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      09-02-2017, 07:38 PM   #1
desertman123
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3.23 to 3.73 differential swap DIY AKA poor man's supercharger

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 E9x 328i from a junkyard. You can find them for $100-$300, depending on age and condition.

We need to remove (2) 18mm, (1) 21mm and (16) E12 bolts, so get the right size bits. It's a bit cumbersome so I had to use short and medium length extensions, a 3/8" and 1/2" breaker bar, 18/21mm box wrenches and various socket adapters.



I sourced mine from eBay from an '06 325i. Mine didn't have the axle flanges for whatever reason, but those swap right over from the old diff. Just grab a crowbar (a giant flathead won't work sadly), get some leverage and pop them out. They're held in with a C clip (p/n below and you should replace them). They go back in even easier, just oil it up well, line the splines up and either hit it with a rubber mallet or step on it, it'll pop into place.

Since the diff is old, it leaked, so I had to drop the diff a second time. Don't be me lol.



Grab some RTV, new seals and threadlocker (if you wish for the driveshaft). These seals have to be taken out carefully so you don't score the sealing surface, and pushed in at a certain depth. Too far deep and you wasted that seal, since you cant really remove it without ruining it. I just brought my diff to a local BMW tuner and had them deal with it.

Driveshaft seal 31507609535
Output flange seals 33107505604
Lock ring for the output flange 33131214961

I chose to apply new RTV to the cover as well. There's a million guides on how to do this, I torqued my cover to 30ft/lbs. edit: service manual says 40lbs, but I've had no issues



Looked good in here! No chewed gears or bearings. Let's install it.

First, jack up the car, get the rear as high as you're comfortable with and take off the rear wheels. Pull the e-brake and put it in gear.


The slow 3.23 we need to remove.



Remove the 10mm bolt and bend the heatshield away from the driveshaft. Remove the E12 bolts. Grab a friend to sit in the car operating the e-brake so you can break a bolt loose, turn the shaft, then break another bolt loose.



Use the E12 and remove the axle bolts. One axle will sit on the exhaust but the other will hang, so I just held it up with zipties. Same deal as the driveshaft, but there's a lot more bolts lol. The E12 bit gets squished right next to the CV boot, so this part is a pain in the ass.




There are 3 bolts holding the diff into the rear end. 2 in front (18mm) and 1 in back (21mm). Also circled the heatshield bolt.



Doesn't really matter which bolt you remove first but, once you take the first one out, break the others loose and hold the diff up with the jack, then remove them. Definitely get a friend to operate the jack as you guide the diff out. It's heavy but balances well if you put the jack in the right spot. This part is mostly finesse. 2 of the bolts have nuts so you're gonna need both a box and socket wrench.



It's out!

And now the new diff goes in just like the old one came out. Jack it up, thread in a bolt, adjust the jack and thread in the others. Just take your time. Edit: for the torque specs I just made them all "gutentite" but if you can fit a torque wrench up there the specs are:




https://www.ecstuning.com/b-genuine-...s/33207572716/

https://www.ecstuning.com/b-genuine-...h/26117571956/

The diff uses 75w-90 API-GL5 oil, just over 1qt of it. Easiest to fill 1 full qt while it's out, then top it off when you install it. The drain plug is a 14mm hex.

Now go rip it! This swap is a lot of fun for not a ton of cash. It really wakes up 3rd gear.

Added a PDF so when the pictures inevitably get deleted or imgur explodes, this guide is still available.
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File Type: pdf 128i 323 to 373 diff swap DIY guide.pdf (1.80 MB, 695 views)
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Last edited by desertman123; 08-06-2020 at 12:28 PM..
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      09-02-2017, 08:46 PM   #2
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Even better is to do an LSD at the same time, but I realize that this is a lot easier and cheaper. When I did the same ratio swap, a lot of people thought I was crazy for going "too aggressive," but I really think it's a great upgrade. Not too extreme at all, IMO. Like you say, really just wakes up the car, just makes it feel lighter and more responsive. Enjoy!
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      09-03-2017, 03:19 PM   #3
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Been thinking of exactly this. Would really like to improve initial acceleration and this seems just the ticket at an affordable price. Just wondering how this affects your highway cruising? At 120 km./hr (about 75 mph) I'm currently about 2700 rpm. What are you experiencing with the 3.73 gear at this speed?
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      09-03-2017, 06:12 PM   #4
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Quote:
Originally Posted by Olgeezer1 View Post
Been thinking of exactly this. Would really like to improve initial acceleration and this seems just the ticket at an affordable price. Just wondering how this affects your highway cruising? At 120 km./hr (about 75 mph) I'm currently about 2700 rpm. What are you experiencing with the 3.73 gear at this speed?
The difference between 3.23 and 3.73 is ~15.5%. Just multiply the RPM at a given speed by 1.155 to get the new RPM. So you'll now pull ~3120rpm at 75mph.

