Real HP & torque gains across entire RPM range. Underdriven pulleys are a performance product that actually net additional engine torque and bhp by reducing parasitic drag caused by belt-driven accessories, but more importantly by being lighter and reducing rotational mass. This modification also allows for quicker gear changes due to the RPM being more responsive. Free up 12-15 BHP These are some of the best bhp improvements you can find for your money. The kit has been designed to allow you to remove the auxiliary belt tensioner, a/c idle pulley and all of the casting they are bolted to, so in total 4.65kgs can be lost as a direct result of fitting this kit.