Best Diff for my setup

C

charlesm3

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Hi all,
I am looking at buying an e9 with a built 3.5 M88 and a Getrag 5 spd. From driving it and by the sellers admission it has the wrong diff in it. I do not know the ratio for what's there but can anyone advise what would be the best setup for this combo?
Best Charles
 
Some things to thinks about

Hello and welcome.
The 'side loader' diff should have a stamp somewhere, 11/38 etc....

More info would be helpful.

Is it a OD or CR 5spd?
What diff is in it now?
What do ya want it to do? be quick or have higher top end?

Long autobahn legs, seek out a 3.07 or 2.93
Very quick canyon carving or track: 3.91 or higher

A common midrange diff is a 3.45

diffsonline.com does BMW diffs and has a good calc to play with.

HTH
 
As shanon said, it really depends on the transmission. There are 2 five speed Getrag box styles, an OD and a CR. The CR is usually paired with a 3.07 diff to compensate for the lack of an overdrive. The OD trans is usually paired with a 3.45.

It also depends on what kind of driving you intend to do. If you want to do a lot of backroad, fun driving, a higher diff would help a lot without sacrificing much, given the drive cycle. If you intend to use it on the highway a lot, then combining the 3.07 with the OD would give you great gas mileage at the expense of some acceleration.
 
Thanks all for the feedback. These are good questions that I can answer when I get the car in my possession for the final inspection. I'll look forward to more shared info in several weeks. Best!
 
There are a total of 5 five speed trannys available for the big six non m engines and also the early 4 speed tranny. Including the m engine you have 6 trannys. Not counting automatics of course....
Diff ratios ran from the 2.93 on the m106 engine, 3.07 on the m102 engine along with the cr tranny all the way up to the3.91 diff on the later e24 cars. The 535i e28 got a 3.25, the e9's got 3.45 and 3.64 and the early e24 got the 3.71. Slushboxes went as far as the 4.27 diff in the late e34.
Try to match the weight with the power band with the gearing. A m88 engine is a pig with a 2.93 - the factory provided the correct diff for the states.
 
I'm still wondering WTF: my '75 CSi had a 4.10 diff in it, with a 4-speed. I never drove the car (it was an unrestorable barn find) but I took the diff out anyway in case I ever need a replacement (right..that'll happen.) So if anybody wants one, lemme know.
 
A 3.25 (stock CSi setup) with a close ratio and the M88 would be perfect in my opinion, if you have an overdrive 5 Speed then the 3.45.
 
I'm running an S38 with a 3.45 and 265CR. I also had the diff in the car with a 280OD. It was far too low numerically for my type of driving (90% city or twisties/10% highway) with the OD. Might be a hair too low still with the CR - haven't fully decided - but if I move it would only be to a 3.64 and not much higher. Right now I get ~3000RPM at ~80MPH.
 
I've got a 3.07 limited slip side loader sitting in my garage that I won't use. It's a sideloader, and it's out of an early euro 635csi. To put it in an E9 all that you have to do is swap the differential flanges for the CV's with your present differential.

Gary
 
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