Anyone adjusting tire height in the TCM?
 

With this car being TQ based and with all the issues I have seen with TB blade closure and random TM events going down the track that I can't explain, I am curious to see if anyone has adjusted the tire height table in the TCM for their drag tires.

The stock tire is 26.83 on the car if you lay a straight edge across the top of it at full pressure and measure from the ground up. The TCM verifies this as 26.83 is what is entered in your speedo calculator. My M/T tire lists 28" but is really right at 27.5" at 18psi on the car. Roughly .70" difference but going through the traps my speedo says 116 when I am going 122-125. In my mind that has to be effecting something in the TQ tables which in turn could be effecting the TM tables. I know when I put the stock tires back on and leave my track tune in with TM turned back on, the car shifts about 200rpm higher than commanded and where it shifted at the track.

I have heard that when guys try to run a 26" tall front tire it throws the car all out of whack. These are big shops that live and die at the track. They said it messed up the front to rear ratio and TM kept kicking in. They had to go back to a 27.5" front runner to get the car to run. It would seem to me that if shortening the front does this then raising the rear would have the same effect.

Currently, no matter what I do TM comes in on my 3/4 and 4/5 shift. The funny thing is that the higher the rpm the more intrusive it is. I have every TM table I can find deleted for track runs and all of my allowed TQ tables are set above what the car is telling me it is generating.

My only hypothesis is that as mph and rpm diverge as the car goes faster the TCM/ECM think something is going on and try to slow the car down or get it back into line. In the lower gears the separation from what it is expecting to see is small and possibly allowable. As things get further out of whack as the speeds increase the computers don't allow it any more.

The other issue I could see is if you are commanding a certain tq from pedal position it could be bounced off of rpm and mph as an equation to decide the final allowed output. You can see this effect when you first smash the gas and the TB blade takes forever to open. The ECM/TCM are managing TB opening to give the greatest velocity based off of tables I may or may not have figured out yet. There is a table that sets the max allowed TB % open vs. RPM. It is way jacked up in the Vett's.

I haven't had time to work on any of this theory and it would take a bit to rework all of my shift mph's at the track for that tire to see what it is worth.

So if you have messed with this then please pipe up.

You should adjust it.

Stupid question: Is this with the traction control full-off (press and hold for 5 seconds)?

If so, it sounds like not all torque tables or settings are unlocked with the tuning software yet. Or, try a tire height change in the settings and see how that impacts your TB closures and TM events.

I don't seem to have any issues BUT I did have the same question as you because I don't want my odometer to read more miles than my car actually has on it. I'm curious about an answer to this as well.

Thanks Laynlo. Are you saying I should try it to figure it out or that you have done it and I should as well? Wither way I think I am going to make some adjustments and see what happens on the street.

I turn all the TC off. I hold down the button till both lights come on and then I hold it another 10 seconds or so.

Interesting the issue about the 26" Front Runner....I haven't heard or seen anyone mention an issue with it. Guess I shall find out if it is indeed an issue, as I have a set right here, although I'm running 26" in the rear as well. Was the issue of 26" Front runners paired with 28" rears?

I run 28 fronts and 28.5 rears

I have very limited data on what you are asking but I did adjust the tire height in the TCM to reflect the actual size.

Iirc I was seeing 6250(avg) on the shifts when it wasn't corrected, I made 1 pass after I entered the correct tire size and was seeing 6400. I can double check my log later tonight.

I did plan on experimenting more on this, my only pass though tells me that the correction did indeed raise the shift points.

Edit; my rear DR is listed at 28.7 but it only measured 28.5 on the car.

I went back and adjusted mine to 28.1 for my street and track .

Do it pray. Go into edit for the tire wizard and it will let you adjust it. Go baby go. I'm gonna run 10.50's this weekend so you better get that lt4 sc on there or your gonna be toast.

I believe there was a mismatch. Not sure if same front and back would fix it. Or if somehow the computer is reading mph or rev per mile for the front tires as well.

I know you will do it bro. You keep raising the bar and motivating me. I may have one more step to tackle before the blower. I have a theory I want to test on the N/A side before I totally give up on it. There is something I haven't seen others try and you know I have to be different. If it works the pay off could be pretty big. Only one way to tell. I have to bite the bullet and do it. After this last venture plays out then I will put my effort into the blower build. Hopefully I will have it done by the Fest.

This sounds like "Engine Drag Control" (EDC), a separate subsystem independent of competitive driving modes. It's described on pg 236 of the Owner's Manual

I haven't messed with the tire height settings - didn't know they were available - but I may have to do this too after getting slightly taller MT's. Thanks for bringing this up. It didn't occur to me 'til now.

--Cal

Good information. Thanks.

I hadn't considered this yet either. I'll have to ask Jordan about doing this for me. We're running the 20" 305s in back and conti dw's in 275 in the front. I'm sure I could use some adjusting in the diameter settings.