DOWNshift timing tables. ??? [Archive]

killerbee

November 3rd, 2011, 08:41 AM

It is very clear that turbo spoolup relies on significant fueling as well as rpm. It stands to reason, that a TCM tune with good downshift logic is going to be very beneficial, as this is the surest way to get the desired exhaust flow. The problem can be that a downshift can take as long as a full second, in which time, fueling is inhibited for "protection".

I noticed, for example, that the timing tables imply only control over upshift, e.g. "4>5". Are there tables that sep control downshift time, 5>4?

THEFERMANATOR

November 3rd, 2011, 05:49 PM

For a downshift the TCM must first evaluate which gear is needed, and then go down through the gears to get to it. The ALLISON cannot simply jump from say 5th to 2nd on a downshift as this is mechanically impossible due to the way teh valve body shifts. It has to do every downshift in between 5th and 2nd before it gets there. I have found that this is a large portion of the lag in downshifts, and is simply the nature of the beast per say. With the way ALLISON uses the 3 shift solonoids, 2 trim valves, and the hand off of main line to either hold the C1 or C2 clutches, there is alot going on when a downshift is commanded from 4th or 5th back down to 1st, 2nd, or 3rd. It would be nice if we could get access to the 0% throttle tables though as the 0% in the shift tables does not change the 0% shifts and lock-up in all scenarios I have found. I know there is ALOT of info inside the TCM that EFILIVE does not show us though, as I'm sure they could spend months trying to unlock even just half of the code in it.

As for the actual shift times if I understand you correctly, your wanting to know if you can change teh amount of time a downshift takes, or are you wanting to change the clutch to clutch shift time? The current shift table times control how long the TCM takes to basically hand off from clutch pack to the next to avoid a tie up and destroy the internals of the trans. The actual time a downshift takes is largely due to the TCM's limitations in how long it takes to calculate which gear to go to, and how many steps to make to get there.