An underdrive pulley replaces your stock crank pulley. it's smaller and lighter.. it lessens the amount of power supplied to run the electronics in your car and gives that power back to the engine.. so you'll have a little more power because the engine isn't getting robbed of all the power it was before in order to power your electronics.. if you have a big stereo, don't get an underdrive pulley.
It does slow down the alternator and power steering. I have had no problem, other than the lights dim at idle. It makes most of its power by lightening the load on the crankshaft, not by underdriving. Lower rotational weight=less power loss.
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