[Nezil]

First off SeanWRT, thank you so much for giving your opinion. I know you've said previously that you don't have much experience with 6MT, but you do have a lot of experience reading logs and understanding how our DME works - and that is super valuable!

The other thing I want to thank you for is having a second opinion. I've been looking at logs since going with BM3 (as you know), and I've got my own opinions about what's going on, but without anyone to bounce these ideas and opinions off, it's very difficult to make any progress.

Originally Posted by SeanWRT

Nezil, I've dug a bit more and my theory is this:

During shifts, if anything, the ECU is trying to SLOW DOWN the turbine when you think you need it more. Because:

1) Although DV dumps boost, the turbo shaft speed is still high. It's already high enough to support top end charge air (check MAF) which is too much for next gear.

I've checked the logs as you suggested, but MAF doesn't seem too high to me. After the shift, MAF stays at the same point until WGDC hits 100%, then rises as boost rises. MAF seems quite aligned with boost, and consequently low after the shift.

Assuming the shaft speed is too fast, are you suggesting that the waste gate is effectively letting the turbine free-wheel a bit to avoid a shock which would slow it down if the waste-gate were to close?

2) The dumped (via DV) charge air goes into turbine housing, adding more pressure.

It sounds to me like you're saying we've lost boost but the turbine is still spinning too fast. Would a DV which vents to atmosphere help slow the turbine down so that the DME doesn't prevent boost from building?

I'm not sure if BMW uses turbo shaft speed sensor though I know for sure Nissan does with the great VQ30.

I don't think BMW has that. There are certainly no additional connectors on the turbo housing.

ECU simply decides 80+% is the maximum WG duty to give when you need to SLOW DOWN the turbine, or there'd too much overshoot to deal with after boost target is met.

As RPM rises and turbine slows down, it gets to a point where turbine is not overloaded (for the RPM). Then 100% WG duty is allowed.

How could the DME know this if it weren't for a shaft speed sensor? You've suggested that MAF is too high, but it doesn't look like it in the logs... unless I'm missing something.

I do not think 80% makes a much slower spool up unlike what you see from your log. It could be the DV working in conjunction on 100% duty when allowed.

But you do agree that boost is building slower with a quicker shift than a slower one, based on my logs. I realise that boost gets to target quicker when the shift is quicker, but the rate boost builds is actually slower if the shift is faster.

I've been logging boost behavior on DCT shift for a long time, because it plays a key role in acceleration and safety. My finding is that overshoot is a much more serious issue and takes 500-1000 RPM normally to settle. More boost headroom you leave, more poised the engine will be taking it up, hence the less power loss or you even make more power (by properly eating more boost!). Halim has a "boost guideline" tuning strategy (I made up that name and won't go deeper on it), with that there are even more uncertainties.

In any cases, the priority on shift is always boost dumping and turbine adjusting/decreasing. There is no way for 100% WG duty instantly on tap, unless you're already significantly under-boost before shift, which would be a much more horrible scenario.

This isn't what the logs show... the log for the slower > 2.0s shifts show WG duty instantly going to 100%. Boost was above target before both shifts 2nd to 3rd & 3rd to 4th.

In this sense, to "fix the lag" will be on less pedal down time. DCT fixes it by shifting impossibly fast, not by forcing 100% WG duty.

Isn't DCT fixing the lag because there is no period out of WOT, not because of the speed of the shift. Obviously the shift speed is what allows this, but it's not the shift speed that's significant, it's the constant WOT that is.