MarcusOReallyus
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That's because the smaller pulley spins the alternator faster, thus (potentially) creating more power... but the regulator clamps it down to the correct output. The smaller pulley put more load on the engine, so putting the correct size on there is a good thing.I swapped out sheaves on the drivers side alternator so they both sheaves on the alternators are approx. 78 mm in diameter. The rebuild alternator originally had a sheave of approx. 68 mm in diameter which is why I had no adjustment left even with a new belt, now it is great. As cucvrus said the, rate of charge did not change at all with the sheave diameter so other than the belt needing to be 1-1/4" shorter, the sheave size has no affect on the rate of charging ....
If you put a much bigger pulley on there, you will spin the alt slower, and if you get the pulley big enough, you will get to a point where you don't spin the alternator fast enough to put out the correct power. There's not enough mechanical energy being transferred from the engine to create the required electrical energy.