APR's Supercharger Drive Pulley Upgrade decreases the diameter of the factory pulley from ~63.25mm to 57.75mm, which in turn spins the supercharger ~9.5% faster. This results in a healthy bump in horsepower and torque across the entire RPM range with the supercharger operating at a higher rate of speed. The pulley is CNC machined from billet stainless steel, e-coated black for an OEM appearance and is bolted to the factory supercharger unit.APR's Supercharger Crank Pulley Upgrade increases the diameter of the factory pulley from ~162mm to ~187mm, which in turn spins the supercharger ~15.4% faster. This results in a larger bump in horsepower and torque across the entire RPM range, as the supercharger is spinning at an even higher rate of speed. The pulley retains the factory harmonically balanced crank pulley and bolts on top of the existing unit. It is CNC machined from light weight billet aluminum with weight saving pockets throughout. It has an anodized black finish and includes new crank pulley bolts.Upgrading both the supercharger drive and crank pulleys results in the factory supercharger spinning ~26.4% faster than stock. This speed was chosen after extensive engine and chassis dyno testing with various pulley sizes and supercharger speeds. The result is an even higher horsepower and torque gain across the entire RPM range. However, at the airflow levels achieved with a dual pulley setup, maximum power is severely restricted by the factory throttle body unit. The APR Ultracharger Throttle Body Upgrade is necessary to take full advantage of the dual pulley configuration, and is available in platform specific variations for the B8/B8.5 A4/A4/S4/S5, Q5/SQ5 and C7 A6/A7.Belt IncludedSoftware sold separately