The M Coupe diff' cover comes with a new bushing already pressed in.
This bushing is completely different from the one that the E30 M3 originally
came with. It appears to be quite a bit stiffer - an observation which is
confirmed by the additional harmonics that have been added to my car under
bad my old e30 m3 bushing looks. It is certainly "long-in-tooth". And this bushing has
only 55,000 mls on it. Nonetheless, 10 year old rubber is 10 year old rubber. Also note
the way both bushings use aluminum inserts to help provide stiffness, but in different ways.
The vertical ridges along the sides of the M Coupe bushing are actually curved aluminum plates.
But they get covered with rubber during molding. The e30 bushing (I do not believe that there
is a unique e30 m3 bushing) also has these plates - but only one per side vs. two. Keep in
mind the forces that the diff bushing must cope with. Vertical forces are applied during
braking and acceleration and lateral forces are produced during cornering. Both bushings
are obviously "tuned" to react differently to braking, acceleration and cornering.
The M Coupe bushing has two fat rubber bumpers both above and below the metal insert.
These make the bushing stiffer during acc/braking than the e30 bushing. This stiffness
can be "heard" in the form of increased resonance during off-throttle coasting. I am
not sensitive enough to actually "feel" it just driving on the street.
Should anyone want to try just the M Coupe bushing as an upgrade, the part number is:
33 17 2 228 425