What exactly does an underdrive pulley do. Some people tell me they dont do anything, but then some say they reduce engine lag. Can someone answer that for me?

 

its slows the engine accesories down and basicly frees up some power. i like htem and have used them before i had no problems with a/c cooling and or electrical problems. But i have a optima battery in my truck and now its in my mustang.

 

They are one of the best bang for the bucks.