Dinan's underdrive pulley is designed to increase power (11 horsepower 8400 RPM and 7 lb-ft torque 8400 RPM on a stock car) by reducing parasitic drag on the engine from spinning the alternator, water pump and AC compressor. This is accomplished by slowing down the speed of these accessory drives that rob power from your engine. Dinan has done extensive hot weather testing so you can be assured that your engine is safe. The Dinan underdrive pulley is made of aircraft quality 6061 alloy. is anodized black and does not modify your harmonic damper as some underdrive pulley kits do which can be harmful to your engine. 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. Note: BMW changed the belt configuration from a single-sided power steering and AC belt to a double-sided belt for M3's that were produced after May 2008. Please check with your dealer to find the correct Dinan part number below.