Downshift time for multiple gear downshift 8L90E

[JR2010]

In the attached file did you change any of the transmission settings yet or is it 100% stock? It looks like the 2018 vs 2016 factory tunes are quite different in some areas. Just wondering if it is due to better info or feedback from the field. And what harm would it do to make changes to the 2016 tune to match the 2018 calibration.

Here is the current tune I am running in my truck. Even with outside temps running in the 90's with the ac on I am getting 18-20 mpg in mixed city/highway. It pulls good from a stop, shifts well but with the occasional downshift clunk when coming off the highway.

O would be interested in your thoughts on this.

[WS6FirebirdTA00]

It is 100% stock. Yes, they are quite different. I was surprised.

I compared mine to the 2016 and most areas I was slower, but I was a tick faster than my file. I think it feels good, some of the 4th gear downshifts from high gear take a while. I just don't want to push it

I will compare yours to mine and post my latest tables for comparison as well.

[paulsmithsr]

I have a good trans tune

[WS6FirebirdTA00]

Here is my stock and modified stock file. I only did the power upshift, downshift and inertia tables I am using. Check it out and if you try them let me know what you think.

[HellKnightHicks]

Ever Figure out if the z06 cal could be flashed in full? Or where the missing tables were that prevented the use of there stock z06 paremters?

Thanks... Good info.

[HellKnightHicks]

I took the lowest times from your tables WS6FireBirdTA00 and put my shortest (factory times) with yours modified times. So I should have the best safe setup I can get from merging the 2
I have also for tow haul changed the desired shift times to no more than .35 across the board.

No pressures But I have changed the transmission shift speeds with BlueCat compensated for the whapping 4600lbs beast I drive.

Posting my tune this has headers and a 2.9L whipple
3 2017 Denali Lowered Shift times 35 TowHaul quick shift.hpt

I do have one problem with my setup. I hammer the throttle and if I lose traction ill bounce off the rev limiter a few times as I pick up speed and itll finally shift gears. Other than not hammer the crap out of it like that I dont know how to stop that.

[cmitchell17]

I took the lowest times from your tables WS6FireBirdTA00 and put my shortest (factory times) with yours modified times. So I should have the best safe setup I can get from merging the 2
I have also for tow haul changed the desired shift times to no more than .35 across the board.

No pressures But I have changed the transmission shift speeds with BlueCat compensated for the whapping 4600lbs beast I drive.

Posting my tune this has headers and a 2.9L whipple
3 2017 Denali Lowered Shift times 35 TowHaul quick shift.hpt

I do have one problem with my setup. I hammer the throttle and if I lose traction ill bounce off the rev limiter a few times as I pick up speed and itll finally shift gears. Other than not hammer the crap out of it like that I dont know how to stop that.

I haven't had any luck finding more info about using the ZO6 OS, I do have a spare TCM so I could just try to flash and see what happens, I don't think it would really brick the TCM or anything though so may be worth a try.

I just recently adjusted my shift times a little shorter, I noticed traction control is now kicking on bad when it goes to shift at WOT downshifts and upshifts. However, I was able to reset the TAPS and it appears it has fixed it. My guess is that since I reflashed the tune with lower shift times, it tried to increase pressure to the off-going clutches to try to meet the new desired times while it wasn't able to adjust the on-coming clutches enough since it is in a slower self correcting ("learning") mode so it may have eventually cleared up and stopped traction control from engaging. However, resetting the taps fixed it I think (unless it comes back) I think the on-coming clutches were just coming on faster than the off-going could adjust which was creating a small tie up which changes the output shaft speed very fast and may have instantaneously reversing the shaft which would definitely trigger traction control on.

I really wish we had more control over traction control but everything with TC is controlled by the chassis control module which we don't have access too.

[HellKnightHicks]

Mine was one of the locked TCMs. Had to be sent off to be unlocked.

If you could post your tune. Id like to see your numbers

TAPS I dunno what that is..... Im assuming it can be reset in the scanner tool.

