Dinan's lightweight billet aluminum crankshaft pulley underdrives each of the accessories by 13 percent, reducing parasitic drag on the engine for a 6 HP @ 8000 RPM and 4 lb-ft torque @ 8000 RPM gain on a stock vehicle. The Dinan underdrive pulley retains the factory harmonic crankshaft damper, eliminating vibration that might otherwise damage the crankshaft. The kit includes all of the necessary mounting hardware, a lightweight pulley, and shorter belts. This product is backed by our industry-leading 4 year/50,000-mile warranty so you can rest assured that you are purchasing the best quality product for your BMW.