6L80 1-2 shift spinning badly at track

[mn_vette]

What would be the "proper" way to reduce the shift impact on my 1-2 shift on my 6L80? I'm running an LS3 corvette with a centrifugal supercharger, no other engine or exhaust mods. I'm still running the Torque Ramp on the Torque Reduction.

Looking at my log, the timing starts recovering well before the engine RPM's have dropped down to the proper level. I would expect the torque converter to take up that RPM difference.

On my 2-3 shift it does the same thing. The only real difference is that the TCC seems to be engaged with line pressure even through the shift. It still gets some slip but then re-locks. I'm running the factory TCC settings.

If someone could give me a quick education on how things should work it would be a great help. I've attached a log file and tune for one of my runs. Thanks for the help.

Run1.hpl

AfterRun1.hpt

[Alvin]

Its odd to me it would spin that bad with that long of a shift time and it appears to still have good TM in it. Is this at the track? Tires?

It looks like you were in tap shift mode. Most of those are programmed to stay locked up TCC from the factory.

[mn_vette]

Its odd to me it would spin that bad with that long of a shift time and it appears to still have good TM in it. Is this at the track? Tires?

It looks like you were in tap shift mode. Most of those are programmed to stay locked up TCC from the factory.

I was in TUTD. This was at the drag strip running Michelin Pilot Sport 4s. Launch does ok, 1.9 60'' with no spinning after a good burn out.

[Alvin]

I would put tires on it. Any drag radial would make a huge difference.

[mn_vette]

I would put tires on it. Any drag radial would make a huge difference.

Yeah, that's not going to happen. Its a street car and I run street tires. I don't go to the track on a regular enough basis to make it worth while. It would just be nice to keep the car straight going down the track for safety reasons.

What can I tweak to get this to act a bit better? Should I lengthen the 1-2 shift times so that the RPM's come down further before the transmission engages?

[TheMechanic]

I have not tried this so take it with a grain of salt.
Maybe try VERY large reduction on ctrl + N "15430" .
As a test I would use something like 100 in place of your 6042 on the 1-> shift. See if it falls flat on it's face. Hopefully if it does you can just start from there until you find your happy place.

[edcmat-l1]

PSA: Not everything can be cured by tooning.......

[TransGo Robert]

I don't know enough about tuning to tell you how to change it but there is something wrong with the 1-2 shift command causing a partial bind. Your low/reverse is staying applied until a little bit after the shift command starts, on normal applications, the low/reverse releases at 3 mph. Now you can have the L/R stay applied longer to help save the sprag since you are racing it, but it must be completely released before you start applying the 2-6 to make the 1-2 shift or else it will bind. PCS 3 is the one controlling the L/R, and PCS 4 is the one controlling the 2-6. If you look at your 1-2 shift event you will see that about 2 seconds before the actual shift happens PCS 2 is commanded on with a desired 21 psi output but the L/R is still commanded fully ON. That 21 psi is the fill phase of the shift, the LR has to be released before that happens, otherwise during that 2 second you are actually slightly binding and slowly burning your 2-6 frictions. I don't know if that has anything to do with what you feel (it looks to me like it is more a traction issue than anything else) but you should try to correct that because it will eventually take your transmission out.

[TheMechanic]

I don't know enough about tuning to tell you how to change it but there is something wrong with the 1-2 shift command causing a partial bind. Your low/reverse is staying applied until a little bit after the shift command starts, on normal applications, the low/reverse releases at 3 mph. Now you can have the L/R stay applied longer to help save the sprag since you are racing it, but it must be completely released before you start applying the 2-6 to make the 1-2 shift or else it will bind. PCS 3 is the one controlling the L/R, and PCS 4 is the one controlling the 2-6. If you look at your 1-2 shift event you will see that about 2 seconds before the actual shift happens PCS 2 is commanded on with a desired 21 psi output but the L/R is still commanded fully ON. That 21 psi is the fill phase of the shift, the LR has to be released before that happens, otherwise during that 2 second you are actually slightly binding and slowly burning your 2-6 frictions. I don't know if that has anything to do with what you feel (it looks to me like it is more a traction issue than anything else) but you should try to correct that because it will eventually take your transmission out.

In BlueCat you can adjust the shift lead times. I think that may help that condition. Maybe a faster release time.