It really isn't bad, I kinda like how I can't drive too fast without murdering my mileage. At 80mph I think I'm at about 3400rpm, which is just before it starts engine braking if you let off. So I have that my artificial speed limit. And before you ask, FL doesn't have 80mph speed limits, but parts of I-4 and I-95 will literally travel at 85+ mph.

Also because of how gear ratios work, you'll get a 15% increase in RPM, but also 15% increase in torque applied

Quote:
Originally Posted by Bimmer-Bob View Post
Even better is to do an LSD at the same time, but I realize that this is a lot easier and cheaper. When I did the same ratio swap, a lot of people thought I was crazy for going "too aggressive," but I really think it's a great upgrade. Not too extreme at all, IMO. Like you say, really just wakes up the car, just makes it feel lighter and more responsive. Enjoy!
You know I would love an LSD, but college kid budget says no . I really do like how LSDs feel around a corner, just consistent power delivery.

And I agree! 3.73 is on the upper end for the ratios I'd live with, but it is still 100% daily-able. 3rd gear went from a snooze-fest to actually exciting. My friend who helped out and has ridden passenger in the car a bunch said the difference in torque was bigger than he thought - that he actually got tossed back in the seat a bit.
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Last edited by desertman123; 09-03-2017 at 07:01 PM..
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      09-03-2017, 06:43 PM   #5
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Quote:
Originally Posted by Olgeezer1 View Post
Been thinking of exactly this. Would really like to improve initial acceleration and this seems just the ticket at an affordable price. Just wondering how this affects your highway cruising? At 120 km./hr (about 75 mph) I'm currently about 2700 rpm. What are you experiencing with the 3.73 gear at this speed?
If you want to know what the difference is like on the highway, next time you're cruising in 6th gear, just drop down to 5th, and that's about where you'd be. The stock gear ratios are about 15% between 5th and 6th, almost exactly the same as going from 3.23 to 3.73.

For me, I hardly ever used 6th gear before. I don't mind keeping the RPMs a little higher most of the time, because I like having the extra torque on tap and I like the extra little bit of engine braking on lift-off. Driving style makes a big difference in how much of a gearing change you'll tolerate, and it's subjective enough that one person's sweet spot could be completely unacceptable to somebody else.

But I will say there were a lot of nay-sayers when I was first considering the swap to 3.73. A lot of people with absolutely no personal experience with gearing swaps let me know that it would make the car "unusable on the highway" and "you'll lose top end speed." My feeling is that it's just one of those things that gets repeated and repeated on car forums and people think it makes them sound smart to say it. I mean, to some extent it's true - you can go too far, for sure - but I don't think this is too far at all. I was almost underwhelmed by the change, just because I had heard so much doom and gloom.

Quote:
Originally Posted by desertman123 View Post
You know I would love an LSD, but college kid budget says no :laughs:. I really do like how LSDs feel around a corner, just consistent power delivery.

And I agree! 3.73 is on the upper end for the ratios I'd live with, but it is still 100% daily-able. 3rd gear went from a snooze-fest to actually exciting. My friend who helped out and has ridden passenger in the car a bunch said the difference in torque was bigger than he thought - that he actually got tossed back in the seat a bit.
Yeah, I'm sure it's still a pretty great mod even if keeping the open diff. And LSDs are kind of stupid expensive.
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      09-05-2017, 12:48 PM   #6
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been waiting to do this but i want to do it all in shot. i have a 3.73 sitting in my basement but the funds are being sunk into the other project at this time.

For me I am looking at doing because of certain corners at the track. I would love to be in the power but with a bit more room to accelerate before the braking zone. Stock gearing sucks at Grattan because I tap the limiter going into turn 2 and 3. I have that 1/2 second i would love to be on the power.

