continued conversation from the e21 forum...
The purpose of a flywheel is to increase the polar moment of the rotating assembly. However, a piston engine's instantaneous output shaft speed varies throughout any given rotation due to the variations in instantaneous driving torques produced by the engine's intermittent combustion events. The increased movement of inertia provided by the flywheel's mass helps to smooth out these speed variations and the torsional vibrations that result from the angular accelerations/decelerations.
An engine with fewer cylinders will require a flywheel with a proportionally greater mass than an engine with a large number of cylinders. And with turbocharged engines, with their higher peak cycle pressures, will usually want a heavier flywheel than a N/A engine.
-Higher (numerically) rear gears
Me? I really wouldn't want to lighten it ....Just my .02s....
My theory on longevity- On a 4 cylinder engine-a heavier flywheel would help out smoothing the shock on the rotating mass...However, if you're running a NA engine with low compression, I think it's a moot point.