An underdriven pulley isn't the same as a lightened pulley. UD Pulleys are different sizes than the stock ones and as used to reduce hp-stealing parasitic resistance from the system. The problem with that is that there are some associated issues with under driving accessories like the AC and the alternator and may cause them to not perform the way they are supposed to. Most of the pulley derived power gains aren't from underdriving though...they are from weight savings. Underdriven pulleys seem to perform better than lightened ones because they too tend to be a lot lighter than stock so they have both added benefits.
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