[hjtrbo]

Drop down the WOT TUTD speed from 72 to 62km/h. Never tried it for this condition. Test only

[mn_vette]

Drop down the WOT TUTD speed from 72 to 62km/h. Never tried it for this condition. Test only

Short shifting first gear might be the easiest way around this problem. I'll have to see if I can dial it in a bit better next time I'm at the track.

I don't know enough about tuning to tell you how to change it but there is something wrong with the 1-2 shift command causing a partial bind. Your low/reverse is staying applied until a little bit after the shift command starts, on normal applications, the low/reverse releases at 3 mph. Now you can have the L/R stay applied longer to help save the sprag since you are racing it, but it must be completely released before you start applying the 2-6 to make the 1-2 shift or else it will bind. PCS 3 is the one controlling the L/R, and PCS 4 is the one controlling the 2-6. If you look at your 1-2 shift event you will see that about 2 seconds before the actual shift happens PCS 2 is commanded on with a desired 21 psi output but the L/R is still commanded fully ON. That 21 psi is the fill phase of the shift, the LR has to be released before that happens, otherwise during that 2 second you are actually slightly binding and slowly burning your 2-6 frictions. I don't know if that has anything to do with what you feel (it looks to me like it is more a traction issue than anything else) but you should try to correct that because it will eventually take your transmission out.

Robert,

I think I see what you are talking about. In first gear the PSC 4 is prefilling almost a full 2 seconds before the shift is commanded. And in second gear, PSC 2 is prefilling about 1.5 seconds before. But that doesn't happen on the 3-4 shift, prefill happens at the shift command.

I've been doing some normal driving around here and I don't see this happening then. I wonder if this is something the transmission is doing because of WOT and possibly because of the TUTD mode. I don't think I tweaked anything that could have caused this. Actually I really have no clue what would control this.

I will try to keep an eye on this and see if I can get a bit better understanding of when it does this. The last thing I need is extra wear on the transmission.

[TransGo Robert]

I think I see what you are talking about. In first gear the PSC 4 is prefilling almost a full 2 seconds before the shift is commanded. And in second gear, PSC 2 is prefilling about 1.5 seconds before. But that doesn't happen on the 3-4 shift, prefill happens at the shift command.

I've been doing some normal driving around here and I don't see this happening then. I wonder if this is something the transmission is doing because of WOT and possibly because of the TUTD mode. I don't think I tweaked anything that could have caused this. Actually I really have no clue what would control this.

I will try to keep an eye on this and see if I can get a bit better understanding of when it does this. The last thing I need is extra wear on the transmission.

I did not realize your were it tap shift mode, that might be why but I doubt it is normal. The prefill is ok, that needs to happen to purge the air out of the system and ensure a nice clean short shift. What is not supposed to happen during that pre-fill phase is the low/reverse brake still be applied. It normally comes off at 3 mph. In your case it stays on all the way to 45 mph. Now that helps save the sprag in the unit so it is a good thing on take off, but for optimum longevity it needs to release before the prefill of the 2-6 clutch starts. When the 2-6 apply the sprag needs to overrun for the shift to occur, with the low reverse applied it can't one element is fighting against the other. if for example the 2-6 where to apply completely the wheels would lock up solid (that's how a trans brake work). So yes, having the low reverse brake applies on take off and keeping it on longer then just up to 3 mph helps the sprag, but it if it does not release before pressure is sent to the 2-6 you are hurting your 2-6. Probably not a whole lot since the prefill pressure is pretty low but still not something I would want, not to mention that it slows you down a nanosecond as well. Now do they all do that in tap shift more at WOT, maybe I have never looked at that. It would be floored if it was normal but it might be. I can't see anywhere in the tune where that can be changed, but I know it can because is is a very popular modification people make in high power application to help save the sprag, they change the tune to keep the low reverse on longer. It is one of the best tune modification you can make on for example a 68RFE in a Dodge. I just don't know if it is something that can be done in a 6L80, I am not very knowledgeable when it comes to tuning. Trying to learn...

[hjtrbo]

It prefills so when you tap up it reduces the delay to shift.

[hjtrbo]

Also not aware of where that behaviour would be changed in HP. The tunes that i've looked at in SCT and EFI also do not have that as a tuning option.