stillen underdrive pulley ?
 

does this pulley have any noticeable power differences, i know stillen says 10 more horse but can you feel it an would it be worth the money to get ?

The search function is very useful on this website. Might want to give it a shot.

Then google the problems with running an un-dampened crank pulley and what harmonics can do to an engines internals.

Yup.. with engines that have a harmonic dampener on the crank pulley, and aren't internally balanced, which the 3.7L 370Z motor is, would be a concern.

The Z motor is internally balanced, and the stock steel pulley (while needs to be extremely balanced, and getting a random pulley from somewhere sketchy like eBay is HIGHLY unadvised..) is heavy and can be improved on by moving to a billet aluminum pulley at a 1/4 of the weight.

There is a small rubber ring separating the internal and external portion, of the cast steel pulley, which acts as a dampener for itself and the accessories.. but the Z does NOT utilize a harmonic dampener.

i would expect the engine to be internally balanced. Never said a word about the aftermarket pulley causing a vibration. What i implied is that it is a HARMONIC dampener, which it is. Can you explain how Nissan invented the first ever motor that creates no harmonics, and can't be effected by them?