With the gearing and LSD it would be great, just got to wait to pull the trigger. My issue will be if can justify the cost given that my project will be more trackable than my 128. Tough to say that because the 128i is a blast to drive and chase down those vettes and stangs that lack grip with a bunch of lbs to shed.

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      09-09-2017, 12:00 AM   #7
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desertman123 and Bimmer-Bob, Thanks for the input, very helpful.
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      09-09-2017, 06:18 AM   #8
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I did this to my Mustang and it was a very worthwhile upgrade. I would drive lightly/like a normal person the first few hundred miles and then replace the fluid to get any metal shavings out from break in. Then let it rip.
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      09-09-2017, 12:00 PM   #9
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I want to do this so bad.......
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      09-09-2017, 03:54 PM   #10
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Quote:
Originally Posted by MGM135is View Post
I did this to my Mustang and it was a very worthwhile upgrade. I would drive lightly/like a normal person the first few hundred miles and then replace the fluid to get any metal shavings out from break in. Then let it rip.
That's exactly what I did. Since I had to drop the diff because it leaked, I had to drain my fresh Redline 75w90 () and fill it up again. After driving probably about 100 miles on brand new fluid, it went from gold colored to black. I've done it again, but the fluid is still goldish. Have put about 500 miles on this swap so far and no issues! I only put cheap Valvoline 75w90 in there for now, gonna swap in some Redline juice whenever its convenient then I'll officially consider the task done.
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      09-09-2017, 11:29 PM   #11
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1. The car needs more power. Gears really won't help you that much.

2. If short on $ don't get a LSD weld the diff. That's what I did and it's awesome.

That is all.
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      09-10-2017, 12:20 AM   #12
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Quote:
Originally Posted by The Wind Breezes View Post
1. The car needs more power. Gears really won't help you that much.

2. If short on $ don't get a LSD weld the diff. That's what I did and it's awesome.

That is all.

^
This guy, lol.
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      09-11-2017, 11:45 AM   #13
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Quote:
Originally Posted by The Wind Breezes View Post
1. The car needs more power. Gears really won't help you that much.

2. If short on $ don't get a LSD weld the diff. That's what I did and it's awesome.

That is all.
Why exactly do you come into the N52 threads? All you do is talk down to 128i owners and complain about the lack of power these cars have.

Just stick to the N54 forum since you never have anything constructive to contribute.
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      09-11-2017, 05:00 PM   #14
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Quote:
Originally Posted by Bimmer-Bob View Post
If you want to know what the difference is like on the highway, next time you're cruising in 6th gear, just drop down to 5th, and that's about where you'd be. The stock gear ratios are about 15% between 5th and 6th, almost exactly the same as going from 3.23 to 3.73.

For me, I hardly ever used 6th gear before. I don't mind keeping the RPMs a little higher most of the time, because I like having the extra torque on tap and I like the extra little bit of engine braking on lift-off. Driving style makes a big difference in how much of a gearing change you'll tolerate, and it's subjective enough that one person's sweet spot could be completely unacceptable to somebody else.

But I will say there were a lot of nay-sayers when I was first considering the swap to 3.73. A lot of people with absolutely no personal experience with gearing swaps let me know that it would make the car "unusable on the highway" and "you'll lose top end speed." My feeling is that it's just one of those things that gets repeated and repeated on car forums and people think it makes them sound smart to say it. I mean, to some extent it's true - you can go too far, for sure - but I don't think this is too far at all. I was almost underwhelmed by the change, just because I had heard so much doom and gloom.



Yeah, I'm sure it's still a pretty great mod even if keeping the open diff. And LSDs are kind of stupid expensive.
The closest BMW comparison was people putting in 3.91s into e36s which made first gear useless and rev high at freeway speeds. With a 6th gear and not quite as aggressive of a ratio should make a 3.73 perfect all-rounder in our chassis.

