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Thread: ZF8 Downshift Delay

  1. #1
    Tuner
    Join Date
    May 2008
    Location
    Austin, TX
    Posts
    60

    ZF8 Downshift Delay

    I have a NA 3.6 Pentastar Challenger, and I'm still quite new to tuning these. I would like some help understanding the tuning of the downshifts. Stock, there are huge delays between mashing the pedal and getting the downshift I'm requesting.

    I believe I understand how upshifts work just fine, increasing the Oncoming Clutch and Fill Offset pressures and reducing nominal slip times should get me a quicker and firmer shift.

    But how do the downshift tables work? Nominal slip times range anywhere from around 200 ms to around 1,600 ms for multiple-gear downshifts. Looks like it's the Offgoing Clutch tables that really determine the shift pressure, but they seem to have less pressure for more performance-oriented shifts. How should I go about tuning out some of the delay on downshifts without risking too much damage to the transmission?

    Thanks for the help!
    2002 Silverado extended cab 5.3L.
    Volant cold air intake
    Gibson cat-back exhaust
    Electric fan conversion


    2016 Dodge Challenger SXT 3.6L
    Hellcat airbox
    Ported throttlebody
    Pedal Commander

  2. #2
    Tuner
    Join Date
    May 2008
    Location
    Austin, TX
    Posts
    60
    v28-trans-test-pressureandtiming.hptChallenger36-Stock.hpt

    Stock and test tune I've been playing around with. Any advice would be appreciated.
    2002 Silverado extended cab 5.3L.
    Volant cold air intake
    Gibson cat-back exhaust
    Electric fan conversion


    2016 Dodge Challenger SXT 3.6L
    Hellcat airbox
    Ported throttlebody
    Pedal Commander

  3. #3
    Advanced Tuner
    Join Date
    Aug 2016
    Posts
    386
    I haven't tuned a ZF transmission (my experience is on the GM side of things), but I suspect its the same challenge here. As much as we want them to, these C2C transmissions simply cannot shift instantaneously. The hydraulics take time to prepare for a gear transition (especially if it's jumping multiple gears). On upshifts, the TCM can predict when the shift needs to occur because it can predict when it's going to reach the WOT shift criteria and as such it can prepare for the shift ahead of time. On pedal-mashing downshifts, the TCM has no idea the driver is about to do that, so it can only begin the process of downshifting once its downshift criteria has been met.

    GM (and probably other automakers) addressed this by adding "PAL" to their transmission controllers. Here, the TCM can detect when the driver is driving aggressively (large pedal input changes, etc) and it will engage alternate shift profiles to hold lower gears to higher RPM. The transmission is already in a more-optimal gear for spirited driving.