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      07-15-2017, 11:48 PM   #1
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3.23 --> 3.73 manual to automatic differential swap for 328i info, DIY, impressions

Hey guys, Joester here.

I recently learned about swapping my 6 speed 328i with the 3.73 differential from an automatic and replacing the 3.23 that came with it for more torque to the wheels at the expense of higher revs and lower gas mileage. I had to try it out!


This is for RWD only, I don't know how it would effect xdrive... I assume you'd have to do both diffs and i don't know about interchangeability.

Manual 328is have 3.23 diffs, auto 328is have 3.73 diffs, and auto 330is from 2006 had 3.64 diffs. As far as I know they are all interchangeable. 335i diffs are different so don't get those.

I got my diff out of an automatic 2009 328i with 40k miles on it. I found it through car-part.com, messaged the seller, and had it on my front porch in 2 days for $130 shipped. Not too shabby!

This weekend I installed it. I didn't take pics but the procedure is not that hard. It did take a while for me though, I don't like rushing things.

First, unbolt the heat shield near where the driveshaft meets the diff and bend it out of the way a bit. I also bent the mounting bracket that the heat shield was mounted to for more room.

Second, remove the six E12 bolts holding each axle to the diff, and the four E12 bolts holding the driveshaft to the diff. The axles can slip off the diff then and be tied out of the way at this point. I only tied my passenger side up, the drivers side I let rest on the exhaust pipe and that worked well.

Third, find the 3 bolts that hold the diff to the subframe. There are 2 in front and 1 in the back. The two in front thread into the diff itself but the one in the back is secured with a nut and bolt making it a bit trickier. I think this back one is a 22mm(?) which I didn't have, so I used a 7/8" socket and an adjustable wrench with success, but it wasn't ideal. You'll want a deepwell socket.

The left side front bolt has a nut on the back that holds an exhaust bracket on that needs to come off. Although not necessary, I also pulled the exhaust hanger off the pipe itself and set it aside to make more room. The left side is the tricky side because that's where the exhaust pipe takes up all the room.

Then, loosen and remove the front left bolt. Loosen but do not yet remove the other two bolts. Support the diff with a jack and a rag, and then remove the bolts. The diff is HEAVY AF! Get a friend to help you out operating the jack while you guide the diff out of the car. Once you have all 3 bolts loose you can get the driveshaft out of the front of the diff before you let it down.

Before putting the new diff in check to see that the bolts thread freely into the holes. My holes were a bit rusty so I ran a tap through them to clean them out. Also check the bore where the driveshaft centering "post" goes and make sure that the driveshaft wont have any trouble going in it.

Put the new diff in just like you got the old one out and you're golden!



IMPRESSIONS: I've only driven the car once with the new diff but I really like it so far. The car just feel like it wants to go places now. 1st gear is still totally usable, no traction issues because our cars are slow as hell no matter what diff you put in it. If you stab the gas in 2nd gear it actually feels like the car is going places where it was pretty numb before. 3rd gear went from feeling like being in a prius to actually having a bit of use. I dare say even 4th gear has some pull to it too!

The only downside is highway cruising; 70mph = 3000rpm so if all you do is highway cruising this may not be the mod for you. I don't mind it though.

Overall I'm very glad I did this.

And because i feel bad for not including any pics here is my clean engine bay.

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      07-15-2017, 11:59 PM   #2
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Im going to do a full SAT conversion at the end of the summer. A daunting task to say the least.
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      07-17-2017, 12:23 AM   #3
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These engines are so smooth it's hard to tell the difference between 2k and 3k rpm with your eyes shut.

Great mod!
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      07-17-2017, 12:23 PM   #4
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Love the quick DIY, thank you!!!

I wonder how this car will be with a 3IM, AA headers, 3.73 diff and a tune.
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      07-28-2017, 03:19 PM   #5
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Hey guys, quick update: I filled up my first full tank of gas since the diff swap today and I hand calculated 22.8MPG, which is right around what I normally get, maybe 1MPG lower.

