What is going on?

[wooddaniel]

I just installed new 18"(back) 17"(front) rims on my Bird. before I got my tranny fixed. Now when I go WOT in 1st, the trans will not shift to 2nd without me letting off... same thing from 2nd to 3rd. 3rd to 4th is fine though... Could the speed sensor be off? I know the tranny is good because it was just rebuilt about 200 Miles ago. WTF is going on??? I am baffled. Never had this issue before. Anyone think a tune would fix it? Thanks for any advice guys. I took the stall out also so the stock torque converter is back in the car. That F&%er was causing the PCM to go nuts. I guess I just need to break down and spend the 500$ on a tune.

[wooddaniel]

Forgot to mention. When driving like a normal human being the transmission shifts fine.

[dklowrider]

not sure if this would be causing something that drastic but did you stick to the oem overall diameter when you bought the new tires? larger diameter would have the car going faster than it thinks it is. i would look into tuned by frost.

[wooddaniel]

not sure if this would be causing something that drastic but did you stick to the oem overall diameter when you bought the new tires? larger diameter would have the car going faster than it thinks it is. i would look into tuned by frost.

The tires are 305 35 18's on the back. I just adjusted the tire height and speedometer with my hypertech and it is still doing the same thing.

[Zboner]

I just installed new 18"(back) 17"(front) rims on my Bird. Now when I go WOT in 1st, the trans will not shift to 2nd without me letting off... same thing from 2nd to 3rd. 3rd to 4th is fine though... Could the speed sensor be off? I know the tranny is good because it was just rebuilt about 200 Miles ago. WTF is going on??? I am baffled. Never had this issue before. Anyone think a tune would fix it? Thanks for any advice guys. I took the stall out also so the stock torque converter is back in the car. That F&%er was causing the PCM to go nuts. I guess I just need to break down and spend the 500$ on a tune.


i would get a tune before anything else, but the trans is also a possibility, i went through about 5 in a period of 3 months it just kept eating them. then i sold the car my .02 cents

[wooddaniel]

i would get a tune before anything else, but the trans is also a possibility, i went through about 5 in a period of 3 months it just kept eating them. then i sold the car my .02 cents

Fu%&$ing 4l60e. I should have bought a Manual.

[Firebirdjones]

Those 305/35-18's are 26.40 inches tall when doing the math.

Depending on what your car started life with......

245/50-16's are 25.64 inches tall.....

275/40-17's are 25.66 inches tall.....

These heights very slightly with different tire manufactures.

The computer does use MPH as well as RPM to command shift points. You mentioned changing tire size in the tune with a handheld. I'm not sure what or how those do it other than punching in tire size.
In HPtuner it's a little more complex than that and sometimes you have to fudge the numbers to get it to work just right.
I would get with Frost and see what he has in mind. It could be just a simple tuning issue, and a mail order tune would do wonders for the car anyway.
If not then you might have something else going on,,,,like MAF issues, if you have a 98 especially,,,,,etc...

[wooddaniel]

Those 305/35-18's are 26.40 inches tall when doing the math.

Depending on what your car started life with......

245/50-16's are 25.64 inches tall.....

275/40-17's are 25.66 inches tall.....

These heights very slightly with different tire manufactures.

The computer does use MPH as well as RPM to command shift points. You mentioned changing tire size in the tune with a handheld. I'm not sure what or how those do it other than punching in tire size.
In HPtuner it's a little more complex than that and sometimes you have to fudge the numbers to get it to work just right.
I would get with Frost and see what he has in mind. It could be just a simple tuning issue, and a mail order tune would do wonders for the car anyway.
If not then you might have something else going on,,,,like MAF issues, if you have a 98 especially,,,,,etc...


I did the chalk line thing and came up with 26.46 Inches so I set the tuner to 26.50 inches. I set the shift points to 6400RPM, as that is where the powerband has been best in the past without a stall. The car was tuned previously by a shop in Jax. It ran fine until the transmission went out, and now that I got it back it is doing all this crazy shit. I am going to get a tune from Jeremy Formato (fasterproms.net) next time he is here and hopefully that fixes everything. He did an outstanding job with my GTO. If not, looks like I will be suing the shop and buying a transmission from performabuilt like I should have done in the first place. Between this and the bumper on my GTO this has been a really really shitty month.:rl:

[Firebirdjones]

The chalk line trick is a little misleading because there is weight on the tire and it's compressed slightly.
Instead of the chalk line trick try this.....it's accurate down to the decimal.


Tire example: 275/40-17


Section width x Aspect ratio (looks like this) 275 x .40 = 110

110 is your section height,,,now

Section height x 2 (looks like this) 110 x 2 = 220

220 is your combined section height,,,now

Divide combined section height by 25.4 (this is a constant number in the tire formula) (looks like this) 220/25.4 = 8.6614173

Simply take this new number and add your rim diameter of 17 inches. (looks like this) 8.6614173 + 17 = 25.661417

And that is your overall tire diameter, hope that helps.

[wooddaniel]

The chalk line trick is a little misleading because there is weight on the tire and it's compressed slightly.
Instead of the chalk line trick try this.....it's accurate down to the decimal.


Tire example: 275/40-17


Section width x Aspect ratio (looks like this) 275 x .40 = 110

110 is your section height,,,now

Section height x 2 (looks like this) 110 x 2 = 220

220 is your combined section height,,,now

Divide combined section height by 25.4 (this is a constant number in the tire formula) (looks like this) 220/25.4 = 8.6614173

Simply take this new number and add your rim diameter of 17 inches. (looks like this) 8.6614173 + 17 = 25.661417

And that is your overall tire diameter, hope that helps.


Thanks. My rim on the back is 18 though. Will that change anything?

