2009 G8GT - Fresh 6l80- downshift issues

[ThatSilverG8]

To make a long story as short as possible my 2009 G8 has a fairly large cam / e85 tuned - Just installed a larger stall up to 3800 form 3400 previously. Complete fresh rebuild on the 6l80.

Rebuild was necessary because a highway pull completely trashed the converter clutch and 4,5,6 were pretty trashed and coned. The old trans had downshifting issues above 40 mph. It seemed to not like to downshift above that speed. I believe that is all in the tune, but no clue where to look.

Current Build- NA G8 making 480-500 RWHP
- Fresh Converter Refresh and update to 3800 stall speed
- Fresh 6l80 Pump
- Sonnax Zip Kit
- TECHM pressure switch rebuild
- Reybestos Red Clutches for 1-4 w / Billet apply piston
-Reybestos Red Clutches for 4,5,6 with Lindered steels - Sonnax HD piston
-3,5,R - Alto G3 clutches with Kolene steels - Sonnax HD Apply piston

I used the Blue Cat tool to adjust shift times and WOT and partial throttle shifts seem to be fine and quick and not overly harsh.

Issues


Main Issue - it just doesn't seem to want to down shift properly at almost any speed

Minor Issue - After cruising for a bit it then coming to a complete stop, upon taking off again it sometimes has slight popping (small backfires ?) under light throttle

Minor Issue 2 - It seems to lug a bit above 35 mph in town and between 50- 55 on the highway

Any help would be greatly appreciated


G8Current.hpt

[jaydubb71]

Can you explain the main issue in further detail?

[Frost]

So when you are just over 40 MPH, the car has to make a pass-through-neutral shift to get from 6th gear back to 2nd at WOT. I can tell you right now that trying to make that power downshift with your Advance>Minimum Spark>Torque Management set to 6.0 deg had to contribute to hurting the trans (if you make a lot of power downshifts here). Before you do anything further, set that table back STOCK. Set it back to -20 across the board. That will give the ECM room to pull the torque that it NEEDS TO during that downshift.

[ThatSilverG8]

@Frost Thank you so much, I will give that a shot. Are there any other adjustments you would look at if that doesn't help?

[ThatSilverG8]

Can you explain the main issue in further detail?

The trans from a standing idle shifts beautifully fairly crisp and quick.no complaints about part throttle or WOT on shift characteristics. However from any rolling speed it doesn't seem to want to (power) downshift at WOT. Sometimes it seems to want to accelerate from the current gear even at highway speeds. Partial throttle it will drop from 6-5 cleanly to accelerate but not from 6th to 4th,3rd, 2nd.

[Xjas]

I would copy the pattern A shift scheduling into pattern B as well, doubt it will fix the issue but wont hurt and rules that out as an issue.
Why is your tyre circumference 2051 inches? Dont think this is used by the trans for speed calcs but it should be corrected regardless.

6L80s don't do a 6-3 shift, they have to do 6-5 and then 5-3, makes for a shitty downshift if you're rolling at that speed where it wants to go back to third, I have mine set up to lock out 6th gear in sport (pattern A) so before I stand on the gas I can tap the shifter over to sport and it drops back to 5th before I go WOT, I think this is a lot nicer on the trans and avoids the double downshift.
Other than that you really need to post some logs of the issue showing the shift timing, clutch pressures and torque management, listing the car/engine mods would make it easier for people to help you also.

The backfiring on light throttle is probably from the trans trying to reduce the torque too much by cutting spark on the upshifts, shouldn't be too hard to fix by limiting the torque reduction in those areas, again a log would help.