3.46 Differential Swap into Manual 128i?

[cerealwars]

Gear swaps are a wonderful way to really wake up a "slow" car. I've been very enticed by the 3.73 swap that so many rave about. However, I have an arthritic left knee and I do a decent amount of highway driving, so I have been stubbornly searching for something in between the tall 3.23 stock gearing and the 3.73 E90 auto gears for my 2010 128i 6mt.

Using 225/45/17 tires, 0.85 sixth gear ratio, here are the given RPMs at 75mph:

3.23 - 2694 rpm
3.73 - 3111 rpm

I've noticed some models of earlier Z4s (E85/86) have 3.46 gearing. This is smack dab in the middle.

3.46 - 2886 rpm

Looking at ebay listings, damn it sure looks like the same case as mine. I'm really tempted to try one out, it seems like I would just swap my rear cover over, refresh it with some new seals, and be good to go. Am I drunk?

Second question - Which diff case does the manual 128i come with? 188L? 188LW? 188K?

Here's some links I found helpful in my searches, for anyone else's reference.

Factory gearing and application table: https://www.moreheadspeedworks.com/bmw_gear_ratios/

Diff Case info: https://www.difflab.com.au/pages/bmw-differential-types - **Note the typo of 168LW on this page, I believe it is meant to say 188LW

Internals application reference: http://www.wavetrac.net/application.htm

[cerealwars]

Alright, well I found some more detailed ebay listing pictures and it seems the Z4 housing is in fact a bit different. The front mounting points are narrower than on the 1er, and the rest of a the casting is a bit different too. So, probably a no-go on the Z rear end and I'll just have to go for the 3.73s, or if I'm lucky maybe I can find a 3.63 out of an E90 330i, since I heard that also works. I'll try and update this thread if I happen to find something useful.

[Spielmp3206]

This topic is quite interesting and on time for my next steps,

I am looking to get an LSD for my 128i and checked the options that diffsonline provide for the 128i and I only saw the oem 3.23m the 3.64 and 3.73, there are some other crazy options like 4.1 or 4.44 but those would be just way too much.

I currently have the 3.73 in my manual 128i and I really like it, even in highway is really not bad to cruise at 75 at a little over 3k rpm. However I would consider the 3.64 if I decide to go with a diffsonline unit it just depends on the budget to continue the project next year.

now that you mentioned that the 330i diff works, I might try to find a 330i 3.64 diff on ebay, buy a wavetrack lsd and put it together myself, the units that diffsonline offer are sweet but pricey

I think we are pretty close if you want to try out a manual 128i with the 3.73 just let me know

[cerealwars]

Originally Posted by Spielmp3206

This topic is quite interesting and on time for my next steps,

I am looking to get an LSD for my 128i and checked the options that diffsonline provide for the 128i and I only saw the oem 3.23m the 3.64 and 3.73, there are some other crazy options like 4.1 or 4.44 but those would be just way too much.

I currently have the 3.73 in my manual 128i and I really like it, even in highway is really not bad to cruise at 75 at a little over 3k rpm. However I would consider the 3.64 if I decide to go with a diffsonline unit it just depends on the budget to continue the project next year.

now that you mentioned that the 330i diff works, I might try to find a 330i 3.64 diff on ebay, buy a wavetrack lsd and put it together myself, the units that diffsonline offer are sweet but pricey

Alright so I looked into E90 330i automatic differentials with 3.64 gears, the input flange/pinion yoke is the three-bolt style, as opposed to our 6-bolt circular flange - but otherwise it looks like the rest would work. 3.46s out of a 135i are also the same story with yet another yoke/flange. I'm fairly confident someone could DIY swap it, but I also know pinion preload is a pretty important thing and grenading a gearset on the highway sounds like a bad time.

Doesn't quite seem worth it for me right now, I'd just go with the 3.73s and know that the thing is set up right. I've set up a more than a few diffs on my old 240, but not sure if it's worth the gamble on my "reliable adult car". I'm becoming older and "sensible". Time to put me down I guess.

Next step would be to verify the pinion gears themselves have the same spline dealio for the flange to swap over, before dropping cash on a unit that won't work. Realoem would be helpful here, maybe I'll look at the interchange tomorrow just for shiggles.

An alternate route, if you're desperate enough: You could use any diff above if you also snagged the companion driveshaft. Then you would then have to take both driveshafts to have a driveshaft shop frankenstein them for a few hundred bucks.

Originally Posted by Spielmp3206

I think we are pretty close if you want to try out a manual 128i with the 3.73 just let me know

Yo that's awesome man! I would love to take you up on that, your car is super sick! Not sure if you winter drive your car or not, but we could meet up out in Highland area sometime and hit up some backroads. PM me if you want.