I have my max pressure set at 1000.. Whats the safe spot for these?

Just ran with the posted tune and between the low end spark and the lower shift times its made a huge difference in the feel of the truck. Shifts are crisp no bang shift. Which I dont mind close to bang shift if I lay into it.

Any shift speed above .35 i dropped to .35 unless it was one related to a multi gear down shift. All of those I just went with the lowest between the stock tunes.

[WS6FirebirdTA00]

I actually raised some factory times because it felt so bad. Some of those areas hit too hard. Light throttle or reducing throttle on the 1-2 was an issue.

The thing I hate most is the lack of tweaks to adaptive settings. I can tell the TCM is tweaking itself to adjust shifting and I really wish I could adjust base values and limit adjustability. The main thing I get is flare in some gears after a reflash. I have tried to tweak what I can, but it seems to think it can do something it can’t.

[8850]

Here's a 2017 Z06 file for comparison.

[cmitchell17a]

I really wish I knew what drives the OEMs to calibrate such different and specific shift times especially when you have calibrations with different shift times with the same exact transmissions and relatively similar weights have such different times and the fact we pointed out they change them between the years.

So I do know that with these clutch to clutch transmissions there are 2 events happening during and upshift (not sure if its the same way for downshifting but I would like to know I can't tell what's happening from a log file) there is a "torque phase" which happens before the part of the shift you feel as the rpms are coming up to the shift point. During this first part one clutch is releasing and another clutch is applying at the same time, they are both slipping at the same time, and as pressure comes off one an equal relative pressure goes to the other one in perfect balance, so basically the transmission is slipping during this phase but everything is so precise that you just see the tachometer keep climbing steadily and don't feel a thing. From what I can tell the initiation for this first phase to start happening is driven by the shift mph speed. Once it gets to where most of the pressure has come off of one clutch and onto the other one it then initiates a "inertia" phase where it completely lets all pressure off the first clutch then clamps down completely on the second clutch, this is where the actual rpms of the engine drop, hence the "inertia", so the "inertia" of the rotating mass of the engine and driveline before the clutch is then brought to the same rpm as the rest of the driveline and wheels. It completely amazes me how the TCM can execute the torque phase imperceptibly. Since law of conservation of momentum the momentum lost when your engine drops rpms during a shift is put into the momentum of the vehicle moving forward but the engine has a lot less inertia than the vehicle, but it is enough to cause a artificial torque increase (chirping the tires during a fast shift) and this is also the reason timing is pulled during this inertia phase so it doesn't double up on torque on the driveline.

As far as I can tell only the "inertia" phase is counted in the desired shift times. I can't believe modern transmissions can execute all this especially when you try to trick it by moving the throttle up and down fast.

My guess is the shift times are different in different gear shifts maybe because some shifts have different inertias? in some gears there is more shafts and drums and mass turning in the transmission than there is in other gears so you have different inertias, but that doesn't really make sense as the TCM should just compensate for each different shift with a pressure increase and shoot for making all the shift times the same so you wouldn't get inconsistent feel from one gear to the next, which is something that drives me insane.

It does make sense that they try to increase the shift time as engine torque increases since heat capacity would become more of an issue and you could make smoother shifts at part throttle since you wouldn't have to deal so much with heat capacity with lower relative rpms between rotating clutches. And by heat capacity I just mean that the transmission has to make clutches slip to make this all work which is a lot easier when its completely submerged in transmission fluid, the fluid can more easily pull heat away from the local areas than a dry clutch in a manual (you can't just let your manual clutch slip).

I think in my last trans tune I made most of my lower gears 2-3, 3-4, and 4-5 the same shift times to try to get a more consistent feel between shift, but its hard to notice a difference. I'm not convinced a longer shift time always 100% means a more "luxurious" "imperceptible" shift and a faster shift time always means a more "firm" and "racing" "chirping" shift. I don't know of a good way to progressively take out torque reduction in the 8L90 TCM tunes, most people just raise the MIN spark table which in my opinion is the completely wrong way to do it.

