Pre-91 750iL cars came with a 3.15 rear gear, most with a limited slip differential. Sometime in 91 they started putting 3.64 gears instead, but with an open diff, so between the higher gear and all the power going to one tire, more chance for uncontrolled wheel spin. Not to worry, those cars have traction control.

But traction control works by limiting engine power and applying the brakes, hardly the way to get maximum acceleration. It would help if power flowed to both wheels, then traction control would have to interfere less.

It turns out it's not impossible to get the best of both worlds for a relatively small amount of money. A professionally build diff is easily more than $1000, if not double that.

It's easier for post-91 cars, because 3.15 LSD units come on sale fairly regularly on eBay, most for under $300 (plus about $100 shipping). You take the LSD out of them, and put it in your diff.

For cars with the 3.15 rear, you have to hunt around for a 3.64 diff, and since they're rare, they're more expensive. Make sure you get a large case diff, not a medium one. You'll take the LSD out of your diff and put it in the new case. You do it this way, not the other way around, so you can leave the pinion gear in it's original case. If you don't touch the pinion gear, you don't mess with the preload and backlash setup.

Be sure to keep the spacers for the output shaft covers (parts #8 here in their original spots, they also set up the gears.

Note: if your diff came from an 850, you may have to drill and tap one of the 3 bolt holes for the front mount on the side of the case. You'll need a 31/64 drill bit and a 14 X 1.50 metric tap. You'll also need to swap your rear diff cover to the new diff, along with the speed sensor.

On the left is my original 3.15 gear with the LSD unit, on the right is the 3.64 gear with the open diff.


Another view. In the back are the output shafts and between them is one of the output shaft covers:


There are 10 bolts that hold the ring gear on the carrier, and they are TIGHT! Use a big screwdriver wedged between 2 bolt heads and a long cheater bar to get them loose. The gear itself is a press fit on the carrier - I threaded in some longer bolts (4 of them) and used a hammer on the bolt heads to get the ring gears off the carriers. Oh, you have to remove the speed sensor rings first (star-shaped thing on the bottom in the pics). They are just pressed on.

I used the same longer bolts to pull the 3.64 ring gear onto the LSD unit.

These are the tightening specs:


Despite what the specs say, you can reuse the old bolts. Note the chart is in Nm - so 100 Nm is about 75 ft-lbs. Turning the bolts an additional 50deg is no problem if you have a long enough cheater bar, the problem is keeping the gear from moving while you do so.

I ended up making a wooden box out of 2X4s to hold the gear


The box is screwed to a piece of plywood from the bottom, and the plywood was anchored to the workbench on the floor in 4 spots (red circles):


The wood keeps the gear in place, and the large screwdriver keeps it from turning


The rest of the assembly was pretty straightforward. I did use some machinist dye to check the gear contact pattern, and it was perfect. I used a new rear cover gasket and shaft seals, and filled it with Royal Purple 75W-90 synthetic gear lube.

The new diff runs smooth and silent, and the acceleration is much improved. It's only been a week, but my mileage so far has NOT gone down, in fact it may be slightly better than before. This includes a 200 mile round trip last weekend and almost a week of rush hour commuting - using the AC for most of it (no AC use back in Jan-Feb).

One difference is that now, economy mode doesn't suck. So now I find I only occasionally use sport mode, while before the only time I went into economy mode was after reaching cruising speed on the highway. And of course, actually *feeling* acceleration above 80 now is pretty cool :)

It may be a while before I can do some timed runs, but they should be interesting!

Erwin 89 750iL Lowered (a little)

Engine / Driveline Wolf chips 19 lb, 4-hole (Design 3) fuel injectors PowerFlow Intake Air Filters Royal Purple 20W-50 Custom Cold Air Boxes 3.64 LSD w/Royal Purple gear lube Royal Purple trans fluid Interior Phone removed, replaced with cut-to-length dash wood trim on hinge E38 Shifter Knob, Console Plate, and Boot M-Tech 2 M-technic Steering Wheel - air bag deleted E38 Self-Dimming Mirror Audio BMW Business CD Player "CD43" Head Unit Refurbished factory amp w/equalizer Bass blockers (300hz/4ohm) on stock front kick panel speakers Rear deck speakers: Alpine SPS-13C2 5.25" 2 way 175 Watt with bass blockers Subwoofers: 2 JBL GTO804 8" 4-ohm subs Subwoofer install: custom sealed enclosure mounted behind ski bag opening Subwoofer amp: Alpine MRP-M450 400w MONO CD changer relocated to driver's door bottom pocket actuated by bio- mechanical arm controlled by semi-intelligent nut behind the wheel Suspension H&R Lowering Springs (1/2 coil removed from fronts) Koni adjustable front shocks Stock rear shocks (self leveling) Racing Dynamics Stress Bar Stock front sway bar Dinan rear sway bar - 21mm M5 aluminum lower control arms Front tires: 245-40-18 Goodyear Eagle F1 GS-D3 on 8.5" EuroTech rims Rear tires: 275-40-18 Michelin Pilot Sport PS1 on 9.5" EuroTech rims Exterior 25% Side tint 5% Rear tint BMW Mud flaps - rears trimmed to tire width Rear fender lips rolled Smoked side markers (after pic taken) Euro Smiley Headlights (after pic taken) Zymol wax Way too many paint chips touched up Brakes Front Rotors: 850CSi, 324 X 30 mm, cross-drilled and slotted Rear Rotors: stock size, cross-drilled and slotted Pads: Hawk Performance Ceramic (may switch to HPS) Stainless steel lines Racing Blue brake fluid

• Differential LSD swap (pic) (url) - erwin 2008-05-16 01:35:58 (132 views)
  • Re: Differential LSD swap - rv8flyboy 2008-05-18 15:21:39 (24 views)
    • I didn't take the LSD apart - erwin 2008-05-18 17:21:47 (21 views)
      • what, we have *know* what we do before we can fix our cars? LOL!! (nt) - rv8flyboy 2008-05-18 22:30:05 (19 views)
        • HAH !! You got me :) - erwin 2008-05-19 14:08:07 (25 views)
  • I am still learing the ways of the differential so bear with me! - Ed 2008-05-18 03:01:07 (23 views)
    • I bought the open diff to get the 3.64 gears - erwin 2008-05-18 12:18:05 (18 views)
      • My 03/90 735i manual was delivered with 3.64 LSD - cspargo 2008-05-20 02:36:32 (16 views)
        • Yeah, but 735s have medium case diffs - erwin 2008-05-20 10:38:24 (12 views)
      • Very cool. Good luck with everything. (nt) - Ed 2008-05-18 17:30:02 (17 views)
    • BMW quit because of cost. - kfister 2008-05-18 05:24:01 (39 views)
      • You know it is weird, the 750 did seem to do better in snow - Ed 2008-05-18 17:29:28 (16 views)
  • Re: Differential LSD swap - my406cid 2008-05-16 09:12:12 (27 views)
    • No - speed sensor is in rear - erwin 2008-05-16 09:57:05 (34 views)