I might go with an auto open diff for the time being since is stupid cheap and quick to swap in.
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      09-11-2017, 05:27 PM   #15
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The diff, minus the cover, looks the exact same as an E46 diff. Though, the axle bearings inside the case look beefier. I wonder if they're interchangeable? Nice DIY, always tempting to pick up a 128i for drift work. Guessing a welded 3.73 would be a lot of fun.
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      09-11-2017, 11:02 PM   #16
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Quote:
Originally Posted by desertman123 View Post
That's exactly what I did. Since I had to drop the diff because it leaked, I had to drain my fresh Redline 75w90 () and fill it up again. After driving probably about 100 miles on brand new fluid, it went from gold colored to black. I've done it again, but the fluid is still goldish. Have put about 500 miles on this swap so far and no issues! I only put cheap Valvoline 75w90 in there for now, gonna swap in some Redline juice whenever its convenient then I'll officially consider the task done.
if I have a 3.39 final drive ratio in my car equipped with an 8 speed auto, what sort of ratio would I want if I add another 60hp? 100hp? (300, or 340HP)

3.10, 3.15? something in that ballpark? is there some sort of calculation you can do to determine what would be optimal? thank you
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      09-12-2017, 12:32 PM   #17
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Quote:
Originally Posted by German Straight-6 View Post
if I have a 3.39 final drive ratio in my car equipped with an 8 speed auto, what sort of ratio would I want if I add another 60hp? 100hp? (300, or 340HP)

3.10, 3.15? something in that ballpark? is there some sort of calculation you can do to determine what would be optimal? thank you
why would you change if your adding that much HP? I would love to see 340 NA n52........

Despite not wanting to do the swap till I fork over the cash for an LSD, I'm putting my 3.73 in with the finned cover before my trackday on 9/30. I hope that the shift points work well. I just don't want land in that proverbial hole between gears at corner exit. I know that nothing is perfect and that my car is not a race car so there will be trade offs. The car will just have to rev more on the highway and shouldn't be that obnoxious at 3200rpm for 80.

Last edited by gixracer; 09-12-2017 at 12:38 PM..
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      09-12-2017, 02:19 PM   #18
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Quote:
Originally Posted by gixracer View Post
why would you change if your adding that much HP? I would love to see 340 NA n52........

Despite not wanting to do the swap till I fork over the cash for an LSD, I'm putting my 3.73 in with the finned cover before my trackday on 9/30. I hope that the shift points work well. I just don't want land in that proverbial hole between gears at corner exit. I know that nothing is perfect and that my car is not a race car so there will be trade offs. The car will just have to rev more on the highway and shouldn't be that obnoxious at 3200rpm for 80.
I was thinking of going the same route and adding the e60 (iirc?) finned cover to an open 3.37 now. I was planning on doing it when I get an LSD but it will easily swap over when I finally throw down a few grand for it. Which one are you going to use?
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      09-12-2017, 02:22 PM   #19
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Quote:
Originally Posted by gixracer View Post
why would you change if your adding that much HP? I would love to see 340 NA n52........

Despite not wanting to do the swap till I fork over the cash for an LSD, I'm putting my 3.73 in with the finned cover before my trackday on 9/30. I hope that the shift points work well. I just don't want land in that proverbial hole between gears at corner exit. I know that nothing is perfect and that my car is not a race car so there will be trade offs. The car will just have to rev more on the highway and shouldn't be that obnoxious at 3200rpm for 80.
well, it wouldn't be NA. I would supercharge it.

if not,then I will get only catless headers.

what would be a good final drive ratio for 285hp? 340hp?
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      09-12-2017, 03:54 PM   #20
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Quote:
Originally Posted by ubersteuer View Post
I was thinking of going the same route and adding the e60 (iirc?) finned cover to an open 3.37 now. I was planning on doing it when I get an LSD but it will easily swap over when I finally throw down a few grand for it. Which one are you going to use?
when I have the cash, diffsonline.

Straight -6 what 8 speed trans and what motor in which car ?
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      09-12-2017, 05:37 PM   #21
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Quote:
Originally Posted by gixracer View Post
when I have the cash, diffsonline.

Straight -6 what 8 speed trans and what motor in which car ?
Should have clarified, I meant the cover.



..and 8 speed never came on e9x/e8x, you might be in the wrong forum. Sounds like f30 to me.
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      09-12-2017, 09:04 PM   #22
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Quote:
Originally Posted by ubersteuer View Post
I was thinking of going the same route and adding the e60 (iirc?) finned cover to an open 3.37 now. I was planning on doing it when I get an LSD but it will easily swap over when I finally throw down a few grand for it. Which one are you going to use?
Just FYI there is minor modification required to make the E60 cover fit our cars - like a spacer and a longer bolt. I don't know exactly because I bought the diff already built by Jim Blanton.
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