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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Suspension | Brakes | Chassis > Swapping Xi front subframe for RWD model



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      05-28-2019, 02:43 PM   #1
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Swapping Xi front subframe for RWD model

Got a 2006 e91 that is currently AWD that I will be doing an LS motor swap to shortly just gathering parts in the mean time. Will obviously be converted to RWD but I am trying to figure out the pro's and con's of keeping the Xi front subframe or swapping in a RWD model.

The Xi has a front diff support along the back side that possibly could be left alone and not interfere with the oil pan. If it is cut away I imagine I will lose most of the rigity I need. Is there an advantage to the suspension design on the RWD subframe how the control arms are mounted? This will be a daily driver with occasional track and drag use.

Main thing I am concerned about is strength differences, better suspension design and neccesity of the swap.

And no I will not be selling the car for a RWD model. I got this wagon for very cheap in great condition and do not mind the added work.
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      05-30-2019, 02:30 PM   #2
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I could be wrong on this, but I think the RWD subframe is aluminum, so it should be lighter than the xDrive subframe that I'm 99% sure is steel. Also, the RWD subframe gives you the ability to run M3 control arms, as well as a faster steering rack, the stock xDrive rack is super slow. The stock RWD is a little quicker, and the M3 rack is like 12:1 or something super quick.

The other thing to mention is that the trans tunnel is different on the xDrive cars, and so is the transmission/transfer case mounting. I doubt you'll be using the stock trans with an LS swap though, so you'll have to fab your own trans mount anyways. Just don't try to measure a RWD car and use that on yours, it won't fit.
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      06-03-2019, 08:40 AM   #3
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Quote:
Originally Posted by rothwem View Post
I could be wrong on this, but I think the RWD subframe is aluminum, so it should be lighter than the xDrive subframe that I'm 99% sure is steel. Also, the RWD subframe gives you the ability to run M3 control arms, as well as a faster steering rack, the stock xDrive rack is super slow. The stock RWD is a little quicker, and the M3 rack is like 12:1 or something super quick.

The other thing to mention is that the trans tunnel is different on the xDrive cars, and so is the transmission/transfer case mounting. I doubt you'll be using the stock trans with an LS swap though, so you'll have to fab your own trans mount anyways. Just don't try to measure a RWD car and use that on yours, it won't fit.
Good to know about the steering rack. They look identical but the PN are different. I'm looking for a parts car so I can get the whole front end with suspension as well since most that stuff isn't compatible.

I imagine the trans tunnel just has a bump out for the transfer case? I am between a t56 magnum or the 6l90 out of a CTS-V. 6L90 will need some tunnel massaging but it will fit though.
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      06-05-2019, 09:10 AM   #4
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agree on the slow xDrive steering rack (I think 14:1 or even higher ratio).
on the point of steel vs. alum, you can see some differences of xDrive vs. RWD front subframe and corresponding suspension components here:
http://dreamingin302ci.blogspot.co.a...ve-handle.html

BUT....why not build an AWD LS E91?
LSx AWD stuff is available as far as the FAD and the TC goes.
Are you planning to retain E9x DSC?
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      06-05-2019, 02:05 PM   #5
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RWD is aluminum, xDrive is steel.

Being able to run M3 FCA is huge imo.

xDrive steering is always 16:1, ZSP RWD is 14:1, M3 is 12:1
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      06-05-2019, 03:41 PM   #6
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Quote:
Originally Posted by Antetokounmpo View Post
RWD is aluminum, xDrive is steel.

Being able to run M3 FCA is huge imo.

xDrive steering is always 16:1, ZSP RWD is 14:1, M3 is 12:1
Awesome, thanks for those numbers. I've found some cheap racks on eBay for a M3 as well as the FCAs so will attempt to go that route if possible.

Quote:
Originally Posted by renizer View Post
agree on the slow xDrive steering rack (I think 14:1 or even higher ratio).
on the point of steel vs. alum, you can see some differences of xDrive vs. RWD front subframe and corresponding suspension components here:
http://dreamingin302ci.blogspot.co.a...ve-handle.html

BUT....why not build an AWD LS E91?
LSx AWD stuff is available as far as the FAD and the TC goes.
Are you planning to retain E9x DSC?
An AWD wagon with an LS would be absolute bonkers and I'd love it but the amount of work and money it would require doesn't seem worth it to me.

