Disavantages to medium case diff?

[alpinewhite318i]

I have come across a medium case 4.10 limited slip differential from a 1989 325I convertible.  Are there any disavantages to this larger differential like larger ring gear? I am putting it in a 84 318i. Any feedback greatly appreciated!

[DesktopDave]

I just swapped to a 3.73 medium case.  It's heavier, bulkier and cumbersome.  I'd go for it, the install isn't too bad.  Fits perfectly and I'd suspect it's more durable.  Also see if the diff ear bushing is OK.  My 325IS had a solid bushing that was in great shape - the old webbed 318i diff bush was cracked and weak.

It's also good to have a helper do gear & brake while you're under the car.  I made three observations when I put it in:
1. Easier to fill it with gear oil out of the car, just put a little block of wood under the input shaft to level it a little.
2. The driveshaft studs are sometimes misaligned.  Pound them into the driveshaft and then just tighten them really good afterwards.
3. When you're levering that big old thing in, bolt the driveshaft on pretty well, THEN try to tighten the four upper bolts.  You'll need a 19mm box-end wrench to get them off, a socket won't fit the top two.  I had to use two wrenches in tandem to get them loose.  Do the half-shafts next.  Do the diff 'ear' mount last.  I know it's tempting to get that bolt back in there, but trust me, do that one last.  I also had to use a jack to push it sideways about 1/2 inch.  It just refused to cooperate.  

I also unfortunately had to shorten the drive shaft a little.  I'd have dropped the exhaust to do this but it was all rusted together.  So I pulled out all the little heat shield bolts and got to the big threaded collar in an hour or two.  That was a PITA, don't want to do it again.

[alpinewhite318i]

Does the differential seem to heavy in the rear?  Did you loose performance?

[DesktopDave]

I didn't notice any difference at all.  The higher ratio makes the car slower, but the gears are more useful.  I should pick up a few miles every tank too.  I'd figure the diff might weigh 15 pounds more, not much different.  I'm hoping that it's added durability, and the car could use a little weight in the rear anyway.  You can see that they're not really that much different:


I also forgot to mention that BMW changed that little speedo sensor connector on the rear between the diving board bumper models and the plastic bumpered ones.  The sensors swap, but be sure you can get out those two little 10mm bolts.  I rounded one off (I should have used a flare wrench) so I decided to splice the old connector into the 318i rear harness.  Seems to be accurate.  I matched the wiring colors too, but I'm not sure that's required.  One lead is brown/red and the other is just brown.

[tjts1]

I measured 25lb different between small case 3.64 and medium case 3.73lsd. Not worth the extra weight to me.

[DesktopDave]

I measured 25lb different between small case 3.64 and medium case 3.73lsd. Not worth the extra weight to me.

I heard that autox'ers are swapping to the lighter unit for the weight savings.  I'd bet a small case diff, insulation removal, motorcycle gel cell and losing the spare tire/trunk carpet/back seat you could lose 100+ pounds off the back end.

As for me, it's a daily driver/work car.  It'll keep the heavy diff, dual-mass flywheel, insulation and maybe even get power seats and a decent stereo.  I'm tempted to pull the P/S pump and A/C for increased mileage, but the wife's comfort trumps my performance every time!

Not to spread heresy, but I'm into this car to teach her about how much fun driving can be...and maybe do a little wrenching to relax.  Don't let her know, but it's a stepping stone on the way to that Z3 that I see far in the future!