WOT shifting problems at the track

[Skinny]

I've been chasing down this problem for a year now. Immediately after launch (at WOT) it will shift to 2nd, then immediately 3rd. After a second or two it will downshift to 2nd and then continue the run. Here's an example of a pass I made last night:

After WOT, shift to 2nd at 4400rpm, 16mph.
Shift to 3rd at 4500rpm, 24mph.
Downshift to 2nd at 4800rpm, 37mph.

I'll work on posting the tune and the log of that pass. The only other issue I see occuring is about 4* of KR right at the launch (which I'm pretty sure it attributed to a banging exhaust), but that wouldn't cause it to immediately shift, would it?

[Skinny]

Here's the tune and the log for that pass:

[MMGT1]

It may be the conflicting table in your shifts. You have different values for your 100% shift in WOT and Normal. I would copy and paste the WOT shift speed numbers to your part throttle 100% shift numbers. Might be the problem?? You should blend the part throttle shift speed from 75% up to 100 as well so it equals out at 100% throttle.

[Skinny]

I never thought of that. Just assumed it would ignore the part throttle table when at WOT. I made the changes. A shame that it will be a week before I can see if it helps. I really think something else is the issue here though. There's no reason it should ever shift to 2nd gear at 14mph at 100% throttle. The lower end of my part throttle shifts even has it at 18mph. And the 2-3 shift occurring at 24mph. At light throttle driving through town it shifts later than that.

[MMGT1]

Have you changed tire sizes? If you have, double check your math on the changes to make sure your reading correct as well. I had mine off by a bit and RussK cleaned it up for me. No more shift problem(I had a similar issue)

[Skinny]

At the track I run a 26x10.0 ET Drag. On the street a 315/35/17 radial. Tire size is set at 25.69. Should I change it to 26 when I'm at the track? I kinda figured they were close enough.

Since this problem started for me, I have changed tranny's (one built 4L60E to another), changed tires (27" ET Street to 26" ET Drag), changed rear end gear (4.10 to 3.42), and I've changed WOT shift points all over the place to no avail. I tried zeroing out the shift RPMs and leaving only the MPH table. I tried zeroing out the MPH table and leaving only the RPM table, all with the same results.

[MMGT1]

I replaced a transmission that had nothing wrong with it because of my tire size. That really sucked!!

[Sneakyws6]

I can tell you right now I just found this same issue last night in my tune as well.

I would jump on it at 65 on the highway and te truck would drop down into 3r, then it would drop into 2nd and fee like it was barely pulling until if shifted back into 3rd.

I would take the shift speeds for the WOT map for upshift and down shift, take those speeds and input them into the part throttle map in the 100% part of the map, then work backwards from there adjusting the part throttle maps until the shift speeds and throttle perchatages are in line.

I would bet this will fix your issue.

[Sneakyws6]

Here is what I see now looking at the map...

WOT map

1>2 - 46
2>3 - 93
3>4 - 255 (remove this as it is the top speed limiter for the vehicle)
2>1 - 42
3>2 - 90
4>3 -155

Part throttle settings now for 100% map:

1>2 - 38
2>3 - 66
3>4 - 140
2>1 - 35
3>2 - 63
4>3 - 155

This is exactly what is causing your issues. the WOT shifts dont match the 100% shift points in the part throttle map. Figure out what shift points you want in your WOT map, then put those in the 100% part throttle map and like I said above work backwards to the 0% part throttle map for each gear.

Just a reference like the top speed limiter, even though someone may remove it form thespeed limiter section, it is still referenced in maybe 3 or 4 other maps as well which will still limit you to the top speed by doing something like not letting the tranny shift

Also go into your shift properties tab, change your positive force motor setting for the 100% map to 0, you went the wrong way on that map as well. Same thing for the negative map as well.

Next raise the max pressure in that same tab to 96 as well.

Now if you have not changed your desired shift time tables there may be some room there as well to decrease the amount of time it takes the tranny to shift into the next. This will make the shifts more abrupt but it will not slip the shifts as bad.

Now go to the TCC tab, change the max duty cycle to 100%, then change the minimum to 90%

Maybe Russ will chime in on anything I missed. He is the one that really helped me the most on my tranny settings.

[Skinny]

I made that change as suggested by MMGT1. Updated file attached.
As mentioned above, I can't really test the changes until next wednesday at the track, as that's the only place that I can get a real launch from a stop.

[Sneakyws6]

See my above edited response as well, there are some other areas you can adjust and improve the tranny tune as well.

With those setting you realize that the tranny will never shift from 3rd to 4th with the speed set to 255 dont you. Are you doing this to keep from shifting into 4th right before the traps?

