Rear end Differential swap

[632 Regal]

Was looking for a lsd rear pig for the newer E34, ouch! So I figure whoever buys the 94 won't really appreciate a 3:64 LSD differential in an old rusty car so I decided we will swap the LSD from the 94 to the 95. I put a 3:64 in the 94 to replace the open 3:15. It was a little bigger gear than I wanted but thinking about it the difference between a 3:64 and a 3:42 isn't much at all.

Anyways, I figured out why I had a problem last time. The 94 has a different driveshaft and input flange on the differential. The 95 has a smaller spline driveshaft and smaller differential input flange. I swapped the input flanges with no problem except the 3:64 seems to have some non smooth bearing feel to it. That's the rear end I didnt get the crush washer in time to install and as that was 100K ago I have no clue where it is now.

Whatever I used loctite again and snugged the front flange down until I felt a little drag from the front bearing. The 95 had the stock 3:15 which is very smooth. My son says the lube in the POSI unit is really thin so I will change that out.

I had a few really cool write ups here but they have become disloged with the different servers and hosts over time. I will try to get all this to work again along with pictures on this write up.

Anyways, I am sore, these things are a challenge without a garage and proper setup. So far I am swapping both cars out with just the rear tires up on ramps on a sloping driveway. Had to spin one tire on a side to get to the other output shaft hex nuts. That was interesting.

Other than that I will revise this thread.

[MBXB]

The 3.15 wasn't LSD?

[632 Regal]

No both original differentials were the 3:15 open.

I finished the swap yesterday, I don't know why but I was always afraid of busting the bolts loosening the rear and front support brackets. Decided to give it a check and sure enough they are tight but did loosen up. This makes a 4 hour job a 1 hour job, hardest part is getting the front support bracket above the rear end support. Bolt the back side in and it just about aligns itself. Man to think I did this 4 times the hard way.

[ahlem]

Here in these parts d to d rears at the local LKQ Self Serve are under $200. Are the diffs the same on 525, 535 and 540?
As I understand it the M5 and 535 Automatics had 3.91(?) LSD ratios. There is a 540 in there right now that is tempting me. If you spin one rear wheel and the other spins in the same direction does that mean it's LSD?

[genphreak]

Here in these parts d to d rears at the local LKQ Self Serve are under $200. Are the diffs the same on 525, 535 and 540?
As I understand it the M5 and 535 Automatics had 3.91(?) LSD ratios. There is a 540 in there right now that is tempting me. If you spin one rear wheel and the other spins in the same direction does that mean it's LSD?

Yep. Especially if the input shaft is locked (eg engine in gear with the clutch engaged). An open will spin the other side in the opposite direction. A 540 will have a large case diff (210mm gear) as does the M5. The sixes (and 530 V8) otherwise have medium (188mm) diffs. I'd grab that entire rear end, the driveshaft and if it has a 6 speed and M60 keep going... take the damn lot... lol!

[632 Regal]

540 and M5 are large case rear ends, you can do an ebay search and find about every gear ratio known to man but not the one you need. The output shafts and driveshaft inputs vary. I have both a 94 and 95 530 and the driveshaft input to the pig is different. The other 530, 535, 525 etc, are small case units. I am not sure of the difference in changing from small to large, I think it has to do with the mounting hardware and flanges, but I have not personally compared them. I can't really see BMW changing the rear subframes to accommodate a gear change so it is most likely as I said.

Gen is dead on how to tell posi vs non. I used the new rear fluids made for limited slip and have no chatter! Yea for technology last time I needed to add stuff. I'm running mountains and curves so LSD is recommended, one slip could mean your going backwards fast.

[genphreak]

540 and M5 are large case rear ends, you can do an ebay search and find about every gear ratio known to man but not the one you need. The output shafts and driveshaft inputs vary. I have both a 94 and 95 530 and the driveshaft input to the pig is different. The other 530, 535, 525 etc, are small case units. I am not sure of the difference in changing from small to large, I think it has to do with the mounting hardware and flanges, but I have not personally compared them.

Beware, what Jeff what is calling small is what most call medium (he's talking about 5 series only). In general terms small bmw diffs have 168mm gears and are fitted to 4 cylinder BMWs as well as the smallest of the 6-pot 3 series (320i e30 & e36 for example). More modern ones as well as diesels have higher torque output so probably have bigger diffs than before. The medium diffs use 188mm gears and are used in bigger engined e30 and e36 as wel as e32 and e34 except M5, Alpinas, 540, 740 that all have the big case units.

Never swapped to large myself but have talked to people who have- I think one can swap a medium case car to large without too much trouble. I'd do it if upping output FI otherwise the medium diffs are exceedingly strong anyhow. With good oil they'll outlast a hard-driven car. The mounts may vary for each case size but would be easy to swap out if so. I don't think they do much. Output shafts on the large case are all larger diameter as is the input shaft hardware, bigger bolts, output flanges, etc. With gearing the rule at BMW seems to be to customise the entire drivetrain for each engine type.

The e34 subframe caries on M5 to allow for the narrower track, but it also does provide more space for the diff. I think we'd all suggest you check out the parts listings if you decide to do a swap to help you check you have everything you need from the donor before passing anything up.

I don't know why but they just love changing drivetrain; on e36 and e46 you get medium case diffs with 6 and 8 bolt output flanges, related output shaft differences, LSD differences, etc.

At least on e32 and e34 it is fairly simple: In terms of ratios, 4 speed cars work best with 3.64 & 3.91, 5 speed cars 3.07, 3.15 & 3.23, 6 speed 2.65-2.93.

Did you get the parts off yet?

[632 Regal]

The e34 subframe caries on M5 to allow for the narrower track, but it also does provide more space for the diff.
Did you get the parts off yet?

Under the car looking around there is plenty of room for a bigger case, if the subframe is different which I think it is I don't know why. Perhaps the big cases are longer which would make sense. I also think the rear subframe has a different number in realoem.