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Discussion Starter · #1 ·
Can anybody shed some light on how the WOT LTFT (22) gets updated. I seem to remember that C4C5 said the latest PCM software looks at O2s during WOT for 25% of the input.

What is the other 75% - MAF input?

Are the part throttle LTFTs (1-19) used in the algorithm for WOT (22) input?
 

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Interesting thread, but it also said that the computer eventually "re-learned" his new fuel pressure and brought his HP back to the starting point.
 

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Roger, Good to know your around to try to decifer what is really going on. The Z06 programing can and does seem to do strange things when measuring performance at the track. The effects on times is strange when trying to use a weather station. Humidity is an even biger factor than with any "real" race car, and the way you drive on the way to the track has a big efect. Trying to bracket race mine is testement to my mental illness.;):D Ric
 

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Discussion Starter · #6 ·
I know what happens when the dew point gets within 7-10 degrees F of the ambient temp and about the 100 g (.33 in hg) effect on the combustion process. I know how to tune around that. But I don't know how to predict the performance degradation. Is this what you are talking about?
 

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Roger Ramjet said:
I know what happens when the dew point gets within 7-10 degrees F of the ambient temp and about the 100 g (.33 in hg) effect on the combustion process. I know how to tune around that. But I don't know how to predict the performance degradation. Is this what you are talking about?
Can you let me know what happens? I mean I know what I feel in cold wet air, but I don't know if my ass is an appropriate measure of performance.

thanks
 

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Discussion Starter · #9 ·
Mike -

The water vapor condenses in the intake tract because of the lower pressure and combines with the fuel. The degraded fuel reacts differently - more slowly. When I have control, not the PCM, I add 2 degrees of ignition lead and add 2 or 3% fuel. This seems to minimumize the high water vapor presence. Of coarse this requires a weather statiion to monitor the conditions.

I don't have enough information to tell you how much performance is lost. We need bracket racers with weather stations to provide real data. SAE totally avoids this subject. They just say 'Dry Air'. Which means that you have to subtract the water vapor content form the barometer reading.
 

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Wouldn't the water vapor in the air act like water injection? What I would expect (and from what you've said) you can run more timing/compression/whatever as the engine won't detonate as easily.

I know the car feels much much stronger beneath 50 degrees, and even better when its a bit humid. In warmer weather, first gear won't spin the tires at higher rpm, but in the cold wet air, I get wheelspin starting about 5000rpm.

I don't know what just cold air does (well, ok so I do, but not at low altitude) without the humidity.

I was assuming that cylinder pressures would be higher, due to the dense air + water vapor.

I've been pretty interested in water or water/alcohol injection for the past few years, and it seems like foggy conditions would somewhat replicate it.
 

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Roger Ramjet said:
I don't have enough information to tell you how much performance is lost. We need bracket racers with weather stations to provide real data. SAE totally avoids this subject. They just say 'Dry Air'. Which means that you have to subtract the water vapor content form the barometer reading.
SAE does address humidity effect in their air correction formula. They have picked 0% RH as the standard baseline, but if you look at the formula you can see that the actual atm vapor pressure is subtracted from the baro pressure to give an effective "dry-air' pressure.

How do you know that the total effect of RH isn't covered by the SAE air correction formula? Seems to me if there were other significant effects of humidity, SAE would have addressed it.

It's hard for me to believe that water vapor is actually condensing inside the intake manifold. At WOT the pressure inside the intake system isn't much less than the atm pressure. And if the intake manifold is hot or warm, the incoming air will heatup some on its way to the intake valve. By the time the cylinder is filled at BDC, I highly doubt there is any condesed water in the mix :D.
 

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Discussion Starter · #12 ·
I think low humidities do contribute slightly to indicated HP. It's the higher partial vapor pressures that cause the problem. There is both a temp and pressure drop in the intake system. When operating close to the dew point the air can become saturated. I spent 2 days on a temp and humidity controled dyno to come to these conclusions.

OBTW I'm still looking for the WOT (22) algorithm!
 

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Roger, Yes that's what I was after. My tag weather station does a good job on predicting et's for my bracket car but is way off for my Z06 in hot humid conditions. I seem to slow down much more than I should. The station is within .02 for my camaro day-day but for the vette you can't even use runs from a good day to predict on a hot-humid day. My feeling was that the differance was probabley in the computer program of the car not the weather station. Ric
 

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Discussion Starter · #14 ·
PowerShifter -

TAG uses the slope/intercept method of predicting ETs from several past runs at different weather conditions. I would suggest that you try a system that does not use that method. Computech has a 1 past run prediction system.
 

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Functions of long term fuel trim

Hi there,
Well, after watching this post, and getting a mail from Roger, I felt the need to explain the functions of the LTFT, as you are curious.
Now, please understand, this only applies to closed loop operations. And, when you are NOT WOT. You see, when you are WOT, your engine is in power enrichment, and defaults.
Explanation follows.
LTFT is a group of cells, which are arranged by rpm and MAP sensor readings. Each cell acts like the STFT. As the operation of the engine changes, your cell changes accordingly, to determine on time of the fuel injectors.
Now, while the LTFT cell is chosen, it also compares STFT readings. If, for any reason, the STFT is out of 0%, the PCM will change the LTFT readings by 1 percent, and recalculate the equation. This process continues until the STFT is back at 0%, and stabilized. Both cells LTFT, and STFT, have limits within the calibration for your particular vehicle. If the LTFT, and STFT are out of ideal 0% by enough of a percentage, you may see a SES light, and you will have a lean or rich code, for which ever bank is having problems.
As for the o2 sensors, per page 125 of the fuel injection control manual, for LS1/LS6, your pre-cat o2s will be looked at for about 75% of the STFT equation. The rear o2 sensors will account for 25% of the STFT equation.
Under WOT, the PCM defaults the STFT to 0, and does NOT compensate for the added fuel. If this did not happen, the PCM would continue to lean out the engine, until the enrichment stopped, and then, the LTFT would be so far out of ideal 0%, that there would be drivability issues.
This is alot of the reason that the MAFT does not really work as well at WOT, as the LTFT does not use the MAF for cell calculations. IT will work at other times, however, as described above, not at WOT, for this reason.
Besttoall, as this is fact, and is not meant to be abrasive in any way. c4c5:eek2:
 

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Discussion Starter · #16 ·
Thanks C4C5 for the information.

I thought that LTFTs 1 through 19 were for part throttle operation but LTFT 22 was didicated to WOT. If that is true, is cell 22 only a fixed power enrichment value or does it get updated via some algorithm?
 

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Hi there,
I do not know what cell is used for the WOT pulsewidth programs.
It is always a fixed value, and is not updated, unless you have an updated calibration value installed by reprogramming the PCM.
Besttoall, c4c5:eek2:
 
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