Differential internals: 135i DCT vs 120d

[NonoxX]

Hello,
I have a 2010 135i DCT which I really enjoy, and consider fitting a LSD into it. My choice would be a Quaife one, as I think it is a good compromise.

The problem with the 135i DCT's differential, is that it has a welded crown. This means you can't simply DIY. You have to take it to a professional that will machine it to be able to assemble the crown to the differential body itself.

Also, I have a spare 120d differential, which has a bolted crown. The output flanges seem similar, and the ratio is the same (2:56).

I think you can start to guess my question... Would it be possible to use the crown of the 120d diff in the 135i diff?

This would mean that all dimensions are equal, especially the module of the gear and axial plays...

[NonoxX]

Hello,
I have contacted Quaife, they forwarded my message to someone from Birds Auto which could provide me with an answer.

Even if I was physically possible, crownwheel and pinions get manufactured together and are paired. You're not supposed to break the pair for any reason.

Google said you would most likely run into reliability issues.

Let's maybe take the problem from another perspective: let's say I fit this diff with a Quaife, as it is. All I would need is a driveshaft with proper flanges on both length, the correct length and that can handle the 135i's power...

[JCip91]

Have you taken a look at Mfactory's limited slip options? They are pretty reasonable.

[juld0zer]

Doesn't the 120d have a different pinion input design?
You would have to make a frankenshaft out of your existing front half and a 120d rear half, get it balanced etc assuming the splines are the same.
Are both pumpkins the same size?

Id speak to a diff shop and ask their opinion about using the pinion and crown from two used diffs together. If they were brand new parts then it would just be an adjustment but if both have been used, wear patterns will be different and you cannot rule out the possibility of noise.

[NonoxX]

Originally Posted by JCip91

Have you taken a look at Mfactory's limited slip options? They are pretty reasonable.

Seems so yes. But still, the issue is that disassembling the crown requires machining.

BTW, MFactory says:

Originally Posted by MFactory

E8X DCT - 215MM DIFF HAS RING GEAR WELDED TO IT. If you buy our 2.56 Final Drive (MF-TRS-02E25) you can then install the LSD easily.

They at least propose that option: still expensive, but DIY friendly and makes rolling back to stock configuration possible.

Originally Posted by juld0zer

Doesn't the 120d have a different pinion input design?

Yes they do, that's why I thought of taking the crown only.

Originally Posted by juld0zer

You would have to make a frankenshaft out of your existing front half and a 120d rear half, get it balanced etc assuming the splines are the same.

Yep, maybe not the best idea ever.

Originally Posted by juld0zer

Are both pumpkins the same size?

I did not measure, but they seem so. At least, DCT pumpkin is not a very small one, like on MT.

Originally Posted by juld0zer

Id speak to a diff shop and ask their opinion about using the pinion and crown from two used diffs together. If they were brand new parts then it would just be an adjustment but if both have been used, wear patterns will be different and you cannot rule out the possibility of noise.

That's what my research converges to: it could be noisy and unreliable.

[Luftmeister]

I'm in the same boat. MFactory looks like the way I want to go.

I have a street car that I take to the track. Not the other way around. Mostly Autox and a couple of HPDE per season. Not ever winter driven.

Is there a good "plug and play" option where you can simply swap the pumpkin? It would be something I'd take to a shop because I wouldn't feel comfortable doing it without proper equipment .

Thoughts?

Thanks and regards,

[Luftmeister]

This looks like the one?

https://www.ecstuning.com/b-team-mfa...e92-215-256kt/

Quoted $500 core charge.

-J

[NonoxX]

Originally Posted by Luftmeister

This looks like the one?

https://www.ecstuning.com/b-team-mfa...e92-215-256kt/

Quoted $500 core charge.

-J

Yes it does. In doubt, just drop them a message on their chat, or an e-mail just to double check that the input and output flanges are the ones you need. But I'm quite sure it's the one.