[SpencerC_6MT]

I recently had a Wavetrac limited-slip diff installed on my 6-speed M240i, which included a ratio change from the stock 3.08 to 3.46. Although the car drives fantastically after startup, I've ran into a peculiar issue soon after the car starts. It should be noted that the car is bone stock besides the diff and an intake, and there's no check-engine light or warnings from iDrive. I always drive in DSC OFF, so it's likely not traction control related.

When the car is first started, cold or not, I lose acceleration in-gear after about a second. The turbo will start spooling and the car will accelerate normally for a second, but immediately cut power after, even if the gas pedal is pressed to the floor. The car will still accelerate, but at a painfully slow rate, almost unsafely slow. If I let off the gas and floor it again, you can feel a momentary blip of power before it cuts again. If you look in Sport Displays on iDrive, it shows a very low power number, indicated the ECU knows exactly what it's doing. If I shift gears, whether upshifting or downshifting, the car will behave normally again for a second before the same behavior returns. I'm not sure exactly what causes the behavior to go away, but the car will suddenly begin behaving exactly as expected between 10-60 seconds after beginning a drive.

I called my mechanic who installed the diff, and described the behavior to him. He is unsure what caused this, as he said the engine systems were untouched, which makes sense. Our leading theory is that the ECU is confused by the wheel speed not matching the expected RPM due to the new axle ratio, and cutting power in an attempt to diagnose the issue. I find this very likely, as I've noticed the ECU cutting power if I have excessive clutch slip during an aggresive gear shift. Perhaps the car is trying to prevent what it perceives as clutch slip? My other theory is that it has to do with some sensors in the suspension. To install the diff, the shop had to disconnect the entire rear multilink suspension system, and they replaced a bent front control arm unrelated to the diff.

I'd love to hear from any other forum members who may have had this issue, and how they fixed it if they managed to. I know that, even among owners who swap in a limited-slip diff, the ratio change is uncommon, so I don't expect to find any solid conclusions.

EDIT: a video link to demonstrate the issue. https://www.youtube.com/shorts/EnszNYXpfBY

[JMcLellan]

To your specific question/request I do not have experience with this exact issue, or have experience with resolving but I wanted to chime in.

You need to get some diagnostics done to pinpoint what the issue is. I tend to agree with the working theory that the change in gear ratio is causing this but these cars are really complicated and it potentially could be other things. There has got to be someone who can go drive it and/or hook up something to your OBD port to do some readings.

Bummer to hear about this. I've heard great things about the wavetrac and how it transforms the driving experience of the car. Hopefully you get this resolved soon.

[freakystyly]

Sounds like most people have kept the stock final drive from reading through random posts. Out of the ones who decided to change the ratios, most if not all are experiencing issues with wheel speed sensors and other malfunctions.

I know on previous chassis this wasn't an issue so it is a relatively new problem. Keep hunting and maybe someone will post a fix. It will likely involve coding of some sort (possibly ECU/TCU tuning?). If you have access to coding tools maybe take a look and see if anything jumps out at you.

[SpencerC_6MT]

After running a scan, my mechanic confirmed my suspicions that the car thinks my clutch is slipping.

[dradernh]

Originally Posted by SpencerC_6MT

I recently had a Wavetrac limited-slip diff installed on my 6-speed M240i, which included a ratio change from the stock 3.08 to 3.46.

Why did you choose to change the final drive ratio to 3.46?

Did that have anything to do with your driving the car in DSC OFF all the time?

Just curious – wanting to understand your thinking.

[SpencerC_6MT]

Originally Posted by dradernh

Why did you choose to change the final drive ratio to 3.46?

Did that have anything to do with your driving the car in DSC OFF all the time?

Just curious – wanting to understand your thinking.

I got in the habit of using DSC OFF since it disabled auto rev-matching. Once I coded it out with Bimmercode, I was used to not having the harnesses on and driving with traction on felt wrong.

The 3.46 gears are because I don't do a lot of highway driving, and I wanted to be able to wind out the gears without going too fast.

[aerobod]

I believe E9x cars had manual transmission gear ratios (no final drive ratio, but a value for each individual gear) configured in the DME, this could well be the case with F2x cars, too. One characteristic of changing the final drive ratio on an E9x manual without updating the DME/DDE gear ratios was that cruise control would no longer work.

Most useful path to confirm if F2x DMEs have the gear ratio values in them is to talk to one of the tuning companies, as an update would require a custom tune.

[JMcLellan]

I have the XHP trans tune and tried to connect pretending i have the 6 speed transmission. It errored out probably cuz my car is the 8 speed auto. I was wanting to see if there were custom gear ratio settings.

I wonder if you could track down another member who could login to their XHP and see if there are any gear ratio settings in there for the 6mt cars?? If there are those settings then you could get the XHP license and go from there.

[aerobod]

Originally Posted by JMcLellan

I have the XHP trans tune and tried to connect pretending i have the 6 speed transmission. It errored out probably cuz my car is the 8 speed auto. I was wanting to see if there were custom gear ratio settings.

I wonder if you could track down another member who could login to their XHP and see if there are any gear ratio settings in there for the 6mt cars?? If there are those settings then you could get the XHP license and go from there.

There isn’t a TCU on manual cars, the DME would need to be used if gear ratios are stored, so it is part of the engine tune as opposed to a transmission tune.