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Thread: 6L80 Power On Downshift - 500+ whp

  1. #1
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    6L80 Power On Downshift - 500+ whp

    All,

    What are people doing to keep cars alive on power on downshifts?

    I tend to not allow the cars not to downshift lower than 3rd on 6L80s and high power applications.

    Just looking what other people have had luck with.

  2. #2
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  3. #3
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    in short, depending on power. I do a similar thing. with more torque available and depending on speed i generally prevent shifts from 6th to anything lower then 4th.


    if they want 3rd they should use tapshift down to get into gear and go up for wot. asking for it to do a wot 6-3rd shift is asking for trouble

  4. #4
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    What's the best method for locking out power downshifts (6l90)? Does disabling neutral skip downshift do it or do you need to max out times or torque factors for the multi gear shifts? I have mine set to upshift in TUTD at wot and I don't let it downshift but I'd like to have it locked out just in case.
    Last edited by gmorris; 03-28-2018 at 05:12 PM.
    69 Chevelle 418" LS3, 11.25:1,PRC 260 heads, TSP 235/239, Longtubes, dual 3", 4L80, 3.42
    2010 Corvette Grand Sport A6, Heartbeat blower, 78mm pulley/10% lower, Kooks Long Tubes, 224/232 blower cam, ID1050x's, ECS stage 1 fuel system, alkycontrol meth system

  5. #5
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    just need to adjust your downshift speeds

    just think, if your in 6th gear doing 100mph and go wot, the tcm looks up the 100% value to see what it should do:

    for example: 6th-5th might have 160, so that means it needs to go back to 5th gear because its BELOW 160.

    now what if 5-4th has 120, well it also needs to go down to 4th, and so on. So if you set your 4th gear 100% to say 95mph, then it wont go lower than 4th because your already above the speed to enable a downshift.

  6. #6
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    I was trying to make it too hard as usual...I was thinking it would have to be in the power downshift tables somewhere not just the part throttle shift tables. Thanks!
    69 Chevelle 418" LS3, 11.25:1,PRC 260 heads, TSP 235/239, Longtubes, dual 3", 4L80, 3.42
    2010 Corvette Grand Sport A6, Heartbeat blower, 78mm pulley/10% lower, Kooks Long Tubes, 224/232 blower cam, ID1050x's, ECS stage 1 fuel system, alkycontrol meth system

  7. #7
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    So if the TCM is getting proper input I don't see why WOT skip downshifts would be any worse with extra power? So if I am thinking about this correct the 2 common scenarios would be a 6-3 and a 6-2. For a 6-2 I would think it would be simple, 2/6 clutch is already applied in 6th so to go to 2nd all it has to do is leave the 2/6 clutch on and just apply the 1/2/3/4 clutch. So in this scenario there should be a simple one clutch applying downshift or is that not correct?

    For a 6-3 I think it would be more complex and more risk for torque reversal and tie-up. The 2/6 clutch would already be applied, it would have to release the 2/6 and handoff to both the 1/2/3/4 and 3/5/R clutch. So I am not certain what the TCM does in these scenarios, does it handoff one off-going clutch while simultaneously applying 2 on-coming clutches at the same time? I know trying to observe my 6L80 it feels like it will go from 6th gear to some intermediate gear then to 3rd gear, however sometimes it feels like it goes directly from 6th to 3rd gear?

    Also taking the above 6-2 scenario as an example, trying to observe the PIDs we have in VCM scanner and observing off-going and on-coming clutches we don't have enough PIDs or they are not correct, does the TCM momentarily neutral itself so it can rev match the engine RPMs to the correct gear ratio for 2nd? If it is in-fact as easy as only applying the 1/2/3/4 clutch to engage 2nd from 6th directly, the behavior that I observe doesn't' really correlate. It definitely feels like it goes directly from 6th to 2nd, but there is an obvious neutral no power being transmitted to wheels feeling while the engine is revving up to match. So maybe the TCM will momentarily release the 2/6 clutch and be in neutral until engine RPMS get high enough then apply the 1/2/3/4? I guess if it didn't do this and just applied the 1/2/3/4 I guess you would get thrown forward even more on the downshift as the vehicle momentum would have to be transferred into the engine which would cause maybe too much vehicle speed loss?