[Russ K]

To get rid of the delayed WOT downshifts on the A8 trans, go to Trans>Torque Management>General>Downshift Red Tq Accel & raise the whole table to 300 ft lbs (stock is 15 ft lbs).

Russ Kemp

[cmitchell17a]

To get rid of the delayed WOT downshifts on the A8 trans, go to Trans>Torque Management>General>Downshift Red Tq Accel & raise the whole table to 300 ft lbs (stock is 15 ft lbs).

Russ Kemp

Thats interesting, I didn't realize the values in that table was a limit value, I thought it was the actual amount of torque reduction so a lower number would be less but I guess not.

I was scared to set it to 300 ft-lbs right away, so I set it to 148 ft-lbs since the downshift torque reduction when decelerating is 148 ft-lbs. I noticed no difference so far trying some WOT downshifts except one downshift felt really weird, it felt kind of normal then you could feel a large torque reduction then all of the sudden you could feel the torque reduction turn off but it still hadn't kicked down to a low gear then it finally did and it was pretty abrupt and jerky when it did. I am going to try 300 next. I don't get why this table isnt torque dependent, it seems to me it still must be a limit table that dosent allow an engine torque above that amount during a downshift.

I've never looked to really see how the 8L90 works hydraulically, but does anyone know if it can skip gears? The calibration has shift schedule tables that skip one gear, like 1-3 or 5-3 so does that mean it only has the capability to either shift sequentially or skip only one gear? I'm not sure of any transmission I know of can go straight from one gear to another and skip all the gears in between, I've never understood that limitation and why its not hydraulically possible. I notice after being so used to the 8L90 and driving a 4L60 its so much more responsive and almost feels like an instant WOT downshift?

[NotSure]

To get rid of the delayed WOT downshifts on the A8 trans, go to Trans>Torque Management>General>Downshift Red Tq Accel & raise the whole table to 300 ft lbs (stock is 15 ft lbs).

Russ Kemp

Is this chart specific to the A8 trans, or is there something similar on the A6 as well? Same issue for which I cannot seem to find a resolution.

[WS6FirebirdTA00]

Thanks Russ. I will try that out. I completely missed that table with the scroll bar.

[WS6FirebirdTA00]

I was scared to set it to 300 ft-lbs right away, so I set it to 148 ft-lbs since the downshift torque reduction when decelerating is 148 ft-lbs. I noticed no difference so far trying some WOT downshifts except one downshift felt really weird, it felt kind of normal then you could feel a large torque reduction then all of the sudden you could feel the torque reduction turn off but it still hadn't kicked down to a low gear then it finally did and it was pretty abrupt and jerky when it did. I am going to try 300 next. I don't get why this table isnt torque dependent, it seems to me it still must be a limit table that dosent allow an engine torque above that amount during a downshift.

Interested to see what you find here. It just did it that one time? I was tempted to bring this up to something more reasonable than 15 ft-lbs, but after tweaking some downshift times I am halfway pleased. The worst part is cruising in 8th and trying to go down to 3rd.

[cmitchell17]

Interested to see what you find here. It just did it that one time? I was tempted to bring this up to something more reasonable than 15 ft-lbs, but after tweaking some downshift times I am halfway pleased. The worst part is cruising in 8th and trying to go down to 3rd.

Yes there is some times usually its around low 40 mph not sure which gear its in but the downshift can take almost 4 or maybe even 5 seconds. Its like it will literally just sit there doing nothing.

Yes it just felt weird on that one downshift one time but it hasn't done it since, and everything feels normal like it did before, I can't really say I notice any quicker downshifts but to be fair I haven't timed them and the placebo effect can be very strong both for and against so they could in fact be faster than before.

[HellKnightHicks]

Heres what I ended up coming up with. The TCC tables are pretty agressive. Might want to keep your own on that. Basically feels like a gear shift with lockup occurs.

Ive smoothed it out for the most part while driving. Im still fine tuning it here in there. But this has really come a long way from where it stared.

8L90E 3200 stall shift settings 1500 GMC.hpt