1979-93 Ford Mustang 5.0L Underdrive Pulley Set. Chrome plated billet aluminum 6 groove serpentine alternator, crank and water pump pulley set. The pulleys are CNC-machined from the lightest, most durable materials possible: 6061-T6 billet aluminum. Underdrive pulley systems increase engine horsepower by reducing the amount of power required to drive external engine accessories. By replacing factory-sized crank and accessory drive pulleys with re-sized pulleys, the accessories are slowed, resulting in more horsepower available to the drive wheels. Typically, a set of CFR pulleys will increase horsepower by 11-15, and increase mileage by 2 to 3 MPG. All pulleys are manufactured from billet steel or aluminum, exceeding factory quality.