So far it doesnt seem like this higher ratio diff has any large effect on efficiency. My commute is a state highway with tons of stoplights. I cruise ~50mph but all the stop lights have me starting and stopping almost the whole way.
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      07-29-2017, 11:35 AM   #6
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Quote:
Originally Posted by Joester View Post
Hey guys, quick update: I filled up my first full tank of gas since the diff swap today and I hand calculated 22.8MPG, which is right around what I normally get, maybe 1MPG lower.

So far it doesnt seem like this higher ratio diff has any large effect on efficiency. My commute is a state highway with tons of stoplights. I cruise ~50mph but all the stop lights have me starting and stopping almost the whole way.
How has getting used to the adjusted shift points been? Do you feel on day-to-day cruising you need to be in 1 gear higher than you used to be?
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      07-31-2017, 08:39 AM   #7
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Quote:
Originally Posted by desertman123 View Post
How has getting used to the adjusted shift points been? Do you feel on day-to-day cruising you need to be in 1 gear higher than you used to be?
It's different but not that different. Sometimes I catch myself cruising in 4th (like I used to with the lower gearing) where I really should be in 5th, but for the most part your mind subconsciously takes care of the shift points for you.

Some turns and things along my regular route I had to retrain myself though. I used to turn into my subdivision at high revs in 2nd gear and now I turn in at a bit lower revs in 3rd gear.

I wouldn't say you need to be 1 gear higher, the 15% change in gearing does not offset a full gear's worth of rpm change if that makes any sense. You just shift a little sooner, that's all.

Its been a few weeks now and I still like the new diff, unfortunately I've already become used to the extra bump in torque; I feel like if I were to go back to the 3.23 diff I would be very disappointed.
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      07-31-2017, 02:26 PM   #8
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Diff swap is a great mod for RWD N52's. I have the 3.64 diff in my 330i (which is also a 15% change from the stock 3.15 final drive ratio) and boy do I love it. This mod is a no-brainer because you truly feel the difference in torque no matter how you drive. Whether cruising at 1K RPM or 6K RPM, you know if you mash the gas pedal, you will feel the car pull harder than before. Congrats and enjoy!
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      08-01-2017, 11:24 PM   #9
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I have an auto and now I'm wondering if I swap into the manual diff would I sacrifice a bit of torque feel but gain fuel mpg generally speaking?
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      08-02-2017, 06:52 AM   #10
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Quote:
Originally Posted by yupetc View Post
I have an auto and now I'm wondering if I swap into the manual diff would I sacrifice a bit of torque feel but gain fuel mpg generally speaking?
Given my pretty much unchanged fuel economy, I would have to guess that you would just be sacrificing torque for nothing. But who knows, try it if you want!
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      08-02-2017, 09:38 AM   #11
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Quote:
Originally Posted by yupetc View Post
I have an auto and now I'm wondering if I swap into the manual diff would I sacrifice a bit of torque feel but gain fuel mpg generally speaking?
Fuel economy has more to do with driving habits vs. just engine speed. If you're hammering around, getting on the gas hard you'll naturally burn through more gas. Gas consumption isn't solely connected to engine RPM, so although you may see an improvement (especially if you do a lot of constant speed hwy driving), it wouldn't be worth losing the torque to the ground.
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      08-02-2017, 12:21 PM   #12
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Quote:
Originally Posted by yupetc View Post
I have an auto and now I'm wondering if I swap into the manual diff would I sacrifice a bit of torque feel but gain fuel mpg generally speaking?
Quote:
Originally Posted by Volasko View Post
Fuel economy has more to do with driving habits vs. just engine speed. If you're hammering around, getting on the gas hard you'll naturally burn through more gas. Gas consumption isn't solely connected to engine RPM, so although you may see an improvement (especially if you do a lot of constant speed hwy driving), it wouldn't be worth losing the torque to the ground.
Agreed, swapping the already short-geared auto diff for the manual diff is a tremendous waste. The auto diff has shorter gears to aid with off-the-line acceleration when the driver is likely not revving as high into each gear (unlike a manual). Torque is a big deal and hang onto every lb ft you can get like your life depends on it. Car will not drive like a proper BMW with reduced torque.
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      08-02-2017, 12:32 PM   #13
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Well explained, everyone. Just curious is all.

