BMW E30 Limited Slip Differential Health

I'm in the process of parts gathering to re-build my 318ti (let's just call it an E30 - 168 mm small case) rear differential.

When I test the LSD breakaway torque, it registers 31 lb-ft. Intelligence from the internet shows 50-55 is new condition, and 60+ is cheaty in spec E30 circles.

My question is, how much of an issue is my weak 31 lb-ft unit? I'm slowly building it up for Lemons/Champcar and will have the whole thing apart to do all the seals and bearings. Is it worth the ~$150 for new clutch plates to refresh the LSD too?

In street driving, it does function as an LSD - but I'm not sure what the difference in performance would actually be if it was "new."

Have you looked for a Z3 torsen unit? Beefier mid case, beefier axles and a better diff.

Sorry to not answer your question. My 318ti has a weak LSD too.

Because of Champcar rules, I'm not interested in swapping to the medium case differential - the points penalty gets pretty steep for the diff and axles. Maybe one day if longevity becomes an issue.

Maybe you can add a beercan shim?

That'd be pretty grassroots.

I ran a 3.91 small case on my e30 for rallycross for 5 years...it never had more than about 35 lb-ft on the lockup - though for rallycross it doesn't matter as much. IDK what help that is though, lol....

irish44j said:

I ran a 3.91 small case on my e30 for rallycross for 5 years...it never had more than about 35 lb-ft on the lockup - though for rallycross it doesn't matter as much.

If you can't tell the difference between 35 ft-lb, and 200ft-lb, and welded diff... you need to get rid of that IRS and get something that makes grip!

It was a big learning curve when I went to a welded diff from a clutch pack limited slip, and that limited slip was "OVER" 200ft-lb.  I easily drove the car with a broken axle.  Huge difference noticable too between 200ft-lb and sub-80ft-lb.  Basically, the tighter the diff, the less the steering wheel does anything useful on corner entry, and the more you better have the car pointed at where you want to be when you accelerate.

Yes its worth it.  Add more friction plates and less metal discs to increase lockup beyond what the stock setup was.

I use a BMW small-case LSD on my 79 3-series coupe.  50-55 pounds for stock sounds really good.  The last time I checked mine it was at about 45 pounds breakaway.  For me, that's useful on the track, and not in the least bothersome on the road.  The car dynos with 140 HP at 6K rpm to the differential.

But I feel like it's no where near race nick.  You're right I think Spec E30 (the bmw medium-case diff) is capped at 65 pounds?  And I think those guys kinda grind their teeth about that limit; they would really like more. 

The medium-case LSD on my E36 track car is currently at 95 pounds, and it leaves me wanting more, since it's a V8 and the torque is what makes things fun.  Rebuilding it and adding a 3rd friction disc is on my list for sure.  But there's a long list ahead of it.

Thing is, I'm not sure whether the small case can stand up to anything more than spec.  Tracking my E21, with an E30 M20B25 motor in it, I have already broken one LSD cover.  It's just a one-ear mount.  If the 318i is the same way, I'd be leery.