What differential do I buy?

[rammstein]

My 318 has an E46 M3 6 speed going into a 3.25LSD inside a 325is diff case.

Forgive my ignorance, but I bought the car as it is, and this I am not sure about the following- does this mean that I can buy any differential in a 325-sized case and bolt it right up? Or do the internals matter?

The issue is that my current LSD has given up on the whole limiting slip part :(

This became apparent to me on my last two track visits, and I confirmed this last night after jacking the rear of the car up and turning the wheels (and seeing the other wheel rotate the wrong way).

Anyhow, will any 325-sized case work? Any advice on getting a replacement?

[sharky]

get a e28 3:25 lsd and swap your rear diff cover and flanges.

[rammstein]

I've got a reputable guy offering me an e28 diff @ 3.46 which isn't too far off as far as the ratio goes. I just need to get an undestanding of this "change the cover and flanges" bit.

Wouldn't the e28 box come with its own cover and flanges?

Sorry I don't know more about these things. Definitely a noob with anything drivetrain related. I've gotten goos with brakes though :D

[sharky]

3:46 is a pretty desirable ratio, the E28 rear cover wont fit on E30, swap directly over your E30 rear cover (get a new gasket) swap your E30 output flanges over from the E28 output flanges. Search, this has been covered.

[rammstein]

Thanks for the advice. I wasn't even aware of this swap until now. So I did the search, and it correlates with what you've written. Seems like a good option.

Let me ask you this-
If you could buy the E28 diff as is, and swap the parts yourself, for $375, or have someone else send you the E38 diff with the e30 housing and output shafts already changed, so all you had to do was unbolt the old and bolt in the new, for $500, is it worth it? I'm learning my way around fixing things on the car, but I am a bit fearful of doing this myself.

[cabin]

Its too easy of a swap to justify spending an extra $125 IMO. There really isn't anything you can screw up.

[BigD]

If you're going to the track then I wouldn't buy an E28 diff. The newest one is 22 years old, used in a heavy car with a 3.5L. Do you think any of them have much life left? Find a diff from an E36 MCoupe/MRoadster, they bolt up except for the flanges, 3.23 ratio, and easy to find low miles. They also come with the finned cover everyone drools over, and sells for $250+.

But if you're getting the 6 speed, I would look for something even higher. Maybe find a cheap 3.73 and have it rebuilt (or call up Dan at Diffsonline.com to see how much he'd sell you a refurb for - or if you got the $$$$$, buy a variable ramping one, better than any torsen/queef). The reason everyone wants ~3.25 is that it's the best compromise of top speed (ie highway revs) and acceleration. 3.46 is too high. But that's with the E36 5 speeds with a 1:1 5th. Your 6 speed has an overdrive gear.

[rammstein]

There really isn't anything you can screw up.

You don't know me well enough ;-)

If it wasn't clear, I already have the 6-speed.

Diffsonline has a stellar reputation, no doubt. But when I was pricing out a differential, I came out to $2k without even trying. Now if Dan is out there, and he feels like sponsoring me... then giddyup.

[BigD]

Well a proper diff from them is expensive. But ask how much for a normal refresh of a diff.

[e30polak]

If you're going to the track then I wouldn't buy an E28 diff. The newest one is 22 years old, used in a heavy car with a 3.5L. Do you think any of them have much life left? Find a diff from an E36 MCoupe/MRoadster, they bolt up except for the flanges, 3.23 ratio, and easy to find low miles. They also come with the finned cover everyone drools over, and sells for $250+.

But if you're getting the 6 speed, I would look for something even higher. Maybe find a cheap 3.73 and have it rebuilt (or call up Dan at Diffsonline.com to see how much he'd sell you a refurb for - or if you got the $$$$$, buy a variable ramping one, better than any torsen/queef). The reason everyone wants ~3.25 is that it's the best compromise of top speed (ie highway revs) and acceleration. 3.46 is too high. But that's with the E36 5 speeds with a 1:1 5th. Your 6 speed has an overdrive gear.

The E46 M3 6 speed is nearly identical in gearing compared to the ZF 310/320 in 1st-5th gear. The 3.23/3.25 gearing will still be a good choice, with the 3.46 being optimal with the 6th gear.

Depending on what you are/will be using the car for, a 3.73 may be too much. 1st gear will be shorter than it's worth, doesn't leave you with enough time in power band. Then 2nd will only get you to maybe 45 mph. Too short IMO unless you have an extremely over-sized tire and wheel combo to lengthen the effective ratio a bit.

As far as paying someone to do the cover and flange swap DON'T. The cover is a few bolts and a gasket. The output flanges pop right out with a screw drive, by prying against the diff case, in between the circular shaft and the diff case. then you carefully pop the E30 ones in and you're good to go!