I tend to drive with a heavy foot anyways, not looking forward to losing that torque.

thanks, all.
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      08-02-2017, 09:52 PM   #14
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Finally have all maintenance up to date, or at least parts incoming, and started looking for a dif for my 06' 330i. :-)
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      08-03-2017, 07:37 AM   #15
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Quote:
Originally Posted by bushwa View Post
Finally have all maintenance up to date, or at least parts incoming, and started looking for a dif for my 06' 330i. :-)
Remember though, your 330i should already have a 3.64 diff in it, so installing a 3.73 would be about a 2.5% change, almost negligible.

In my opinion it's definitely not worth the effort for you.

EDIT: DISREGARD THIS POST, IT IS INCORRECT, MANUAL 330i HAS A 3.15 DIFF MAKING THE MANUAL TO AUTO DIFF SWAP WORTHWHILE.
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      08-03-2017, 10:00 AM   #16
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I think bushwa has a manual, so has a 3.15 final drive.

I have an auto pumpkin (3.64) to install in my 330i.... Someday that project will bubble to the top!

Kevin M.
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Quote:
Originally Posted by Joester View Post
Remember though, your 330i should already have a 3.64 diff in it, so installing a 3.73 would be about a 2.5% change, almost negligible.

In my opinion it's definitely not worth the effort for you.
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      08-03-2017, 11:13 AM   #17
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Correct, mine is a manual. Good point though about the difference between 3.15 to 3.73. Might be bit too much of a change. Although I don't frequent the highway too much, and I was used to my 5spd 2.5 Rabbit, which was revving over 3k at highway speeds, and that never bothered me.
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      08-03-2017, 11:14 AM   #18
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Quote:
Originally Posted by ktm137 View Post
I think bushwa has a manual, so has a 3.15 final drive.

I have an auto pumpkin (3.64) to install in my 330i.... Someday that project will bubble to the top!

Kevin M.
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Oh, damn, good call. I guess I must be tired today. You're right.
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      08-04-2017, 04:53 PM   #19
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Did you have to reprogram the computer?

What did you use to recalibrate the speedo?
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      08-04-2017, 05:29 PM   #20
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What did you use to recalibrate the speedo?
Don't need to.
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      08-05-2017, 04:57 PM   #21
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I actually just did this swap in the backyard this morning. Picked up a diff locally, took about 4 hours to do. Theres the two speedo cables and they are so in the way the whole time and they are so stiff. Exhaust had to be lowered a lil too. Lining up the diff is super easy with just you. I used my feet to pump the jack up while lining up the diff and bolts.

Was it worth it?
Yeah, I think 3.9 or 4.10 wouldve been even better. I dont go on the freeway much, if at all. It pulls much harder and lettin out the clutch you can feel it rotates quicker. If you have air tools or electric drive impacts, do it, its not hard and totally worth it. Also, get some 18mm wrenches, sockets, and e12s. Wobble extensions or wobblys are necessary.
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      08-07-2017, 07:20 AM   #22
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Quote:
Originally Posted by Peter944 View Post
I actually just did this swap in the backyard this morning. Picked up a diff locally, took about 4 hours to do. Theres the two speedo cables and they are so in the way the whole time and they are so stiff. Exhaust had to be lowered a lil too. Lining up the diff is super easy with just you. I used my feet to pump the jack up while lining up the diff and bolts.

Was it worth it?
Yeah, I think 3.9 or 4.10 wouldve been even better. I dont go on the freeway much, if at all. It pulls much harder and lettin out the clutch you can feel it rotates quicker. If you have air tools or electric drive impacts, do it, its not hard and totally worth it. Also, get some 18mm wrenches, sockets, and e12s. Wobble extensions or wobblys are necessary.
Speedo cables? I think maybe you are thinking of e-brake cables. Speedos haven't been cable driven since like what the 80s?

On my LCI car I didn't have any issues with any cables. Maybe one of them was slightly in the way but was more an annoyance than a real problem. I also didn't have to touch the exhaust. Perhaps non-LCI cars are slightly different?
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