Please enlighten me

Trips

Lighter pulleys help in the unsprung weight on the crank and therefore giving you a small hp gain. There are two types Just a Lighter pulley or A Lighter Undersized pulley with a matching Alternator pulley in which you would also need a smaller belt. NST? Sells this version as a set and also Stillen? Sells the Lighter and Undersized type. You need to decide if its worth the expense and what type would work for you? I've been using the NST lighter undersized type with no problems and I did notice a slight off idle difference.


NST (NonStopTuning) 370Z VQ37HR Pulley Kit Review w/ Dyno Charts
NST - The Leader In Nissan 370Z Pulley Development



Stillen Underdrive Pulley - Review/Pics/Dyno/Vid
STILLEN : STILLEN Performance Lightweight Pulleys

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Much more info here... NST (NonStopTuning) 370Z VQ37HR Pulley Kit Review w/ Dyno Charts
NST - The Leader In Nissan 370Z Pulley Development

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ChipsWithDips

A lightweight crank pulley reduces the rotating mass, but that is not the same as unsprung weight. Much of the unsprung weight also happens to be a rotating mass (wheels/tires/brakes), but the difference is that unpsrung weight also affects handling by changing the inertia that your springs must overcome to keep your tires stuck to the pavement.

But yeah the other advantage of an underdriven crank pulley is that your belt driven accessories(Power steering, A/C, Alternator) sap less power from your engine, at the cost of slightly reduced performance of said accessories.

The NST kit includes an overdriven alternator pulley, so that the net effect of overdrive + underdrive = normally driven alternator, so only the power steering and A/C would be underdriven.