3.23 to 3.73 differential swap DIY AKA poor man's supercharger

[atmosphericM]

Some info regarding exactly which carrier units are swappable for those interested: http://www.e90post.com/forums/showthread.php?t=1466828

[arhoads335i]

So what I read in this thread is you should maybe go for the 3.64 that you can find in a 330i? That might be the perfect balance for a daily plus track toy

[Boudreau1589]

Please forgive my pathetic naivety on this subject but I'm interested in doing a diff swap (hopefully with an LSD inclusion) but I spend significant time at 80-90 MPH and I'm looking to explore opportunities to have my cake and eat too.

In terms of feasibility, difficulty of the project, cost, unforeseen issues, and overall effectiveness, if I were to swap the rear end for something else (and hopefully include an LSD) could I adjust the 6th gear ratio to something that would allow 6th gear cruising at 85ish MPH and RPM at or below 3000 RPM?

1. is this possible?
2. could a motivated semi-mechanically included individual pull this off?
3. are parts available?
4. Is this crazy talk?

Go ahead and rip into me.

Thanks

[Bimmer-Bob]

Originally Posted by Boudreau1589

Please forgive my pathetic naivety on this subject but I'm interested in doing a diff swap (hopefully with an LSD inclusion) but I spend significant time at 80-90 MPH and I'm looking to explore opportunities to have my cake and eat too.

In terms of feasibility, difficulty of the project, cost, unforeseen issues, and overall effectiveness, if I were to swap the rear end for something else (and hopefully include an LSD) could I adjust the 6th gear ratio to something that would allow 6th gear cruising at 85ish MPH and RPM at or below 3000 RPM?

1. is this possible?
2. could a motivated semi-mechanically included individual pull this off?
3. are parts available?
4. Is this crazy talk?

Go ahead and rip into me.

Thanks

A simple test: Go out for a drive on the highway and find your cruising speed in 6th gear. Then shift into 5th. If you can live with the RPMs at wherever they land, you will be happy with a 3.73 final drive. If you think the RPMs are too high for cruising, consider longer gearing, e.g., a 3.46 final drive.

[Boudreau1589]

Hey Bob,

Thanks for the quick feedback. I have found that 5th at 80 is just too high, unfortunately. I know I would need a transmission shop to weld the taller gear onto the output shaft and that's probably the more painful cost but I know there are some other obstacles I'm overlooking here.

I'm just wondering how painful those oversights are likely to be as well.

Thank you again though,
Kyle

[arhoads335i]

Can anyone confirm that a 330i auto rear diff would fit a 128i and that it has a 3.64 ratio?

[gixracer]

Originally Posted by arhoads335i

Can anyone confirm that a 330i auto rear diff would fit a 128i and that it has a 3.64 ratio?

They are the same pumpkin but the output is set up for a guibo

[mechavore884]

sooo yeah I know what my next move is this month LOL

[ornicar]

325i auto rear diff (7566221/7566222) is plug&play -->with no modifications, but the ratio is only 3.46 the same as euro 130i.

Edit : Europe 325i equipped with N53 only

[chris_flies]

Originally Posted by ornicar

325i auto rear diff (7566221/7566222) is plug&play -->with no mofifications, but the ratio is only 3.46 the same as euro 130i.

I'm pretty sure the 328i and 325i auto have the same 3.73 diff

A really crazy upgrade would be the 328xi auto diff. It's a 3.91...

[desertman123]

Originally Posted by chris_flies

I'm pretty sure the 328i and 325i auto have the same 3.73 diff

A really crazy upgrade would be the 328xi auto diff. It's a 3.91...

There's a guy over on E90Post who just had a 3.91 LSD built. I think BigInBoca

And yes I'm running an 06 325i 3.73 in my car and have been for a few months.

I would love a 3.55 or to find an LSD with the 3.46. I do a metricfuckton of highway driving and I don't mind being at 3500rpm at 80mph, but it would relax the car and throttle a little bit.

[crowtrobot]

Completed my own diff swap a little while ago, and even though the job was pretty straight-forward, this thread was a really helpful as a sanity check. Thanks!

[markslc1]

Originally Posted by desertman123

This only applies if you have manual transmission!!

So the manuals for N52s all come with a 3.23 ratio diff, but the autos have a 3.73 in the exact same case. The 330i's have a 3.15/3.64 (respectively) and I'm told you can swap them all around. But the the easiest thing to do is buy a 3.73 out of a 2007-2013 328i from a junkyard. You can find them for $100-$300, depending on age and condition.

Are you sure the diff from a 2012-2013 328i (F30) will work? I would assume that was a different platform. It would be nice given these would be lower mileage parts. Can anyone confirm?

[chris_flies]

Originally Posted by markslc1

Are you sure the diff from a 2012-2013 328i (F30) will work? I would assume that was a different platform. It would be nice given these would be lower mileage parts. Can anyone confirm?

Not the same diff in the F30. E90 ran to 2011, I think OP was just confusing E8x model years with E9x mode years. I do the same sometimes...

[markslc1]

Originally Posted by chris_flies

Not the same diff in the F30. E90 ran to 2011, I think OP was just confusing E8x model years with E9x mode years. I do the same sometimes...

Been driving with the 3.73 diff for a month now. I gotta day it's something I wish I did long ago. The car pulls much harder out of corners almost like having the car In sport mode. (Like my x5 has). Best $150 I spent on the car. Highway mileage is still 30mpg too.

