E38 Custom Rear Differential Questions

[Rhein Amacher]

You will need new seals and the crush washer/spacer for the pinion gear. You may also need new shims for proper spacing. Lastly, you will want someone to measure the runout and back lash (I think that is what it's called). Have to make sure the ring and pinion gear teeth line up properly. When I put a 3.62 in the M5 diff when I built my LSx 427 touring, I got lucky and the stock shims worked perfectly.

Brandon, you have done the same mod I am attempting? Putting the internals of a e46 M3 3.62lsd into the casing of a 1995 e38 3.15 open diff?


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[Rhein Amacher]

Once again, people are confused with the e38 and e39 diff case sizes. The 96+ e38 and pretty much all e39s other than the M5 have 215k diffs (V8s that is), while the M5 has the 210 diff case, much like the old e32 and 95 e38. The 215 gears and 210 gears are not interchangeable and neither are the cases.

The first question for the OP is whether or not the e39 4.10 diff he has is from a 540i, or one of the I6 cars. If it's from an I6 car (528i, 525i, 530i), you can go ahead and scrap the idea, because they have a 188k case, which is not interchangeable at all.

So, what options do you have if the 4.10 gearset is a 210? Well, this has been said many times in many threads, but you can get a 95 e38 differential (has 210 gears, but open diff and is already a 3.15 ratio) and swap the gears to the 4.10 and if you want at that point, stick a 210 LSD unit in it from either an e32 (88-89 750il), e39 M5, or e46 M3. There might be others with this case size, but those are the most common. Put it all together and you can install basically plug and play into your e38. I think 4.10 in an e38 might be a bit too short of a ratio, but that's up to you to decide. Most e39 M5 guys stick with about 3.62/3.64 or so, but some have gone shorter. The point is, you still need to buy two more differentials to make your 4.10 gearset work in your car, which will end up being very pricey, though not as bad as a built-to-spec diff I guess.

I think another option you might want to consider is to get a 95 e38 differential and swap the internals from an e46 M3 into it, making a 3.62 LSD. Again, still requires buying 2 differentials, but you can sell your 4.10 to help fund it and the e46 M3 differentials are easier to find and not terribly expensive.

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Oh, and you'll need specific output flanges to work with the LSD and your current axles as well. The easiest option there is to get the e39 M5 flanges and use the pre-2001 (I think) 102mm axles.

If you don't mind modifying the rear subframe a bit, you can also just buy an e39 M5 LSD differential and install it in the e38 by cutting and rewelding the rear diff mounts closer to the center of the subframe. Ask 007008 about that.

So Blackknight,

I currently have the 95 e38 3.15 Open Differential and a 2003 e46 3.62 LSD from the M3. So if I swap all the internals from the M3 into the 1995 e38 carrier assembly, my car would now have a 3.62 LSD, not a 3.15 LSD correct? On a different post, Sabreslight said I would need to swap the 3.15 ring gear in addition to the 3.62 LSD internals. So what would I have at the end? A 3.15 LSD or 3.62 LSD?

[blackknight530i]

So Blackknight,

I currently have the 95 e38 3.15 Open Differential and a 2003 e46 3.62 LSD from the M3. So if I swap all the internals from the M3 into the 1995 e38 carrier assembly, my car would now have a 3.62 LSD, not a 3.15 LSD correct? On a different post, Sabreslight said I would need to swap the 3.15 ring gear in addition to the 3.62 LSD internals. So what would I have at the end? A 3.15 LSD or 3.62 LSD?

3.62 LSD would require both the ring and pinion to maintain the 3.62 ratio, meaning if you swapped the 3.15 ring gear, it wouldn't work with the 3.62 pinion. If you just want the M3 variable LSD with a 3.15 ratio, then just use the LSD with the 3.15 ring and pinion.

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[Rhein Amacher]

3.62 LSD would require both the ring and pinion to maintain the 3.62 ratio, meaning if you swapped the 3.15 ring gear, it wouldn't work with the 3.62 pinion. If you just want the M3 variable LSD with a 3.15 ratio, then just use the LSD with the 3.15 ring and pinion.

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What would you recommend? I am leaning towards keeping the e46 M3 internals and making it a 3.62 limited slip differential instead of keeping the 3.15 gear ratio which came as the factory option for the car, albeit as an open differential. What differences would I notice with a 3.64 limited slip differential instead of the 3.15 open differential I have currently? (besides being more a gas guzzler)

[blackknight530i]

What would you recommend? I am leaning towards keeping the e46 M3 internals and making it a 3.62 limited slip differential instead of keeping the 3.15 gear ratio which came as the factory option for the car, albeit as an open differential. What differences would I notice with a 3.64 limited slip differential instead of the 3.15 open differential I have currently? (besides being more a gas guzzler)

Can't really help you there. I've never driven a car with that short of a ratio. Sorry.

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[Timm]

I have a small program that will show you what revs you will have verses road speed and differential ratio:

https://www.meeknet.co.uk/E31/Timms_Gear_Calculator.htm

[RVAE34]

Brandon, you have done the same mod I am attempting? Putting the internals of a e46 M3 3.62lsd into the casing of a 1995 e38 3.15 open diff?


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I did it in my LSx 427 e39 touring and was able to reuse the 3.15 shims.