The photo shows the BMW Motorsport Evolution II lightweight flywheel next to the stock 2.3L Evo I flywheel.
The Evo II flywheel has a lot of machining done around the outer perimeter in an effort to reduce the polar
moment of inertia.
Reports have the Evo II flywheel weighing in at around 12 lbs whereas the stock flywheel weighs around 17 lbs. Of course, simply comparing the masses does not give the complete picture. The distribution of mass around the axis of rotation is important as well. The Evo II flywheel appears to start out as a stock unit which is then machined to create an Evo II part. There are other lightweight flywheels on the market which possibly reduce the rotational inertia to a greater extent than the factory Evo II piece, but I do not have any direct experience with them. Some of these alternatives are machined from aluminum. Note that neither the stock 2.3L flywheel nor the Evo II flywheels are "dual-mass". They are one-piece designs. Because of the reduced mass on the E2 flywheel, many owners report a reduction in idle smoothness once it is installed on the car. A small price to pay for increased performance in my opinion. |
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Note that the center portion of the flywheel is machined so that it is thinner where the flywheel bolts go. This necessitates the use of the shorter factory bolts (22mm vs. 28mm). |
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The stock flywheel has p/n 11 22 1 311 862 The Evo II flywheel has p/n 11 22 1 312 519 The Evo II flywheel was also used on the Evo III (Sport Evo) and it is available from your BMW dealer, as well as other locations such as Motorsport Imports and Turner Motorsport. |
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Here it is apparent that BMW took a lot of mass off of the backside of the Evo II flywheel as well as the front. The orange oval highlights the machined portion. The right-hand photo shows this same area on both the stock and the Evo II flywheels. As pointed out to me by E30 M3 owner Mike Roznovsky, on some stock 2.3L flywheels, the starter ring gear is pressed on (and removable). The starter ring gear is an integral part of the Evo II flywheel, and also appears to be an integral part of the stock 2.3L flywheel that came off of my car. |
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