That, and a little smoke and the occasional mirror make it all work.
Donald.
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