Delayed Downshift?

SUMMITWHITE1SS · 06-18-2019, 11:05 AM

So i did a quick search and couldnt find any info on this issue... Ok.... So this happens very rarely. But lets say im cruising at around 70-80mph and i want to overtake a car. I dont stab the throttle but i kind of just gradually go to WOT and i hear the engine go under load but the car does not downshift for a good 2-3 sec. So then i let off the throttle and then it downshifts. When i try and replicate it after this happens it won't do it again. Car in sport mode when this happens. Car has long tubes and tuned on e85..

cmitchell17 · 06-28-2019, 06:22 PM

Unfortunately this is a normal characteristic of the 8L90, especially at the speed you are talking about, and it actually seems to happen worse if you roll into the throttle like you are describing.

I've tried experimenting by copying the downshift times from a ZO6 8L90 since they are significantly quicker (.2-.3 seconds quicker), but once you go about .1 seconds quicker the transmission starts binding gears and will eventually trigger stablitrack on especially on a WOT downshift to 3rd. We may be able to full flash the operating system from a ZO6, but I haven't investigated if this is possible

It makes no sense to me mechanically why the transmission is so slow on downshifts, yes it may be fast on upshifts but GM marketing never mentioned the downshifts. My guess is the limitation is hydraulic maybe, but I have never been able to understand how modern automatics are able to have dynamic hydraulic pressure and reduce pressure at cruise, but then be able to instantly respond to full torque from the engine and correspondingly ramp up pressure.

I know the 8L90 operates with a distinct low pressure pump for efficiency I'm not sure if this has anything to do with it, my best guess is maybe there is an accumulator circuit that has to fill a empty volume before a clutch can come on and this maybe the limiting factor in downshift speed. I also have not been able to tell if the 8L90 can skip gears when downshifting or if it has to sequentially shift. There are tables in the calibration that allow it to skip one gear, 8-6, 6-4, 4-2, etc, but we don't have access to the right PIDs to actually see what its doing in logs.

06-18-2019, 11:05 AM	#1
SUMMITWHITE1SS Drives: 2017 A8 1SS Join Date: Jul 2017 Location: So cal Posts: 43	Delayed Downshift? So i did a quick search and couldnt find any info on this issue... Ok.... So this happens very rarely. But lets say im cruising at around 70-80mph and i want to overtake a car. I dont stab the throttle but i kind of just gradually go to WOT and i hear the engine go under load but the car does not downshift for a good 2-3 sec. So then i let off the throttle and then it downshifts. When i try and replicate it after this happens it won't do it again. Car in sport mode when this happens. Car has long tubes and tuned on e85..

06-28-2019, 06:22 PM	#2
cmitchell17 Drives: 17 2SS, 8L90, Cam, Heads, E85 Join Date: Dec 2016 Location: US Posts: 1,201	Unfortunately this is a normal characteristic of the 8L90, especially at the speed you are talking about, and it actually seems to happen worse if you roll into the throttle like you are describing. I've tried experimenting by copying the downshift times from a ZO6 8L90 since they are significantly quicker (.2-.3 seconds quicker), but once you go about .1 seconds quicker the transmission starts binding gears and will eventually trigger stablitrack on especially on a WOT downshift to 3rd. We may be able to full flash the operating system from a ZO6, but I haven't investigated if this is possible It makes no sense to me mechanically why the transmission is so slow on downshifts, yes it may be fast on upshifts but GM marketing never mentioned the downshifts. My guess is the limitation is hydraulic maybe, but I have never been able to understand how modern automatics are able to have dynamic hydraulic pressure and reduce pressure at cruise, but then be able to instantly respond to full torque from the engine and correspondingly ramp up pressure. I know the 8L90 operates with a distinct low pressure pump for efficiency I'm not sure if this has anything to do with it, my best guess is maybe there is an accumulator circuit that has to fill a empty volume before a clutch can come on and this maybe the limiting factor in downshift speed. I also have not been able to tell if the 8L90 can skip gears when downshifting or if it has to sequentially shift. There are tables in the calibration that allow it to skip one gear, 8-6, 6-4, 4-2, etc, but we don't have access to the right PIDs to actually see what its doing in logs.

User Name		Remember Me?
Password