basically spins your accessories slower so you dont use as much hp turning them. Would Havent seen them on a gtp, great for a gt or turbo car. check with intense
Gains are usually in the 1-5 hp range. Your best bet is to just pull off that resistive supercharger pulley. That pully alone can cause a 30+hp loss at the crank. Personally, I got rid of that pulley and gained at least 30hp