BBK SFI Under drive Pulley Kit for Ford Mustang 4.6L-3V GT - 2 Piece features SFI approved performance pulley kits with a SFI approved steel crank damper. It is built from high quality CNC machined aluminum. Each steel BBK kit is designed as a direct bolt on to replace the stock pulleys that rob the engine of horsepower. Each SFI pulley kit is designed specifically for each vehicle. This pulley kit for the 2005-2010 Mustang GT is designed as a 2 piece kit. As with the traditional pulley systems the horsepower gains are approximately 9-12 rear wheel horsepower. It is a result of the reduced drag which also helps with overall fuel economy. As with all BBK parts these SFI approved pulley kits offer BBK's distinctive quality that guarantees many years of top performance. The crank pulley is a steel SFI one piece replacement. Additional pulleys are CNC Aluminum.