MT revs upwards when hard driving in between shifts

[FlammasWRLD]

I've noticed this happening when I do pulls as well. I never was able to re-create it when I turned off the auto rev match.

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[ColonelAdama]

the car probably assumes you want to be in second then rev matches.

Then why would it rev to higher RPM than the peak RPM of the pull? That would mean the car is gaining speed after pushing in the clutch.

 

[bk5]

Yeah, I see what you mean now.

 

[lucky phil]

Bringing data like a boss...

So you declutched, and just left it in gear? Then after 2 seconds (?), the engine revs back up?

Kind of makes sense. If you're cruising in second, declutch to come to a stop, but don't move the gear lever, after some amount of time, the car probably assumes you want to be in second then rev matches. I've always gone 2->N->2 in these situations because I assumed the auto rev match wouldn't accommodate for this situation.

I'll try to take mine out today and replicate this. But I think you've uncovered what's going on.

Or if you declutch and don't move the gear lever the ECU revs the engine back up again to stop the car getting light in the tail if you re engage the clutch again? Imagine if you rev it out in second between corners on a twisty mountain drive for instance, declutch, let off the throttle to change up a gear then change your mind about the gear change. The rpm has dropped to idle or somewhere a lot less than the 6000rpm you revved it to and now you release the clutch still with the car in second approaching a corner entry. Might get a little taily on the way in or even send you off the road on corner entry? I'm just hypothesising. Plenty of stuff is applied to the engine/driveline calibration by the people that do it the rest of us don't understand. My FIL worked with the person responsible for the transmission Cal at GM and he never really liked various aspects of some of the calibration of their autos. trying to get it changed didn't work though. They had their own ideas.

Phil

 

[ColonelAdama]

Or if you declutch and don't move the gear lever the ECU revs the engine back up again to stop the car getting light in the tail if you re engage the clutch again? Imagine if you rev it out in second between corners on a twisty mountain drive for instance, declutch, let off the throttle to change up a gear then change your mind about the gear change. The rpm has dropped to idle or somewhere a lot less than the 6000rpm you revved it to and now you release the clutch still with the car in second approaching a corner entry. Might get a little taily on the way in or even send you off the road on corner entry? I'm just hypothesising. Plenty of stuff is applied to the engine/driveline calibration by the people that do it the rest of us don't understand. My FIL worked with the person responsible for the transmission Cal at GM and he never really liked various aspects of some of the calibration of their autos. trying to get it changed didn't work though. They had their own ideas.

Phil

Phil, what do you think about the possibility of it just de-pressurizing the charge pipe by opening throttle?

 

[djf]

I guess the potential saving grace is that if this is related to the auto rev matching, then it sounds like a bug that could be fixed with a software update. Unless this is somehow expected behavior but I can’t think of why that’d be the case

 

[Joker328]

Then why would it rev to higher RPM than the peak RPM of the pull? That would mean the car is gaining speed after pushing in the clutch.

It might just be over correcting. The rev match isn't perfect.

 

[lucky phil]

Phil, what do you think about the possibility of it just de-pressurizing the charge pipe by opening throttle?

Well if that is the strategy ( and I don't know if it is or not, but I'd be interested to know) it would be a bit annoying for sure. It seems the B58 system doesn't use a recirc or blow off valve to dump manifold pressure so is pulsing the throttle how they dump pressure if the throttle is slammed shut on boost?

Phil

 

[ColonelAdama]

Well if that is the strategy ( and I don't know if it is or not, but I'd be interested to know) it would be a bit annoying for sure. It seems the B58 system doesn't use a recirc or blow off valve to dump manifold pressure so is pulsing the throttle how they dump pressure if the throttle is slammed shut on boost?

Phil

yeah, my guess is like a protection against compressor surge maybe?

 

[lucky phil]

yeah, my guess is like a protection against compressor surge maybe?

Yea good point. On a gas turbine engine we have variable VSV's and bleed valves to control the compressor stalls. Slam the thrust levers shut like you do on touchdown and the Bleed valves open to prevent decel stalls. Same at top of decent when the thrust comes back to idle. A turbo is pretty much the same issue. The flow stops abruptly and the stall margin reduces so you need a mitigation strategy of some sort I guess. But what happens if you go WOT in second gear and jump off the throttle without depressing the clutch. The same scenario pretty much as far as the engine is concerned but having the throttle open momentarily again to control the turbo compressor stall wouldn't be acceptable.
Phil

 

[Saiahhduhh]

Welp, here you go. Conveniently happened on my drive in this AM.

See #1, I lift off the pedal completely after a 2nd gear pull
#2 throttle blips back open briefly
#3 RPM start spiking to redline without any pedal input

I did not even move the shifter, just clutch in.

Thanks for this

 

[Canfilm]

I've noticed this happening when I do pulls as well. I never was able to re-create it when I turned off the auto rev match.

How did you turn OFF auto rev matching?

 

[ColonelAdama]

How did you turn OFF auto rev matching?

I believe it is under the "Sport Individual settings" page. Gear shift assistant or something like that.

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