Nitty gritty on Large vs. Medium case diff swap

[TheStigg]

I took some detailed dimensions noting the differences between a large e32 diff vs. medium case e34, for my friend Andrew, who has also has a boosted e32 735i 5-spd:









Note: medium case axle hubs may come in either small or large diameters. Large case axle hubs WILL NOT swap with medium case axle hubs, but medium case small hubs WILL swap with medium case large hubs.

Also note that medium case input hubs WILL NOT swap with large case input hubs. A medium case manual input hub might have the same stud size and bolt circle, which would work, but the splined pinion shaft on the large case is a larger diameter than the medium case and will not swap.

[moroza]

To clarify:

Big-case flanges are for big-case diffs only. Medium-case flanges are interchangeable with each other.

Big-case axles will interchange with the larger medium-case axles (same part number, in fact); smaller medium-case axles do not interchange.

Hub flanges (at the wheel hubs) have the same boltpattern as the diff output flanges.

There are at least two styles of large-case input flanges. 540iA and 540iM have a larger-diameter boltpattern than the 750iL, whose boltpattern is the same as the 530iA's, and probably most other E34's.

E32 diffs are the same as E34, but there are variations in input flanges (see above). E32 hub (wheel) flanges are the same, too. E32 axles are unique and do not interchange.

(why on earth did you use inches?)

[535ipower]

What would i need to run a medium diff in a large diff car?

Trying to go 3.64 but just cant justify the price of a 3.64 large case.

[moroza]

What would i need to run a medium diff in a large diff car?

Trying to go 3.64 but just cant justify the price of a 3.64 large case.

All the info you need is above. Do some quick math and logic work to get your answer (hint: it'll probably cost more than a big-case 3.64).

[DarkSideofWill]

What case would my 535 manual with 3.46 rear have?

How much power can each size handle? Is there a primer someplace that describes which applications use which size?

Thanks!

[JesterMX6]

i was under the impression that certain e32's had medium case diff's.

aka the diff out of my 88 735i should bolt right into my 90 535i

[moroza]

All M20, M30, M50, M60B30 cars have medium-case diffs; some variation on flange size but not the diff itself. All M60B40, S38, and M70 cars have big-cases.

This only applies to US-spec models; apparently, some Euro 535i had big-cases.

[DarkSideofWill]

How much oomph can you put to the ground through a medium case? Big case?

[JesterMX6]

All M20, M30, M50, M60B30 cars have medium-case diffs; some variation on flange size but not the diff itself. All M60B40, S38, and M70 cars have big-cases.

This only applies to US-spec models; apparently, some Euro 535i had big-cases.

thanks. thats the impression i had as well.

[535ipower]

All the info you need is above. Do some quick math and logic work to get your answer (hint: it'll probably cost more than a big-case 3.64).

I was just in the hope of someone chiming in who maybe already done it. but by the looks of it i would need the diff from a 535i, axles from a 535i, probably a custom driveshaft?

It's hard to find a large case 3.64 and the only place i can find them is at diffsonline and it isn't cheap by any means.

[moroza]

How much oomph can you put to the ground through a medium case? Big case?

By and large, mechanical components like diffs and clutches are rated for torque, not horsepower. These medium-case diffs handle at least 500ft-lb in street-driven cars all day long. I've never heard of a BMW diff breaking. However, tire grip has a lot to do with it, hence the previous qualification of "street-driven": 500ft-lb in 1st gear (1:3.83 on a 535i) through a 3.64 comes to 6971ft-lb at the diff axle flanges. With street tires, you'll be spinning them and not stressing the diff much. With drag radials and a proper hookup, the diff is actually seeing a meaningful load with that kind of torque, and then it might not last.

Unless you're building some barely-streetable monster with slicks and 4-figure power, you'll be fine. Anyway, diffs for these cars are cheap (the common ones, anyway).

I was just in the hope of someone chiming in who maybe already done it. but by the looks of it i would need the diff from a 535i, axles from a 535i, probably a custom driveshaft?

It's hard to find a large case 3.64 and the only place i can find them is at diffsonline and it isn't cheap by any means.

Yup, custom driveshaft due to the different lengths of the diffs. $150-400 depending on what kind of deal you can find. You can use your existing axles.

[DarkSideofWill]

By and large, mechanical components like diffs and clutches are rated for torque, not horsepower. These medium-case diffs handle at least 500ft-lb in street-driven cars all day long. I've never heard of a BMW diff breaking. However, tire grip has a lot to do with it, hence the previous qualification of "street-driven": 500ft-lb in 1st gear (1:3.83 on a 535i) through a 3.64 comes to 6971ft-lb at the diff axle flanges. With street tires, you'll be spinning them and not stressing the diff much. With drag radials and a proper hookup, the diff is actually seeing a meaningful load with that kind of torque, and then it might not last.

Unless you're building some barely-streetable monster with slicks and 4-figure power, you'll be fine. Anyway, diffs for these cars are cheap (the common ones, anyway).

I am aware of the difference between torque and horsepower.
I'm "gearing up" (hahahahaha... lol... umm... chuckle) for a Getrag 265 swap in my 535 ( http://forums.bimmerforums.com/forum....php?t=1485550 ). I was wondering what else I'd need to do to "boost proof" the drivetrain. I'd like to be able to run slicks or DR's.
I am however stepping back the flywheel weight from the stock 30 lbs to about 15, if the numbers I've read are correct.
Also conjecturing that I might want to step back from the stock 3.46 to 3.15 gears in the name of traction. The 265 I have came from an M3, and thus already has 3.38 first instead of 3.82.

[moroza]

535i drag car? Interesting. Yeah, you'll want a big-case diff, and depending on your actual power, there's no guarantee it'll survive what you put it through. Hoping Jon and other folks running lots of power will chime in.

Flywheel weight has little to do with this. By the time you're making anywhere near enough power to raise questions about diff longevity, you won't be using it much to launch anyway.

[E34drifter]

quick question about interchanging flanges. I bought a 3.46 lsd diff from an 89 535i 5 speed. the diff came without the side flanges. So my question is will the flanges from my 92 525i 5speed 3.23 open diff work on the 3.46 lsd?

[attack eagle]

yes. all medium case output flanges interchange.

[moroza]

quick question about interchanging flanges. I bought a 3.46 lsd diff from an 89 535i 5 speed. the diff came without the side flanges. So my question is will the flanges from my 92 525i 5speed 3.23 open diff work on the 3.46 lsd?

Like I said,

Medium-case flanges are interchangeable with each other.

[shogun]

some more diff info
FAQ Medium Case Diff Rebuild
https://www.mye28.com/viewtopic.php?...42750#p1406298

ratios https://web.archive.org/web/20180306.../diffratio.htm
http://www.bokchoys.com/differential/GearRatios.htm
https://www.bimmerforums.com/forum/s...drive-tutorial