325 LSD on M3

Post Mon Jan 17, 2005 4:36 pm

I have a 3.91 LSD - will it fit on the E30 M3.

Karl

Post Mon Jan 17, 2005 5:33 pm

Probably will fit but the ratio is really short for an M3 with the dogleg box. They usually run diffs at about 3.15 or 3.25.

With a 3.91 you'll probably do about 80mph flat out.

Post Tue Jan 18, 2005 12:47 pm

Yeah it should bolt straight up but i think the 3.91 might run a larger diff casing so the prop will need adjusting to suit. Not sure what this job involves but i'm sure one of the experts will post the answer.

There is a gearing calculator on the web which will show you the difference in speeds between the 2 diffs. I don't have the link atm cos i'm at work but i'll post it up when i get home if no-one else has.

Regards, Jon.

Post Tue Jan 18, 2005 6:03 pm

Here you go dude....
Gearing calculator

Can someone clarify what adjustments are required when swapping from a small case diff to a large case?

Thanks, Jon.

Post Tue Jan 18, 2005 7:05 pm

M3 uses the same diff casing as the muscular 6 pots, ive transported both and theyre exactly the same! A 3.91 LSD would fit the M3 without mods but youd have suicide gearing, might be fun tho

Post Wed Jan 19, 2005 10:30 am

The conversion is for a track car - I have been on the gearing ratio website, but of course driving it is different to the theory etc.

Thanks

Karl

Post Wed Jan 19, 2005 12:40 pm

Looking at the gearing calculator you should be ok on paper with a 3.91 diff.

Assuming the 215bhp M3 on 15" rims you'll get about 31mph in 1st, 49 in 2nd, 66 in 3rd, 93 in 4th and 118 in 5th all at 6,750rpm.

Post Wed Jan 19, 2005 7:55 pm

that is too low for on track - I want about 130 mph top speed for some of the tracks.
I will stick with the std diff for now.
Thanks
Karl