Boost spikes during shifting DCT

Asbjorn

Lieutenant
Mar 10, 2018
854
602
0
European, based in China
Ride
Z4 N54 DCT
Anyone had problems with boost spikes during shifting and found a solution?




Key mods
  • GC lite turbos
  • VTT hot side chargepipe
  • 200 cell downpipes, no other cats, otherwise stock exhaust, flaps disabled (always open)
  • Turbosmart Kompact plumb back DVs, green spring (never changed, ie factory spring)
  • M4 DCT flywheel
  • ER FMIC
  • ER cold side chargepipe

We have tried 4-5 tuning revisions to reduce it without luck, only made it worse. Tuner suspects it is a DV spring issue. Turbosmart replied that they think it is a tuning issue, but that many factors could play into this. They don't recommend swapping springs....

Here's some info about the green spring and one that is stiffer. Idle vacuum was around 21 last time I checked a year ago.


 

carabuser

Lieutenant
Oct 2, 2019
894
1
801
0
UK
Ride
Z4 35i & 335i
I've had this forever on my DCT Z4 35i. I'm pretty certain it's normal behaviour caused by the almost instant drop in revs with a wide open throttle. At the lower revs of the higher gear the engine will be consuming less air so pressure increases.

I did try to tune it out but the PID can't possibly compensate for it. Also if you do pull enough WGDC to prevent the spike you end up horribly under boost target after when the pressure spike drops off. I settled on just letting the spike be and keeping the wastegates static to keep a consistent boost pressure post shift after the spike decays.

It's not really noticed much on the stock tune because the boost taper disguises it but it is there. I managed to work around it in my tune by upping the values in the Timing (Failsafe) map to prevent a timing dip post shift. VANOS changes seem to exasperate the issue but that's just because of the increased airflow.

Aside from the massive corrections in cyl1 your log looks OK. I think BuraQ is doing eveything correct there, you aren't getting anything funky like Tq Limit 4 during shifts that the MHD OTS tunes give.

Here's a log of a stock 35is for comparison: https://datazap.me/u/stuartinzg/3rd-gear-runs?log=1&data=1-4-6-21&solo=1
 

WOT808

Specialist
Aug 7, 2017
80
56
25
Ride
E90 335i
I'd rule out the DV issue. Your throttle isn't closing during shift so I'd think the top of the DV is seeing close to the same boost that's in your charge pipe which keeps the DV closed.
 
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Asbjorn

Lieutenant
Mar 10, 2018
854
602
0
European, based in China
Ride
Z4 N54 DCT
Thanks for the replies guys. What @WOT808 wrote definitely makes sense, I am marking this as solved based on what @carabuser wrote. BTW would be good to know what your mod list is just for the record.

I could swear I saw someone in here posting a log saying something along the lines of "this is how DCT shifts should look like" a year or so ago. That log had no boost spikes as I remember it. However, I can't find the post, nor remember who posted it. :tearsofjoy:
 

carabuser

Lieutenant
Oct 2, 2019
894
1
801
0
UK
Ride
Z4 35i & 335i
Thanks for the replies guys. What @WOT808 wrote definitely makes sense, I am marking this as solved based on what @carabuser wrote. BTW would be good to know what your mod list is just for the record.

I could swear I saw someone in here posting a log saying something along the lines of "this is how DCT shifts should look like" a year or so ago. That log had no boost spikes as I remember it. However, I can't find the post, nor remember who posted it. :tearsofjoy:

My engine mods are VRSF downpipes and a 5" intercooler. Cosmetically I just added some 437M wheels. If the boost limit issue with DCT cars ever got resolved I might be tempted to put some bigger turbos.

Boost usually spikes between 2 and 2.5PSI after a shift for me although I'm sure it's proportional so at higher pressures it will spike more. I was initially convinced it was a fault with the tune too but I realised that all the logs with perfect boost profiles were tapering down to the point where the boost target met the spike after the shift.
 
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V8bait

Lieutenant
Nov 2, 2016
501
773
0
Texas
I've had this forever on my DCT Z4 35i. I'm pretty certain it's normal behaviour caused by the almost instant drop in revs with a wide open throttle. At the lower revs of the higher gear the engine will be consuming less air so pressure increases.

I did try to tune it out but the PID can't possibly compensate for it. Also if you do pull enough WGDC to prevent the spike you end up horribly under boost target after when the pressure spike drops off. I settled on just letting the spike be and keeping the wastegates static to keep a consistent boost pressure post shift after the spike decays.

It's not really noticed much on the stock tune because the boost taper disguises it but it is there. I managed to work around it in my tune by upping the values in the Timing (Failsafe) map to prevent a timing dip post shift. VANOS changes seem to exasperate the issue but that's just because of the increased airflow.

Aside from the massive corrections in cyl1 your log looks OK. I think BuraQ is doing eveything correct there, you aren't getting anything funky like Tq Limit 4 during shifts that the MHD OTS tunes give.

Here's a log of a stock 35is for comparison: https://datazap.me/u/stuartinzg/3rd-gear-runs?log=1&data=1-4-6-21&solo=1

This.

You can tune boost to ride a little under target up top, make sure the throttle isn't too sensitive to overshoot in the post shift range, and move on with your life :)
 
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