[b.lam]

I am interested in doing a diff swap into my auto 328i. I know mine has the 3.73. Is it possible to swap the 3.91 from an auto 328xi into my car? I found one at an autowrecker for a good price. Will it bolt right on? Will everything work? What do I need to look out for?

Thanks,

Bryan

[Digitalinkartist]

It should work. On e46's, you usually swap your existing "yolks" or output flanges to the new diff. You may need to do the same on e9x's.

Unfortunately you will only see a nominal performance increase as you will be multiplying your torque output by a very small %. I don't believe the juice is worth the squeeze.

[GTKrockeTT]

Check the input flange as well, that's more of a pain to swap out that the output flange. Most pumpkins have a four prong flange, while others have three.

[b.lam]

Originally Posted by GTKrockeTT

Check the input flange as well, that's more of a pain to swap out that the output flange. Most pumpkins have a four prong flange, while others have three.

Hmm...the 3.91 I have seen has a 3 bolt input flange. Is it possible to convert it to 4 bolt?

[mbanks21]

That's only about a 5% difference in gearing. Not sure if it's worth the hassle. I went 3.15 to 3.64 which is ~14%

[GTKrockeTT]

Originally Posted by b.lam

Hmm...the 3.91 I have seen has a 3 bolt input flange. Is it possible to convert it to 4 bolt?

That's exactly what I had to do. Sourced a 3.38 from an XI manual and replaced my 3.23. Perhaps the XIs have 3 bolt flanges?

[bimmerquick43321]

Not sure this is a good idea in an automatic, yes acceleration will be better in each gear, but you're transmission will need to shift more often which I'm not sure you want considering how "fast " our transmissions shift

[andrey_gta]

Originally Posted by GTKrockeTT

That's exactly what I had to do. Sourced a 3.38 from an XI manual and replaced my 3.23. Perhaps the XIs have 3 bolt flanges?

so this is RWD 328i with 3.23 and you went to a 3.38?

I thought this was not possible because XI have different half shaft mounts into a larger bearings and larger knuckle with the large hand brake shoes.

While the 328i has small hand brake and smaller bearing and as such the hald shafts wont fit the XI diff halfs.

Did you swap half shaft flanges?

[GTKrockeTT]

Originally Posted by andrey_gta

so this is RWD 328i with 3.23 and you went to a 3.38?

I thought this was not possible because XI have different half shaft mounts into a larger bearings and larger knuckle with the large hand brake shoes.

While the 328i has small hand brake and smaller bearing and as such the hald shafts wont fit the XI diff halfs.

Did you swap half shaft flanges?

Correct, I have a manual RWD and went with a manual XI. I did not have to do anything short of having the input flange swapped out.

[pavproch]

Few thoughts, if I may...

3.91/3.73 = 1.048, so the gain would be mere 5%, something you will (probably) not notice at all, where you used to rev 3000, you will be reving 3150

I just did a swap 4.28 -> 5.00 on my other car (Z4 e89 3.0i) ie something like 17% gain (instead of 3k revs I am at 3.5k now) and I am thrilled - whatever gear I shift in, it runs like hell, even against a steep hill climb... to be honest I gotta say the mileage dropped by approx same ratio (with the "summer driver" I don´t care a bit about)

Probably I will go the same way even with my daily (e91 330ix MY2011 EU specs, both diffs to be cut by exactly same ratio of 15%) and will have to live with the decreased mileage cos the fun factor gain is just (in my eyes ) worth it.

[andrey_gta]

Originally Posted by GTKrockeTT

Correct, I have a manual RWD and went with a manual XI. I did not have to do anything short of having the input flange swapped out.

So the drive shaft flange must be swapped from your take of diff pumpkin. Is this hard to do? What tools are used in somethign like this? DIY?

I am asking because i dont want to go to 3.23 to 3.73 and something like 3.46 or 3.38 would be ideal. The problem is that i think someone like BudVlad had to have a drive shaft cut and modified to use the 3.38 diffeential's flange

[GTKrockeTT]

Originally Posted by andrey_gta

So the drive shaft flange must be swapped from your take of diff pumpkin. Is this hard to do? What tools are used in somethign like this? DIY?

I am asking because i dont want to go to 3.23 to 3.73 and something like 3.46 or 3.38 would be ideal. The problem is that i think someone like BudVlad had to have a drive shaft cut and modified to use the 3.38 diffeential's flange

MFactory's partner in San Dimas, Syncrotech Transmissions handled the LSD install for me. They also swapped out the input flanges between the 3.38 pumpkin and my stock 3.23 one. Can't tell you the exact process, but they did this for free for me after I found out the flanges didn't match up.

I didn't want to go to 3.73 either, and found this low mileage 3.38 for cheap. Was doing the LSD also so was a good time.

[andrey_gta]

Originally Posted by GTKrockeTT

MFactory's partner in San Dimas, Syncrotech Transmissions handled the LSD install for me. They also swapped out the input flanges between the 3.38 pumpkin and my stock 3.23 one. Can't tell you the exact process, but they did this for free for me after I found out the flanges didn't match up.

I didn't want to go to 3.73 either, and found this low mileage 3.38 for cheap. Was doing the LSD also so was a good time.

Thanks

Also for the above 8% gain up to 12% is what is typically recommended for differential swaps based on some article of a z4 supercharged 3.0 that I read. This is why I wish to avoid a 15% gain if possible.

FYI
I think e60 pre lci prior to 2005 with the m54 engines had the same pumpkins, they may need also need shaft flange swap, but they do offer good ratios.