? About Timing
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Staging Lane
Joined: May 2008
Posts: 96
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? About Timing Hey guys, I've had my car for a little over a month and i bought the car with the mods in my sig. I was aware that the car had 3.73 gears but figured it had been tuned for them. Well it wasn't, so it hits off the rev limiter unless you let off slightly before it shifts during wot. So i had plans to purchase LT1 EDIT to tune it for the gears. So while scanning the car before i made my purchase i noticed that it sees 36 degrees of timing at wot. Now i have never heard of that much being ran on a N/A V8 but maybe I'm wrong. So my question is what is the most you guys see? I am starting to sense the need for a new opti and i can't really afford both the software and the opti right now so any input would be great, thanks.
at or around 36 degrees at WOT on your LT1 should be okay, assuming you're not getting any knock. just a word of advice get tunercats instead of lt1edit. it's cheaper and has a lot more tables.
Since the pcm gets it's speed input from the tailshaft housing not at a tire, the lack of tuning should not be the cause of hitting the rev limiter. I do not understand why people think it works like that though it is a common mistake. For it to work like that the speed sensor whould have to be driven off a tire.
For your sake I hope there was a failed poor attempt at reprogramming otherwise sounds like a mechanical issue either. Have seen ignition or fuel issues cause a no shift at WOT or could be the tranny itself.
For your sake I hope there was a failed poor attempt at reprogramming otherwise sounds like a mechanical issue either. Have seen ignition or fuel issues cause a no shift at WOT or could be the tranny itself.
On The Tree
Joined: Nov 2007
Posts: 130
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Please straighten me out if I'm incorrect. A stock PCM tune on a '94 A4 with 2.73 rear has 2-3 (WOT) kickdown table set at 92 mph or around 5700 RPM, if gears are changed to 3.73's the same 92 mph shift point would require 7790 RPM. Therefore the tune would have to be lowered to about 67 MPH to stay within stock 5850 rpm fuel cutoff.
The computer has no friggin idea the car is going slower. Like I said this theory would only work if the speedo/pcm were wheel driven.
Look at it this way, the pcm get's it's vehicle speed info BEFORE the axle ratio(or changing it) can affect anything.
Like I said this is a VERY common missunderstanding. One of those things folks just fail/refuse to understand.
Look at it this way, the pcm get's it's vehicle speed info BEFORE the axle ratio(or changing it) can affect anything.
Like I said this is a VERY common missunderstanding. One of those things folks just fail/refuse to understand.
Please straighten me out if I'm incorrect. A stock PCM tune on a '94 A4 with 2.73 rear has 2-3 (WOT) kickdown table set at 92 mph or around 5700 RPM, if gears are changed to 3.73's the same 92 mph shift point would require 7790 RPM. Therefore the tune would have to be lowered to about 67 MPH to stay within stock 5850 rpm fuel cutoff.
It is likely that they are off by roughly the same amount, and so for most people the shifting will still probably work. But if the speedo is off by just a few mph less, than the computer will never see 92mph. Basically since the speedo and shift points are both off when you change gears, it's a toss up as to whether or not it will work. If that makes sense.
Last edited by infinitebird; Jun 29, 2008 at 02:38 PM.
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LOL, yeah that's exactly the problem! Or at least part of it.
Shifts are based on MPH in the shift tables as well as RPM.
The computer still thinks the car is going way faster than it really is, but the shift points are also WAY higher than they should be. That's why it needs to be retuned for different gears.
If you don't retune it, basically you have a situation where the computer does not command the shift until a speed that you may never reach in that gear with the different axle ratio, depending on how much the speedo is off.
You can see this makes sense when the OP says he has to "let off" to get it to shift. When you let off, the shift point MPH is being referenced based on a lower TPS% value, which has a lower MPH requirement that the computer will see in that gear.
The weird thing is, it appears that it's variable depending on the setup. Not everyone will have this rev limiter problem. It depends on if the shift point is off more than the speedo. With larger changes in gears, it appears that the shift points are off more, so this problem is exaggerated. If the shift point is not off by more than the speedo, then it will still shift before redline. Otherwise it won't.
Shifts are based on MPH in the shift tables as well as RPM.
The computer still thinks the car is going way faster than it really is, but the shift points are also WAY higher than they should be. That's why it needs to be retuned for different gears.
If you don't retune it, basically you have a situation where the computer does not command the shift until a speed that you may never reach in that gear with the different axle ratio, depending on how much the speedo is off.
You can see this makes sense when the OP says he has to "let off" to get it to shift. When you let off, the shift point MPH is being referenced based on a lower TPS% value, which has a lower MPH requirement that the computer will see in that gear.
The weird thing is, it appears that it's variable depending on the setup. Not everyone will have this rev limiter problem. It depends on if the shift point is off more than the speedo. With larger changes in gears, it appears that the shift points are off more, so this problem is exaggerated. If the shift point is not off by more than the speedo, then it will still shift before redline. Otherwise it won't.
Last edited by infinitebird; Jun 29, 2008 at 02:40 PM.
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Verdad Gallardo It is as simple as the fact that the pcm gets it's VSS info before the gears can change anything. The car will drive absolutely perfect, only problem from not retuning the the speedometer. Trust me I have done gear ratio change reprogramming a few times.
If the speedo were reprogrammed for and you left the MPH the same then the problems being discussed here would occur, but NOT with an untouched pcm.
If the speedo were reprogrammed for and you left the MPH the same then the problems being discussed here would occur, but NOT with an untouched pcm.
All of this inaccurate information is why you need to get it tuned. All of the problems will go away after that.
Last edited by infinitebird; Jun 29, 2008 at 03:03 PM.
On The Tree
Joined: Nov 2007
Posts: 130
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I agree, I assumed incorrectly that the speedometer was re-programed. If the tune is ALL stock it 'should' shift correctly. The PCM would see correct RPM and speed (VSS) in 2nd and upshift , although the actual speed at the wheels would be +/- 30% slower than the VSS reading. The OP said he needed to choose between a tune or a new opti, probably should start saving up for both LOL.