I'm watching a 330i that T-boned a pole that should be going up for auction in the next couple weeks. I'll be able to salvage the whole front end suspension, spindles and cradle instead of sourcing individual parts. Also the saddle brown sport seats and steering wheel to replace my regular seats. Part out the rest and maybe make a couple bucks.

Just given the M3 rack quicker steering is enough reason for me to swap front ends.
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      06-07-2019, 01:22 AM   #7
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How do you plan on mounting the trans?

An auto trans tunnel is very different from manual:
https://www.e90post.com/forums/showthread.php?t=1603231
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      06-07-2019, 08:34 AM   #8
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Quote:
Originally Posted by squaredphoto View Post
How do you plan on mounting the trans?

An auto trans tunnel is very different from manual:
https://www.e90post.com/forums/showthread.php?t=1603231
Regardless it is going to be a custom bracket but I can either use the existing mounting points depending on how far it is from the trans. If the distance is too great to give the support needed then plate steel with a flange nut welded onto the back side, notch the rails to recess the nuts and stitch weld the plates to the rails. Then bolt a bracket to the new spots.
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      06-07-2019, 08:46 AM   #9
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Quote:
Originally Posted by squaredphoto View Post
How do you plan on mounting the trans?

An auto trans tunnel is very different from manual:
https://www.e90post.com/forums/showthread.php?t=1603231
Auto is the same as manual. The difference is xDrive vs RWD.
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      06-12-2019, 12:37 PM   #10
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Nearly had this 08 M3 bought at auction but ended up having issues bidding and lost it by only a few hundred dollars.. Would have had 95% of the parts I needed to swap over onto mine and still make money parting the rest of the car out.

Going to keep looking for one that can check off all my needs, then it will be looking for a CTS-V for the LSA motor and possibly the 6L90 trans.
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      06-27-2019, 04:08 AM   #11
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Sounds cool, LSA E91 <3 keep us posted
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      10-24-2019, 12:18 PM   #12
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2011 M3 DCT has been delivered. Unfortunately has a seized motor but in immaculate condition otherwise and that allowed me to pick it up for cheap from an auction.

Will be cutting it apart and swapping over parts this winter. LS swap will come when I hit 200k miles on the E91, currently at 193k. I will make a new post documenting the build soon.
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      10-24-2019, 04:15 PM   #13
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Nice! Wish I were close enough to harvest all the rest of the stuff from the M3 - the synergies would be great.

ah welll, keeping tabs on this build. Sounds great. This heavy chassis could use a stump-pulling motor!
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      11-11-2020, 01:41 PM   #14
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Any progress?
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      11-13-2020, 10:45 AM   #15
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Quote:
Originally Posted by Zigarock View Post
Any progress?
Yes, a fair bit. Then I started on body work and that ruined my motivation but I've been slowly cracking away.

Ending up actually buying a 2012 E91 MSport mainly for its updated electronics and newer chassis. Front end is all complete and mounted, every fender bracket had to be cut off the M3 and welded onto the E91, not a single bracket was the same. Rear quarters have been sectioned in on the rear just need body work to complete. I am currently in the middle of splicing the two bumpers together.

I changed transmission plans, will be using a DCT out of a 2016 M4. The M3 DCT that came in my parts car only has a 1:1 7th speed while the M4 is 0.65:1. A Swedish company, Domiworks makes a bellhousing adapter and input shaft adapter for the DCT trans and HTG makes the standalone TCU that I have.

Changed rear end plans from the M3 to a ford 8.8 using seemslegit brackets and half shafts.
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      12-20-2020, 06:42 PM   #16
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Just came across this post, were you able to successfully swap the subframes? I am in the same situation and I have located rwd front and rear subframes.
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      01-11-2021, 07:21 PM   #17
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Quote:
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Just came across this post, were you able to successfully swap the subframes? I am in the same situation and I have located rwd front and rear subframes.
You wont need the rear subframe unless yours is rusted out, awd and rwd have the same rear end

I am curious on this build though. Updates?
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