Underdrive pulley systems increase engine horsepower by reducing the amount of power required to drive external engine accessories. For example, the alternator, power steering, water pump and air conditioner all rob horsepower from the engine. By replacing factory-sized crank and accessory drive pulleys with a resized pulley, the accessories are slowed, resulting in more horsepower available to drive the rear wheels. Typically, a set of underdrive pulleys will increase output by 7-12 horsepower, resulting in quarter mile improvements of .1 to .2 seconds.