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

[desertman123]

Oh wow I didn't expect this to be bumped to the top.

I'm at about 83k miles on the diff swap and still happy with it. RPMs are high on the hwy but I don't drive the car too much these days, I have a work van

[Blau]

I'm a few weeks in on the 3.73 with manual. First gear is useless... Anyone else feel this way?

It's loud AF on the highway too. But 3-4-5 gears are a blast. I'm going to tune the car and see what I think. I worry the car will feel too slow with the 3.23 from the manual.

The auto with a PBX power box felt faster, NGL. This is still way more fun tho.

[mikekilo]

Just finished swapping this into my Australian 125i (stock tune, looking to do 130i tune soon). Sourced a rear 3.73 diff from a local E90 325i auto wrecking.

Took a painful 10 hours or so for me to swap this on jack stands. Excessive faffing about figuring out how to access and break bolts loose, and noticing a leak on the 3.73 diff and needing to reseal it. Took far longer to remove the diff than to reinstall. Managed to reinstall myself by operating the jack with my legs.

The first drive made that well worth it. This car finally has the feeling of torque that I would expect from a 3L engine. The diff swap wakes up the car <4k rpm. The shift points are easy enough to get used to, however I'm still adjusting to the shift timing for smooth acceleration (you need to shift faster).

Keeping in mind that Australian speed limits are low, I was previously using 2-4 the most, sometimes 5, and rarely 6 (6 for long highway and interstate only). I can now easily use 5-6 around town. Maximum speed limits here are 110kmph. Going slightly over at 120-130kmph will equate to roughly 3k RPM (or a touch over). It may drone (have not tested this yet), however given that my car is a convertible, I leave a pair of musician earplugs in it anyway.

EDIT: Forgot to mention the long removal time was likely due to the convertible V underbracing which I did not remove as my car was not sitting perfectly level on left/right. Probably a lot easier to do with it removed or on a coupe.

[One28i]

3.73 should have come from the factory, and should have done this swap earlier on than I did. I have 15K km on it, would not look back. I do notice my winter setup, since I have more wheel torque earlier on, doesn't seem to hook up as it did with the 3.23 setup. LSD would be nice, but I am OCD to understeer, the car is quite neutral except @ the limit, which it still understeers.

[Spielmp3206]

My car came with this diff when I bought it the previous owner did the swap and since the first time I drove it, I fall in love with it. Once the winter season is gone, I am looking to get an lsd install. I saw on difsonline an option to get a 3.64 so I might go with it.

[Suprgnat]

Originally Posted by Spielmp3206

My car came with this diff when I bought it the previous owner did the swap and since the first time I drove it, I fall in love with it. Once the winter season is gone, I am looking to get an lsd install. I saw on difsonline an option to get a 3.64 so I might go with it.

3.64 is $200 more than the 3.73, but I like it.

[80_Mark]

Originally Posted by Suprgnat

3.64 is $200 more than the 3.73, but I like it.

Do you think it’s a significant different? I’m debating betweeen those options and thinking about the usability of 1st gear and RPMs at ~80mph in 6th.

[ZHPizza]

Originally Posted by 80_Mark

Do you think it’s a significant different? I’m debating betweeen those options and thinking about the usability of 1st gear and RPMs at ~80mph in 6th.

Pic attached of RPMs at 75mph

I never drove a manual 128 with the stock diff, but did drive an auto the other day and 5th gear RPMs in that = 6th gear in my manual with the 3.73

[rebornE82]

I've got a 3.73 from a e90 328I sitting in the garage but i'm worried it would be too much if i want to do long trips at 85-90 mph cruising speed, pairing it with a header-back exhaust (laptorr ish) it doesn't sound fun.

I'm thinking a 3.64 would be perfect, and i've found that the e93 325i auto diff is 3.64 and seems to have the same driveshaft connection as the 3,73, unlike the 330i difffs that have the guibo 3 bolt type connection, therefore it should be plug and play

The part number is 33107566174

¿Has anybody installed or know about this diff?

[rebornE82]

Originally Posted by ZHPizza

Pic attached of RPMs at 75mph

I never drove a manual 128 with the stock diff, but did drive an auto the other day and 5th gear RPMs in that = 6th gear in my manual with the 3.73

Thank you for the pic man!
What size wheels and tires do you run? 17’s or 18’s?

[ZHPizza]

Originally Posted by rebornE82

Thank you for the pic man!
What size wheels and tires do you run? 17’s or 18’s?

That picture was on the stock 17in M Sport setup w/ 205f/225r. I've since moved to a 235/40r18, so slightly taller than stock.

