Originally Posted by BigD
First of all there is no extra belt length (due to the smaller pulleys) unless you installed a longer belt. There is more slack with smaller pulleys but that's what tensioners are for.
Underdrive accessory pulleys are larger in diameter then stock, hence the extra belt length (which is irrelevant for the compressor rpm anyway). The greater pulley circumference means less accessory rpm. A smaller crank pulley has the same underdriving effect though.
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