3.73 diff compatibility

[Ohio Enthusiast]

I've been reading several mentions of using the automatic 3.73 diff with a manual transmission (that comes with the 3.08 diff), but I'm not clear on which diffs are compatible. One place stated that E9x non-M automatic diffs are 3.73 and are a direct fit. I'm guessing 1 series automatic diffs are also a direct fit? How about Z4?
I'm interested in a full swap, not looking into opening up the diffs and doing any internal work.

Bonus question - are there any LSD diffs with shorter final drives that are a direct swap into a manual 135i (again without opening up the diffs)?

[cerealwars]

The 3.73 swap is a bolt-in for the 128i manual cars. It is a wonderful pairing IMO.

If you have a 135i, there are other factory diffs that bolt in, but I'm not certain what they are offhand. The input flanges are different on the boosted cars, for whatever German-ass reason.

I would take a close look at ebay listings to see what your factory diff looks like. Pay attention to the case casting as well as the input and output flanges. Make sure the new diff looks identical and you should be good.

As long as you have e-torx sockets, it's a fairly straightforward swap. I think it took me about 3-4 hours at a leisurely pace, also considering a lot of fasteners gave me more trouble than anticipated.

[Ohio Enthusiast]

Good to know about 128i being a direct bolt-in vs. the 135i. I hope other people could share the details of what works with the 128i and what works with the 135i.

I don't own a 1er yet, but being a short-gearing-fanboi the shorter FD swap is really interesting for me, and the ease (and cost) of doing it will add to the pros/cons of buying an E88.

[Ohio Enthusiast]

I dug a little more on this, and looks like I was wrong initially. My understanding now is as follows:
If I want a direct full diff swap (i.e. swapping the pumpkin), I can use the automatic transmission diff in a manual car, but only for the same engine.
So a manual 128i with a 3.23 diff can receive an automatic 128i/328i 3.73 diff for 15% shorter gearing
And a manual 135i with a 3.08 diff can receive an automatic 135i/335i 3.46 diff for 12% shorter gearing.

If this is the case, it tips the scales even more in favor of a 128i for me (I'm a sucker for short gears and a 3.73 FD 128i would be almost as short as my BRZ).

Also, BMW never offered an LSD in these cars so there is no direct swap. That's fine, it was more of a nice to have.

[cerealwars]

Yep, that all sounds correct from my memory. If you're stuck between 135i and 128i, the bigger differences I see are more about having a ton of power with a reliability tradeoff. The NA engines seem to have 1/4 of the problems the boosted cars do. Either will feel like a torque monster coming from the 86 platform.

Something to consider with the 135i is that longer gears are probably more suited to a boosted car.

One other thing I have always wondered about, but haven't had the balls to try:
You may be able to use a boosted car's diff and swap pinion flanges over. I always wanted the 3.42 set, it just seemed like the Goldilocks gears for my use case.

I popped some 3.73s out of an auto E90 328i into my 128i and I actually don't regret it one bit. I see a good amount of highway and although I'm at like 3400rpm doing 80, I've somehow only lost ~1 avg mpg. And the best thing is that third gear is now accessible without doing arrest-me-now speeds.