[Firebirdjones]

Thanks. My rim on the back is 18 though. Will that change anything?

Ya,,,a 305/35-18 looks like this

305 x .35 = 106.75

106.75 x 2 = 213.5

213.5 / 25.4 = 8.4055118

8.4055118 + 18 = 26.405511

[wooddaniel]

Ya,,,a 305/35-18 looks like this

305 x .35 = 106.75

106.75 x 2 = 213.5

213.5 / 25.4 = 8.4055118

8.4055118 + 18 = 26.405511

Outstanding info. I appreciate the help. I don't know very much about transmissions, or the computers on these cars so I have had to play with it a little bit to get it somewhat working.

Just curious, will the hypertech retune the engine to stock? I selected the option not to change the tune as the car is cammed. I hope it didnt mess with the engine tune at all. I have only used this thing once like four years ago and dont remember too much about it. It seems to run the same

[Firebirdjones]

I used a hypertech on my wifes 97 Z years ago. When you flash in the pocket tune it holds the stock tune in the handheld indefinately, and is married to that car.
It does give you the option to reinstall the stock tune if you wish at any time.

I believe you can also do as you described,,,not installing the tune and just using it to adjust speedo settings for gear changes, tire changes etc...In that case it should not have messed with your custom engine tune.

[SSTODD]

What is the front tire/rim size? you need the same general height front to back also to not mess with the ABS and ASR systems.

[wooddaniel]

What is the front tire/rim size? you need the same general height front to back also to not mess with the ABS and ASR systems.

front is 275/40/17.

[SSTODD]

So you have about an inch of height difference from front to back. That could cause some computer issues I would think if the wheel sensors pick that up.

[wooddaniel]

So you have about an inch of height difference from front to back. That could cause some computer issues I would think if the wheel sensors pick that up.

Yeah, I thought the same thing but I didnt have this issue until I got the transmission fixed. It ran fine before with these tires on the old tranny

[PushroD]

I just wanted to point out, from my own experience, if you've had your car previously dyno tuned, and attempt to adjust the rear axle ratio or tire size with a handheld, IT WILL NOT WORK!! The handheld programmer WILL overwrite your ENTIRE tune!! :true:

All told, it cost me just as much money as I spent on my predator to get the car dyno tuned all over again.

I just wanted to save you guys the frustration.

btw, anybody wanna buy a lightly used predator programmer? LOL

[wooddaniel]

I just wanted to point out, from my own experience, if you've had your car previously dyno tuned, and attempt to adjust the rear axle ratio or tire size with a handheld, IT WILL NOT WORK!! The handheld programmer WILL overwrite your ENTIRE tune!! :true:

All told, it cost me just as much money as I spent on my predator to get the car dyno tuned all over again.

I just wanted to save you guys the frustration.

btw, anybody wanna buy a lightly used predator programmer? LOL

That sucks!!! Yeah I ended up having to get another tune. Luckily for me, Jeremy Formato is around my area . He really knows his shit when it comes to LSx tuning.

[clg82]

The tires are 305 35 18's on the back. I just adjusted the tire height and speedometer with my hypertech and it is still doing the same thing.

I had the exact same problem as you stated in your original post. Did you have a shift kit installed. I believe that your tires have nothing to do with it. Long story short I have the same size rims and tires as you, and had my tranny rebuilt added a shift kit as well. I'll elaborate more once you answer my question. And how we fixed it. I don't want to confuse you because I may give you misinformation.~Chris

[wooddaniel]

I had the exact same problem as you stated in your original post. Did you have a shift kit installed. I believe that your tires have nothing to do with it. Long story short I have the same size rims and tires as you, and had my tranny rebuilt added a shift kit as well. I'll elaborate more once you answer my question. And how we fixed it. I don't want to confuse you because I may give you misinformation.~Chris

I had a shift kit installed yes. Tuning was the issue with mine though. It is fixed now runs great.

[clg82]

I had a shift kit installed yes. Tuning was the issue with mine though. It is fixed now runs great.

Ahh I see, what is the tuning issue have to do with the tire size?

[JaycenK]

Did the tunner tell you why it did what it did or just tune and forget?

[wooddaniel]

The problem actually had very little to do with the tire size. It was mostly the shift points were not set correctly and the line pressure was a little low making the shifts all spongy. Also the Rev limiter was set too close to the command shift point so it would not allow the car to shift at WOT.

[clg82]

The problem actually had very little to do with the tire size. It was mostly the shift points were not set correctly and the line pressure was a little low making the shifts all spongy. Also the Rev limiter was set to close to the command shift point so it would not allow the car to shift at WOT.

So when they overhauled the transmission they messed with your tune?

[wooddaniel]

So when they overhauled the transmission they messed with your tune?

Basically yeah. The guy thought he knew better than the tuner and fucked a whole bunch of shit up. Sad thing is it was my dad's chief mechanic who did it when he installed the new tranny (my dad owns a transmission shop here in town) and didnt tell me he messed with the tune.

[clg82]

Basically yeah. The guy thought he knew better than the tuner and fucked a whole bunch of shit up. Sad thing is it was my dad's chief mechanic who did it when he installed the new tranny (my dad owns a transmission shop here in town) and didnt tell me he messed with the tune.

LOL What a dick

[wooddaniel]

LOL What a dick

Yeah. F#$%ed my shit all up. At least it works now. Im not too mad, I guess you get what you pay for. He put the tranny in for free so I cant get too upset.

[98Hardtop]

That is odd. I just had my car tuned by Frost and I have 275/40ZR17 front and 275/40ZR18 rear and my transmission was shifting soft. So soft it was less than the factory settings. He got that fixed very quickly for me :D