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 at 8000 RPM and 4 lb-ft torque at 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, 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.