Yep-in my M5, and I LOVE it... (archive)

The 3.73 diff is much more common and a hell of a lot cheaper. I don't know if the M5/M6 diff will even fit! I don't think the output shafts are interchangeable. The M5/M6 diff is the same as the E34 M5, V8, and V12 cars. It's big and very heavy. I don't know of any other BMW with a 3.91 diff.

Changing to a 3.73 diff results in a 14.7% increase in revs:

3.25 -> 3.73 = 1.147 or 14.7% increase
3.25 -> 3.91 = 1.203 or 20.3%
3.25 -> 4.10 = 1.262 or 26.2%

I use percents, so you can figure it up in any gear.

I have a 3.73 in my 535, and I loved it the few months. But as the years wore on, it got kind of old driving around at 3000rpm at 75mph. But for the track--WOOHOO!! I guess I can live with it. Your gas mileage will suffer by 10-15% with a 3.73 vs a 3.25.

And 1st gear is almost useless with the 3.73, so with a 3.91 or 4.10 (M3), it's like having a 4-speed transmission. I can start from a stop in 2nd gear while hardly slipping the clutch, so long as it's not uphill.

Higher revs will wear out an engine faster. Double the revs means 4x the stress. A 14.7% increase in revs = 31.6% increase in stress:

3.25 -> 1.000 ^ 2 = 1.000 -> 0% baseline
3.73 -> 1.147 ^ 2 = 1.316 -> 31.6% increase
3.91 -> 1.203 ^ 2 = 1.447 -> 44.7%
4.10 -> 1.262 ^ 2 = 1.593 -> 59.3%

Why do you want a 3.91? Just curious.

James
james_535@yahoo.com

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