318is LSD Swap

[fmtpo2002]

I picked up my 1991 318is 6 months ago, and was told that it had an LSD. After some spirited driving I had my doubts and did some tests to see what I've got.

I jacked the rear end of the car up and spun one wheel. The other wheel spun in the other direction. I also crawled under the car and was unable to find a tag on the diff cover. After some more light hooning in the rain today I'm convinced that it is indeed an open diff.

I've done some research and decided that I'd like to go with a 4.27 limited slip diff from a 318i. I've found one from BMRparts and the guy on the phone seems to think it should bolt up with no problem. If any one can confirm or deny this I would appreciate it.

I'd like to get this done soon, since "winter is coming". But, while I have the car up and am taking things apart I'd like to take care of whatever else needs doing.

So, what things should I be doing while I have the car up in the air and am taking things apart?

Your suggestions are appreciated.

Thanks.

[fmtpo2002]

I guess some specific questions are in order.

Would it be a good time to change over to poly bushings? If so what is a good brand?

Should I replace all the axle bolts?

Should I replace the axles?

Should I replace wheel bearings? (not having any problems at this point)

The car is at 166k miles and I plan on autox'ing it this spring.

[SkiFree]

1) Yes, not a bad time since you're int there. I work at IE and run IE bushings so I'm bias, but here's some more help http://www.r3vlimited.com/board/showthread.php?t=133400

2) Not strictly necessary, but not a bad idea if you plan on tracking the car in the future. It's cheap insurance.

3) No, but I would rebuild them (inspect, grease repack, new boots)

4) Again not strictly necessary, but not a bad idea if your budget them in.

As for the differential, there's still a remote chance it's just a very very worn out LSD (cross fingers). Regarding the small case diffs you might find this helpful.

http://www.bmw2002faq.com/_/technical-articles/engine-and-drivetrain/strongest-2002-limited-slip-differential-build-r74

[Ocell]

Wow, that 2002 diff article is fantastic! Thanks for posting.

[fmtpo2002]

Thanks for the information SkiFree. It's really hard to not just do all of it.
I have another question though. I'm trying to figure out which final ratio to go with.
I have the Jim Conforti chip that raises the rev limiter which gets me to a max of about 60mph in second gear with my 4.10 rearend. By changing it to a 4.27 I'm looking at that dropping back down to about 57mph.
I have no experience in autocross so, am I making the right decision in going with the 4.27 or should I stick with a 4.10 for those extra few mph before a shift?

[fmtpo2002]

After further review of the rule book, I'm sticking with the 4.10 ratio. Changing ratio will bump me up into a class where I have even less of a chance of being competitive.

For the time being I'm just going to swap out the diff, and replace the trailing arm, diff mount, and subframe bushings. I'll be using new bolts and nuts throughout.

I still can't decide on a brand for the bushings. I'm just looking for a street/race set-up that won't squeak. Some additional NVH is acceptable.

[SkiFree]

For the diff I was going to say stick with whats in there for now. There's no replacement for personal experience, especially for something where personal preferences come into play. The difference between those two ratios really isn't dramatic. On our race cars we've got a range of diffs from 3.90 to a 4.60 depending on the track.

Well for the bushings, if you email/pm me I'll get you setup (or at least on the right track) of a good setup.