I was actually driving it on the highway today and thinking about how quiet it was, but I have a very tame N55 muffler and no other exhaust modifications.

3.64 would be nice too and better for a daily driver that sees lots of high speed cruising.

[rebornE82]

Originally Posted by ZHPizza

That picture was on the stock 17in M Sport setup w/ 205f/225r. I've since moved to a 235/40r18, so slightly taller than stock.

I was actually driving it on the highway today and thinking about how quiet it was, but I have a very tame N55 muffler and no other exhaust modifications.

3.64 would be nice too and better for a daily driver that sees lots of high speed cruising.

Have you noticed a bit more rpm at highway cruising speed since you changed to 235/45 r18? I run 245/35 r18 on the rear and i’m afraid it would bump the rpm even more

I’m thinking about going 3.64 ratio, but i already have a 3.73 sitting, and i don’t know if its worth the headache just for 80rpms or so (if my calculations are right)

Here’s a pic of my spoiled kid as we say in spain 😂

[ZHPizza]

Originally Posted by rebornE82

Have you noticed a bit more rpm at highway cruising speed since you changed to 235/45 r18? I run 245/35 r18 on the rear and i’m afraid it would bump the rpm even more

I’m thinking about going 3.64 ratio, but i already have a 3.73 sitting, and i don’t know if its worth the headache just for 80rpms or so (if my calculations are right)

Here’s a pic of my spoiled kid as we say in spain 😂

Hell yeah brother I love those wheels!

The 235/40r18 is slightly taller than the stock 225/45r17, so would theoretically reduce rpms slightly, but we're talking on the order of like 5-10 rpm. Definitely not noticeable.

If you already have a 3.73 on hand then no don't waste your time/money on a 3.64. The difference will be miniscule, especially at those higher gearings.

SEND IT

[rebornE82]

Originally Posted by ZHPizza

Hell yeah brother I love those wheels!

The 235/40r18 is slightly taller than the stock 225/45r17, so would theoretically reduce rpms slightly, but we're talking on the order of like 5-10 rpm. Definitely not noticeable.

If you already have a 3.73 on hand then no don't waste your time/money on a 3.64. The difference will be miniscule, especially at those higher gearings.

SEND IT

Thank you boss!
I’ll let you know how it drives when i save up for the quaife LSD and install it!

[Euro2012]

I have a 2009 BMW 128i that I am building into a race car. Looking for a 3.91 differential that I can put a wavetrac LSD into. Am I correct in that the 3.91 from the 325xi/328xi is the same housing but will need the driveshaft joint changed? Is there one that bolts right up to our cars? Thanks!

[mkeiser]

Originally Posted by Euro2012

I have a 2009 BMW 128i that I am building into a race car. Looking for a 3.91 differential that I can put a wavetrac LSD into. Am I correct in that the 3.91 from the 325xi/328xi is the same housing but will need the driveshaft joint changed? Is there one that bolts right up to our cars? Thanks!

manual rwd = 3.23 circle flange

auto rwd = 3.73 circle flange

manual xdrive = 3.38 triangle flange

auto xdrive = 3.91 triangle flange

~~~~

when i say 'flange' I am talking about where the driveshaft bolts up to the differential. so if you have a manual rwd 128i, it has a 3.23 open diff with the circle flange. for a direct bolt on swap, look for a auto rwd 128i/328i and that will get you a 3.73 circle flange diff. swap in your lsd and you're boomin!

[Euro2012]

So there is no direct bolt in for a 3.91 ratio, correct? Do the xdrive 3.91 fit if the flange is changed? Thanks for the info, super helpful!

[Euro2012]

I’ve also read that the rear section of the drive shaft (with the guibo fitting) of an xdrive car is the same length as the manual (4 bolt) one, so it can be swapped out, making the 3.91 an easier install. That, as opposed to swapping out the input flange on the diff. True?

[mkeiser]

You for sure can swap the flange.

Unsure about the driveshaft.

[E82Adam]

Is this the same process If I bought a 3.73 from an e60?

[mkeiser]

Originally Posted by E82Adam

Is this the same process If I bought a 3.73 from an e60?

doesn't look like it. you would at the very least need to transfer over axle's from e90. looks like probably the pinion shaft flange as well (looking at where the driveshaft bolts up to the diff). the diff housing looks ever so different, so it might not even fit or bolt up one-to-one.

if you really wanted to get fancy you could probably open up both diffs and just swap the ring gear over.

IMO just get an e9x diff from junkyard/part out. see my post a couple above for info about making sure you match the pinion flange for easiest swap.