What do underdrive pulleys do? 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 compressor rob horsepower from the engine. By replacing factory-sized crank and accessory drive pulleys with a re-sized pulley, the accessories are slowed, resulting in more horsepower available to the drive wheels. Typically an underdrive pulley will increase horsepower by 10-12 at the wheels. An underdrive pulley kit reduces parasitic power losses associated with high accessory speeds, freeing up additional HP. It also reduces the rotating inertia on the front of the crankshaft allowing for faster revving.