Originally posted by NChatch
you only want to use one of two lightened objects, either the pulley set up or a lightened flywheel...heres why, when a piston actually comes TDC and crests its peak and the plug fires, it actually tries to stop the crank, if you lighten the crank too much this has a play on how efficiently your motor runs, also with the driline lightened too much you lose torque, from AEM
The flame speed doesnt travel instantly to the piston top. And if it did spark where you say it does, and reached peak cylidner pressure there, you would detonate.
Underdrive pullies are mostly for spinning accessories such as a/c, alternator, and power steering slower so they rob less power from the crank. The problem with this is, at idle, the alternator can spin too slow, causing voltage to drop, lights dim, etc etc. You shouldnt need to spin them slower because the alternator has a voltage regulator on it so it only robs as much power as it needs. The a/c has a clutch on it that disengages when not in use. The p/s prolly does rob power all the time, but spinnign the pump slower will make it harder to turn. The pulley itself is lighter than stock, but usually lacks harmonic balancers which hodna builds into the pulley which can be potentially detrimentl to your crankshaft.
To make a long story short, if you want less roating mass, just get a lighter flywheel.