[Chrisred]

While launching 335xi if I get too much wheel spin (with DTC completely off) I get a power cut and a 4x4 malfunction alert. Then car goes back to normal. This didn\'t occur before I replaced my front diff and transfer case. Is this a new BMW software upgrade to protect from harm??

[fdriller9]

Sounds like a problem with calibration or something.

[DRZZZ]

I know this may be off topic, by why did you need to replace your front diff??

[Chrisred]

Cause dealer saw it had a crack in it. Lucky me

[DRZZZ]

I agree with a possible calibration issue. I would scan for codes first, then bring it back to the dealer.

[amclint]

I just installed the JB4, when I launch at 2500 rpm with DTC off my front wheels spin and the engine cuts power then when it has traction it launches forward again. I don't get the 4x4 malfunction light but it's annoying anyway. Does this happen to anyone else?

It might be a non-issue for me now since my wife hit a pot hole and my RFT in front has a huge bulge. I bought new non RFT tires and they're going on tomorrow, I've had them on another car and they grip like crazy

[j2m]

Are you turning DTC and DSC completely off?
This happens to me when I brake boost above 2,600 RPM as well with DTC and DSC off.

Originally Posted by amclint

I just installed the JB4, when I launch at 2500 rpm with DTC off my front wheels spin and the engine cuts power then when it has traction it launches forward again. I don't get the 4x4 malfunction light but it's annoying anyway. Does this happen to anyone else?

It might be a non-issue for me now since my wife hit a pot hole and my RFT in front has a huge bulge. I bought new non RFT tires and they're going on tomorrow, I've had them on another car and they grip like crazy

[amclint]

Got my new tires on today and took the car out in the cool night air, running 99 octane on map 7 for the first time and WHOA is it powerful. From a dig though I'm kind of bugged, still have the same problem with the front tires breaking loose in DS mode, nothing happens when the wheels spin (just a bit more than a chirp really) but at about 6k-6.5k rpm the power is cut and then it jerks forward again still in 1st and shifts to second then no issues through 4th. I logged with JB4 but didn't save the export and my laptop shut off, the graph though showed a really jagged downslope from 1st to 2nd gear change where it's normally pretty smooth looking.

Seems really odd, not sure what to make of it. I've read a bunch the last 2hrs and found a couple threads with the same issue from other XI owners, some possible things to try:

1. Launch with DSC on but DTC off (push button once)
2. Launch in 2nd gear with DSC/DTC off
3. Launch in 1st gear manual mode, but shift at 6k-6.2k
4. Brake boost at 2k rpm so there is no wheel spin

I have done some launches in 2nd gear and the brake boost goes over 3k but the run is a good bit slower (even on map 7) than starting in 1st gear when I don't have the power cut. I did try to shift early in manual mode, but I must have been missing it. It comes up so dang fast I think I'm going to try to re-map my shift light and see if using that helps.

[amclint]

Went back out this morning and tried the stuff I read about:

1. Launch with DSC on
- This was a bit odd, it didn't cut power but I could feel it lag a bit just after the wheelspin, the run was better than when it cuts power
2. Launch in 2nd gear with DSC/DTC off
- Brake boosting to ~3k this did OK with a 4.8 0-60 run
3. Launch in 1st gear manual mode, but shift at 6k-6.2k
- This is hard to get right, but the few times I got the shift right it was my fastest time, the issue seems to be the Steptronic because when you click the shifter there is a delay and the RPMs are going up so quick I'm actually having to try and shift ~5k and the car shifts closer to 6.3-6.5k that way. If I don't get it perfect it shifts closer to redline so it's for sure a practice thing. Maybe moving my shift light down to 4.5-5k would work. This was the FASTEST time I've ever had in my far, no bog and no fuel cut at all. JB4 Map 7 on 99 octane WHOA!
4. Brake boost at 2k rpm so there is no wheel spin
- Didn't seem to matter actually, I'm not sure the fuel cut is due to wheel spin or the car hitting a rev limiter? This was the most odd discover as with no wheelspin but foot to the floot acceleration in 1st I still get fuel cut at the upper RPM in 1st just before the 2nd gear shift. WTH?

0-60 using OBDII reading:
Method 1: 5.0 seconds
Method 2: 4.8 seconds
Method 3: 4.6 seconds
Method 4: 5.2 seconds
DS Mode to redline: 5.2 seconds (fuel is cut so it's a jerky ride)

I only just bought the OBDII device so I don't know how these times compare to what I ran before. My stock time using a camera was ~5.0 seconds and my best with JB+ 100% on 93 octane was 4.5 seconds, so I think these are long by .5 seconds or so.

[amclint]

Got video of some runs last night, OBDII reading is off a bit but go figure it's probably using the speedometer. I hit the stupid fuel/boost cut in 1st gear several times, I'm going to do a new post and see if anyone knows what it is. Very fast times though when I can get everything just right.

0-60:
DS Mode (3.4k rpm launch) shifting @ 6k rpm: 4.03 seconds (no lag or fuel cut)
M2 To Redline (3.4k rpm launch): 4.24 seconds

[amclint]

Oh and I went this morning and looked at my run area, turns out the front tires are leaving about a 2-3ft mark and the rears left a 10ft long one or so. Power must transition to the rear when the fronts start slipping.

The shop that did my tires put like 38 PSI in them so I'm going to drop that down to 25-28 PSI when I go to the track. Anyone else on non-RFT have any tips on PSI to run for best 1/4 launch?

[ajsalida]

Just a guess. Even with DTC/DSC off the car still has computer controlled AWD using transfer case and individual wheel brakes to fake LSD on each axle. It just has no stability control. So it may be there is some logic buried in there that says, OK we're beyond TQ/RPM/throttle amount that braking/TC can distribute, so back off on throttle. I'd be surprised if there weren't, as otherwise you might burn up brakes.

Also what size wheel/tires are you using?

[amclint]

I figured it out by reading a bunch more on the forums, turns out with the large HP gains I'm exceeding the capabilities of the transmission and shift logic. So what happens is the wheelspin, followed by very rapid rise in RPM makes the computer not shift properly causing the car to hit the rev limiter. If I click the paddle early (4.5-4.75k RPM) with the delay it does shift at just over 6k RPM and everything is fine, if I shift just a split second too late (5k RPM +) then it bumps into the 7k or 7.2k RPM limiter and cuts power.

So I guess the trick is try for no wheelspin, launch in M2 or practice shifting to get it in that precise range. I wish there was a way to speed up the steptronic shifts, they aren't all that quick to begin with...

[ajsalida]

Good to know. I also have a AT xi, a Cobb & DCI are arriving in a couple of days.