1983 633 Differential Dilemma

[NWBismarck]

I recently purchased an '83 633 with a 3-speed auto intending to swap in a 5-speed manual, but learned after purchasing it that the previous owner has replaced the original open diff (3.25 I think) with a 4.10 lsd. It makes for a bit more pickup off the line, by with the 3-speed auto 70mph sits around 3800 rpm. I still plan on swapping in a 5-speed, but what do you think about the diff? Should I replace it with something a little bit closer to the original for highway cruising or will the overdrive on a 5-speed be enough to bring revs down a bit?

[Shipper]

There is a good spreadsheet for gearbox/diff ratio/tyre size = speed @ rpm. Let me see if I can find it.

See link below. I'm working in km/h here but, assuming 16" wheels, the 5 speed overdrive box and that 4.10 LSD, at 2750rpm you will be doing 100km/h, or 62pmh. That is a little short but bearable. So keep the diff for now.

I have a 3.71 LSD and 17" wheels so I get an extra 15km/h at the same rpm.

Your link. https://e28goodies.com/gearing-calculator/

Your best diff for performance and long distance touring is the 3.45. To do 62mph you'll sit on just 2250 rpm and to cruise at 77mph you are on 2750rpm.

[DesktopDave]

+1 I'm running a 3.46 (3.45?) on an overdrive manual behind an M30B32 with near-stock tire circumference. It's a bit too tall for urban driving, but it's a great combo of torque & RPM on the highway. Oversize tires will require a shorter diff, all things being equal.

Your short 4.11 diff is worth a lot to the E30 guys. For drag race junkies a really short diff is a great combo with M52 swaps.

I found my diff in a '99 Z3, it fits our cars with a few little mods.

[NWBismarck]

Thanks for the great info! Glad to hear I might be able to get something for the 4.10 from the E30 guys, may just replace it along with the transmission while I'm at it then. Stock tire size so the 3.46 sounds like my best bet for highway cruising and shouldn't be too hard to find.

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[DesktopDave]

One little detail - don't forget to swap/keep the output flanges and rear case before you sell your diff. The E30 uses a completely incompatible rear mount, and it also has smaller bolt circle on the output flanges. Likewise, an E30 buyer can't use your E24 parts, they won't fit an E30. You'll need to hold onto those if you find an E30 diff, to match it up with the E24 half-shafts. The cars are much lighter, BMW managed to save a few pounds in metal on the smaller cars.

One more detail - there were some compatible Torsen LSDs, with desirable ratios, installed in some late Z3 six-cylinders. If you're a fan of that technology (I am), look for an '00 or later 2.2/2.8/3.0 Z3.

I wrote up a blog post about all that stuff, check it out for the nitty-gritty details.

[NWBismarck]

That's a great write-up! I was originally thinking of putting off this swap, but knowing I can sell the 4.10 to the E30 guys and easily find a 3.46 from a Z3 seems like a no-brainer.

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