Haven't logged on in a while and had a few PMs about going to RWD on an XI. I answered them but will post here for everyone's info.
Basically I had fun with it for a bit but got annoyed with how easy it was to spin the tires and have the DSC come on. It behaved just like a RWD 335i and I could burn tires as long as I wanted without any warning lights or throttle cuts; on a 2011 msport 335xi with manual. A mechanical LSD would fix the issue but I haven't really found a clear answer how to do this. I'm a big fan of bolting BMW OEM parts on rather than aftermarket (i.e. if there was a simple way to put an OEM m-diff in, I would do that). DTC and DSC worked exactly the same.
The one thing I notice with the Dynamic Performance Control (FDB) turned off is that the car handles a bit differently in the snow, and not in a good way. Even with the E-diff left coded on and driveshaft installed, its very apparent that the car has open diffs- compared to when the Dynamic Performance Control (FDB) is on. I suspect the reason for this is that the Dynamic Performance Control (FDB) and E-diff work together.
I have zero evidence to back it up but this is what I assume is happening.
Lets say you are driving in the snow with the DSC off and gas it coming out of a corner. With the Dynamic Performance Control (FDB) active, it is very easy and predictable to control the slide. With the Dynamic Performance Control (FDB) off, you can feel the inside tires start to spin and then the rear will step out (awkward delayed feeling). When wheelspin is detected I believe the Dynamic Performance Control (FDB) module cuts power and controls forward/aft torque distribution. The e-diff individually pulses the brakes to simulate a LSD. I believe that the brakes are fairly ineffective at stopping the spinning wheel under power WITHOUT cutting the throttle via Dynamic Performance Control (FDB).
On dry pavement with the DSC off (10 second push) it is very hard to do donuts on asphalt. The inner front and rear tires spin. E-diff is coded ON.
Anyways here is how to do it. I believe step 2 is what causes the 4x4 warning light and abrupt throttle cut during wheelspin with driveshaft removed. Unfortunately I think 2 impacts how well the e-diff works.
1) Pull front Driveshaft off
2) Dynamic Performance Control (FDB) TURN OFF
from
http://www.onelapx1.com/blog
Dynamic Performance Control (FDB)
N55 X1 Parameter: e84_FDB
N54 335i xDrive Parameter: FDB
To disable, set to "nicht_aktiv".
This feature encompasses two things - corner braking designed to "torque vector" and redirecting the power through the xDrive system for a 20/80 FWD/RWD torque split. I am conflicted on this option, and need to do more testing on the implications. Without a doubt, this feature accelerates brake pad wear - if you are driving with a decent amount of slip angle, it will be almost constantly corner braking. Traditional logic holds that corner braking is a worse way to torque vector than mechanical LSDs are and that's probably still true, but recently supercars such as the McLaren 650S started coming with corner braking torque vectoring. Granted, the software in a McLaren is hopefully more advanced than that in an entry level sedan (BMW 335i) but the point holds - there must be something to it if supercar manufacturers are going in that direction. What is frustrating about the X1/335i is that you can't separate the 80% RWD bias (an unquestionably good thing) from the brake-based torque vectoring (possibly a bad thing)? So, what's the upshot? I think it probably goes something like this:
1. Base car without this option - code it on for a nice performance boost!
2. M-sport pack that comes with it enabled, but no mechanical LSD - leave it on
3. Car with an upgraded mechanical rear LSD - ?????
3) OPTIONAL: If you have a mechanical LSD, turn off E-Diff
Electronic Differential (AX_Ref_Diff_Lock)
N55 X1 Parameter: e84_AX_Ref_Diff_Lock
N54 335i xDrive Parameter: AX_Ref
To disable, set to "nicht_aktiv".