What exactly do underdrive pulleys do (Yes i did search) will they make you run faster etc....

 

the under drive pulleys slow down the rate of rotation on your accessories like the water pump, alternator, A/C pump, smog pump and power steering. Each of those accessories has a given amount of turning resistance which robs a bit of power from your motor. If you put on an underdrive pulley, it has a different size( main pulley will be smaller than stock, accessory pulleys are larger than stock) which reduces the rate of rotation.

This slower turn ration saves some power. I think it helps alot more at higher rpms than lower rpms. But it will help through out the rpm range however. Also if you run high rpm's the water pump will "cavitate" meaning theres turbulence in the water which stops it from flowing, which impairs the cooling ability of your cooling system. The underdrive pullies will reduce cavitation if that's a problem for you. I personally don't know how to tell if you're experiencing cavitation.

Dave

 

Read this thread, it'll answer your question.