[Russ K]

At the track I run a 26x10.0 ET Drag. On the street a 315/35/17 radial. Tire size is set at 25.69. Should I change it to 26 when I'm at the track? I kinda figured they were close enough.

Since this problem started for me, I have changed tranny's (one built 4L60E to another), changed tires (27" ET Street to 26" ET Drag), changed rear end gear (4.10 to 3.42), and I've changed WOT shift points all over the place to no avail. I tried zeroing out the shift RPMs and leaving only the MPH table. I tried zeroing out the MPH table and leaving only the RPM table, all with the same results.

You had a lot of problems in your trans tune. Note that 24.91 is the value used for a 275/40/17 or a 315/35/17 tire, not the advertised dia. Then you had the calculated tire dia as 34.61! Your shift speed tables were all off the map!

Your force motor tables were higher than stock (less line pressure). Is there a shift kit installed? Do you have the Corvette servo?

Try this trans tune.

Russ Kemp

[Skinny]

Thanks guys...
To answer some of the questions: I left the 3-4 shift at 100% at 255 as to not shift into 4th at the track (that's really about the only time this car will see WOT). There is a modified corvette Servo installed and a sort of customized Transgo shift kit. The trans is a Century Transmission built 4L60E. THIS ONE

As far as the tire sizes, I typed in 315/35/17 in the tire size calculator and it gave me 25.7, so I put 25.7 in for tire size and 3.42 in for the gear and that's what I got. How do I change the calculated tire size? Also, since I want this tune optimized for the track (where I run a 26x10.0 tire) should I enter 26.0 as the tire size?

Thanks for the changes Russ, I'll try it out. I notice a couple of changes in the TCC apply speeds that might give me problems though. With this huge cam, I cannot drive the car under 60mph with the TC locked in 4th(really bad cam surge). I usually have to put it in 3rd gear and drive around town like that. I wanted to try to have it only lockup in 4th after 60mph, that way I can drive through town at 40-50mph in 4th gear with the TC unlocked. Can I make that happen and what should I change?? (I have another thread in this section about the TCC apply issues)

BTW, I'm flashing this tune onto my car now and I'll go for a drive in a bit.

Thanks again for your help.

[Sneakyws6]

You Just Need To Raise The Tcc Speed Value That It Locks In The 4th Gear Map To A Speed Above 60 For Most Of The First Cells, That Will Keep It From Locking Until You Get Over That Mpg You Put In Those Cells.

[Skinny]

I tried that. Even with the value over 60MPH, I still saw lockup near 45mph. Of course that was before all of Russ K's changes maybe it will work now...

[MMGT1]

Russ has a handle on those tables, believe me! You should be pleasantly surprised when you try his set-up

[Skinny]

Wow, it's actually doing what I tell it to now. Thanks Russ! I set the 4th gear TCC apply speed to 61mph at the lower end and the release speed to 55 and now I can actually cruise through town at 40-50mph unlocked in 4th gear. Before it would still want to lockup at 45mph with the same settings. I actually had to leave it in 3rd when driving under 60mph.

Now one question- Since 25.7" (the number given to me by the tire size calculator) evidently isn't correct for a 315/35/17, does that mean that 26.0 would not be correct for a 26x10.0 ET Drag tire?? What should I enter for tire size with that tire???

[Sneakyws6]

Here is the correct tire formula. I will use your 315/35/17 as a example.

315 x .35=110.25mm
110.24mm \ 25.4 to convert to inches = 4.34" of sidewall.
4.43+4.34+17=25.68"

I will have to dig some more to convert the drag tire into the correct conversion number.

[Skinny]

Here is the correct tire formula. I will use your 315/35/17 as a example.

315 x .35=110.25mm
110.24mm \ 25.4 to convert to inches = 4.34" of sidewall.
4.43+4.34+17=25.68"

I will have to dig some more to convert the drag tire into the correct conversion number.

This is the nuber that the tire hight calculator in the HPTuners software gave me. However, Russ K mentioned:

Note that 24.91 is the value used for a 275/40/17 or a 315/35/17 tire, not the advertised dia.

What I'm wondering is where 24.91 that Russ refers to comes from.
As for the drag tire, the tire is measured and sold by it's height, so the actual tire height should be 26". The only reason I ask what to enter for that tire is the discrepancy between the advertised diameter and the smaller value that Russ K says to use for the 315/35/17.

[Russ K]

24.91 is the stock tire dia used in the GM calibration, I don't know why they don't use 25.7.

The formula for a different dia tire is; new advertised tire dia/old advertised tire dia* the stock tire dia value.
So 26/25.7*24.91=25.2, then enter 25.2 as the new tire dia when using 26" tall tires.

Russ Kemp