    I know thinking about these downshifting scenarios can get really confusing. Unfortunately inner workings of clutch to clutch transmissions and more advanced TCM strategies aren't public knowledge that I can find, and even trying to read through advanced research papers and studies don't really reveal a lot of these scenarios.

  8. #8
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    Do you have PCS1-5 and shift solenoids 1 & 2 (preferably logging at high speed 20Hz+)? If yes would be great to see one of your logs to try and dissect it. I'm going through a bunch of logs now to see if I can find one that has a 6-3 shift in it.

  9. #9
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    Found 1. 6L90. Goodness, it's ugly to look at on the scanner. But from the seat of the pants it feels very smooth. There is a lot of torque management in effect to get this change done. Looks like the shift time is about 0.8s.

    From what I can decipher. Disclaimer, still learning. Not a trans expert. Just a home tuna.
    4-5-6 & 2-6 clutch drop out with a partial hand away to the 1-2-3-4 clutch and full hand away to the 3-5 clutch.
    The 1-2-3-4 clutch is the one that is ramped in for shift feel. The other clutches are coming on and off almost like switches.
    Attached Files Attached Files

  10. #10
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    If you like your transmission, tap shift down to the desired gear, and then go WOT

  11. #11
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    Oh yeah I forgot in 6th the 4/5/6 would also be applied and have to be disengaged. But if the 3-5 clutch came on instantly wouldn't it immediately tie up?

    I am looking at a WOT 5-2 downshift but I can make any sense of what's going on. I feel like I am missing a lot of PIDs I should have on my 2011 Sierra Denali 6.2 and on my 2017 Camaro SS. Looking at this log on the Denali, interestingly enough I get more useful PIDs from the Denali than the Camaro. I only can get a on-coming clutch PID, anyone else notice this?

    From observing the scan tool and looking at the service manual functional diagrammatics I conclude the following:

    C1 - 1/2/3/4 (Dynamic Clutch)
    C2 - 4/5/6 (Dynamic Clutch)
    C3 - 3/5/R (Dynamic Clutch)
    C4 - 2/6 Clutch (Holding Clutch)
    C5 - Low/Reverse (Holding Clutch)

    PCS #1 - Line Pressure Control
    PCS #2 - 3/5/R Clutch Pressure
    PCS #3 - Low/Reverse or 4/5/6 Clutch Pressure (Depending on clutch select valve state)
    PCS #4 - 2/6 Clutch Pressure
    PCS #5 - 1/2/3/4 Clutch Pressure
    Last edited by cmitchell17; 11-05-2021 at 12:54 PM.

  12. #12
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    In my log there is a small delay after the first clutches drop out and about the 3-5 clutch grabs. During this delay the 1-2-3-4 clutch is either prefilling or partially engaged. Just tracing the PCS solenoids is enough to infer what's going on.

  13. #13
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    So I guess the prefilling (volume filling) is what takes it so long to downshift? Well in certain scenarios and certain shifts? I know when they released the 8L90 marketing loved to talk about how it shifted faster than a Porche PDK dual clutch, of course that was only upshifting and definitely not downshifting. Although my 8L90 on some shifts can downshift as fast as the 6L80 and it feels like its going straight from the high to the low gear. They talked about how they moved all the clutch packs forward in front of the gearsets to make the fluid paths as short as possible.

    Its amazing to me that they can calibrate the spool valves and PWM the solenoids in the valve body to consistently fill at that rate everytime, it just seems its so right there on the edge of accidentally filling to slow or too fast and you get a tie up and a torque reversal and something blown up haha.

    In that case you would think they could engineer it to leave fluid in the passages or you would at least think even if they exhaust pressure you would still have residual fluid sitting there held in by gravity maybe?

  14. #14
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    I would assume these are all the available ways the 6L80 can downshift based on the parameters HP Tuners gives us:
    Capture.JPG

    But I don't know maybe the TCM does extra stuff. It does feel like this is true just trying to feel out the shifts, although I swear when doing a 6-3 downshift sometimes it will do it directly, then other times it feels like it goes to a intermediate gear first, but just barely touches that gear then goes to 3rd.