Thursday, February 26, 2009
I have recently had several people ask me what gear ratio differential I am running with my 2.8L M52. I have a S3.25 posi from a 1987 535is E28.Originally my 1986 325 came with a non-posi 2.93. Horrible! Spun the tires at every corner, even with the old 2.7L M20 engine. Even before the engine swap I changed the differential.
There are plenty of gear ratio choices and BMW's to get differentials from. The E28 and E24 diff's will work in a E30, as long as you change the rear cover and the output flanges. Gear Ratios are as follows:
2.79 and 2.93 are about the lowest gear ratios, which while great for the freeway and gas milage, makes for slow starts. You can go all the way to 4.27 which I believe came in the 91-93 318i convertables, but this makes first gear useless in a five speed car. Even the 4.10 which came in a lot of the 325i automatics and also in the S14 M3, make first gear pretty quick and limit top end speed. I have heard good things with the 3.73, very quick, gas milage isn't too bad either.
I believe the S3.25 is the perfect gear ratio, it is both quick off the line and has plenty of top end. I run at 2200rpm at 55mph while in 5th gear . I am getting about 18mpg in commute traffic and I drive with a pretty heavy foot.
All these diffs are available in posi and non posi. Make sure if you buy the posi (limited slip) that there is a "S" in front of the gear ratio. The "S" signifies posi (limited slip).
When changing the diff out, you can put a MZ3 rear diff cover on which adds and extra quart of fluid and has cooling fins that hang down in the slip stream below the car.
If you are doing a 5 lug change over, you can keep the E24/E28 output flanges if you use the 318ti axles. I think you also need the E24/E28 out put flanges for the MZ3 axles and trailing arms.
Important note: When replacing the diff, make sure it is aligned and completely pressed up against the subframe or the bolts will not go back in.