Increase your engine performance with BBK Mustang underdrive pulleys. These kits are a direct bolt on and most include a new resized water pump, alternator, and crank pulley. These kits 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 resized pulley, the accessories are slowed, resulting in more horsepower available to the drive wheels.