Changing diff ratio on E89 35i DCT

[Darke Peak]

Seeking advice. I’m in the process of swapping the stock 2.56 ratio diff on my Z4 35i DCT with the 3.08 ratio diff from a Z4 35i Manual (6MT). I plan to use xHP to change the final drive ratio in the TCU of the DCT, but does anything else need to change in the DME tune?

XHP has a menu option to change the final drive ratio, I believe it is to ensure DCT has smooth shifting. That takes care of the DCT trans control unit.

But will the DME have a problem with the rear wheel speed being different? Will it trigger traction control? Prevent cruise control being activated?

Background: 2009 Z4 35i DCT. Broke the 2.56 diff pinion shaft at the drag strip. Was planning to fit the 3.08 diff to try in future, that plan got fast tracked to today.

 

[pbondar]

On my 20i with the 8HP45 where I couldn't change the ratio as xHP doesn't support it..it threw a meltdown..

I'm assuming the xHP is all you need..as the DME only knows of revs not speed?

 

[wheela]

Not familiar at all with changing diffs, but from what I've read I believe the DSC may not be happy without updating for the different ratio, not sure if updating the diff ratio in xHp takes care of this in the background? Good luck with the swap, hopefully somebody who knows what they're talking about will chime in

 

[wheela]

On my 20i with the 8HP45 where I couldn't change the ratio as xHP doesn't support it..it threw a meltdown..

I'm assuming the xHP is all you need..as the DME only knows of revs not speed?

Sounds like you tried this? How'd you correct the "meltdown"?

 

[pbondar]

Sounds like you tried this? How'd you correct the "meltdown"?

Long story…I was selling the car..I had lots of very expensive bits which I wouldn’t recover on the sale…one was my Quaife LSD…couldn’t find the correct ratio for the auto so put the manual version in instead..different ratio…I had various errors come up associated with powertrain , plausibility and then limp mode…

The way I fixed it was buying another diff with the correct ratio …

 

[pbondar]

Not familiar at all with changing diffs, but from what I've read I believe the DSC may not be happy without updating for the different ratio, not sure if updating the diff ratio in xHp takes care of this in the background? Good luck with the swap, hopefully somebody who knows what they're talking about will chime in

The DSC only knows about wheel speeds ie the rolling radius of all 4 wheels….so in theory DSC doesn’t care about diff ratio…it does care if the rolling radius are substantially different between front and back…allegedly around 2-3% plus difference…not applicable on later designs but so on E90 series

 

[Dumaurier7]

I have XHp on my E89 and there is an input for diff ratio, I also run 20" wheels without issue, dunno if that will help! but I think it will be beneficial to us ALL if you try it out and get a definite answer!