Don't hesitate. Just do it.

[Suprgnat]

Originally Posted by chris_flies

Not the same diff in the F30. E90 ran to 2011, I think OP was just confusing E8x model years with E9x mode years. I do the same sometimes...

E92 was built until 2013.

[Ygrene]

Originally Posted by markslc1

Been driving with the 3.73 diff for a month now. I gotta day it's something I wish I did long ago. The car pulls much harder out of corners almost like having the car In sport mode. (Like my x5 has). Best $150 I spent on the car. Highway mileage is still 30mpg too.

Don't hesitate. Just do it.

I am also about 1 month in with a 3.73 diff and I could not agree more. It’s definitely up there as one of the most rewarding installs I’ve done for any car. I have not been watching my mileage so not sure if any changes there but smile-age has definitely gone up ha ha do you get it

[LcossE82]

Why didn't I get a manual trans 🤦🏽*♂️🤦🏽*♂️

[Sliver_Legend]

it's there an lsd that will fit my 128i? I work for a company that can source salvage parts, I'd like to see if something else will fit. I have a 3.73 already but haven't put it in. I'm in a daily driver but I figure if I'm gonna do it I want to do it once so I guess the question would be if there is a 3.64 lsd that would bolt up what car would have that so I can find me one? also, has anyone used one of those conversion kits? new to all this and very curious about starting here.

[desertman123]

Originally Posted by Sliver_Legend

it's there an lsd that will fit my 128i? I work for a company that can source salvage parts, I'd like to see if something else will fit. I have a 3.73 already but haven't put it in. I'm in a daily driver but I figure if I'm gonna do it I want to do it once so I guess the question would be if there is a 3.64 lsd that would bolt up what car would have that so I can find me one? also, has anyone used one of those conversion kits? new to all this and very curious about starting here.

You'll have to get one specifically made for E82/E88 or E9x chassis, but there are kits that "bolt" in (still need to set runout and whatnot). MFactory, OS Giken, Quaife, etc.

[tree233]

I'm getting my diff rebuilt at the moment with a 3.91 gearset installed at the same time. It's from an E93 325i auto - they have a 3.91 in Australia at least.

The input flange isn't the same as my 130i (3.46), so not a direct swap option, but if you check realoem, maybe you just need to change the part between 6 & 7? Everything else would bolt up. Better do your own research tho, as I think the bolt that holds that part is the bolt that you tighten to set the backlash - but can be marked and replicated for a DIY / good enough job - till you're ready to get a proper rebuild done at least.

https://www.realoem.com/bmw/enUS/sho...diagId=33_1232

I'm super excited and impatient to feel that torque increase!!! 13% extra at the wheels wooooot. Equiv of 356nm vs factory 315nm at flywheel, and based on my e30's gear ratios, the 3.91 gearing in the 130i should suit my fav mountain roads & tight local tracks perfectly. 3rd gear ftw! 2nd gear for hairpins only! I don't care about fuel economy hahaha, I enjoy consuming that consumable.

Feel free to bump me for an update on torque improvement, and I'm happy to take note of rpm vs km/h and average consumption increase.

[jonwilli]

Originally Posted by tree233

I'm getting my diff rebuilt at the moment with a 3.91 gearset installed at the same time. It's from an E93 325i auto - they have a 3.91 in Australia at least.

The input flange isn't the same as my 130i (3.46), so not a direct swap option, but if you check realoem, maybe you just need to change the part between 6 & 7? Everything else would bolt up. Better do your own research tho, as I think the bolt that holds that part is the bolt that you tighten to set the backlash - but can be marked and replicated for a DIY / good enough job - till you're ready to get a proper rebuild done at least.

https://www.realoem.com/bmw/enUS/sho...diagId=33_1232

I'm super excited and impatient to feel that torque increase!!! 13% extra at the wheels wooooot. Equiv of 356nm vs factory 315nm at flywheel, and based on my e30's gear ratios, the 3.91 gearing in the 130i should suit my fav mountain roads & tight local tracks perfectly. 3rd gear ftw! 2nd gear for hairpins only! I don't care about fuel economy hahaha, I enjoy consuming that consumable.

Feel free to bump me for an update on torque improvement, and I'm happy to take note of rpm vs km/h and average consumption increase.

I'll be interested to hear your thoughts on the 3.91 diff. I currently have a 3.91 diff from a 328xi coming from the US to the UK to fit to my E81 130i. My 130i already has decatted headers, full Supersprint exhaust with sports cats, modified intake and StageFP tune. Plan on installing MILVs that are in the garage later in the year which should bump power up to around 290 crank hp. With the diff it would be like a stock car running 330 crank hp which is more than an E46 M3 that also weighs in a hundred Kgs more.

I will fit to the car and see if I can live with the additional revs on cruise but most of my driving is on twisty country roads and we use the F31 335i for longer journeys. If I like the change I will get the Quaife swapped out of my stock 3.46 diff into the 3.91.

I am excited to experience the change and a little nervous it may be money wasted or that I should have gone for the 3.73 diff as more of a compromise. I ended up deciding go big or go home!

The change from 3.46 to 3.91 in the 130i is similar to the change E46 M3 owners make from their stock 3.6 to 4.1 which is meant to be a great improvement...lets see. The diff is currently about to make the journey over the Atlantic!!