I just had a rather interesting thought. I've heard several times from credible sources, that LTFT's are used as multipliers for fuel trims at WOT. When staging at the tree @ a drag strip. usually you've been idling for a little while, so your LTFT's are most likely very deep into the negatives (mine tend to hang exactly at -10.4). When the light turns yellow for the last time, most people thrash the throttle and bring it to WOT or close enough to hit PE mode. Wouldn't this obscure your WOT fuel trims? They will be using the -10.4 as the multiplier for PE mode, which I would think is wrong. Or am I missing something?

 

Your correct. Consider you spend most of the time at the drag strip starting, stopping and idle.
The engine uses the STFTs at 10 minute intervals and each time engine is turned off the STFT is erase. Eash start then and sitting idle the fuel trim average consists of mostly idle so if the engine tends to run too rich or lean at idle then that is now what is used during launch.
Also your STFT lets say is now too rich, lets say -15 which says PCM reduces fuel by 15%, you now do a burnout and that skews the fuel trim more since you do a short blast and then sit at idle again at the line.
Now when you launch the fuel trims are really wacko.

I use a Ease PCM scanner that is bi directional, thus allows me to send a command to PCM and tell it to reset fuel trims so at least it no longer consists of a STFT madeup of idle conditions.

Best case is to have a PCM flash for drag use and bias tuning to that long idle condition so that by launch the STFTs are bias for good performance launch else engine will load up or be too lean causing knock and front end drops.

PE (WOT) adds to what ever trims were right before PE, if it was lean or rich.

 

Wait a second here. I thought that LTFTs only affected PE if they were positive. If the LTFTs are negative they just are not added to PE.

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Nic00Z28M6:
<strong>Wait a second here. I thought that LTFTs only affected PE if they were positive. If the LTFTs are negative they just are not added to PE.</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Who says ?
There is nothing in PCM that gives that condition and its a added value to whatever condition the fuel trim was at at the time it goes to PE.
I have done hundreds of tests and if LTFTs are high or low enough from zero at the time of PE I can see - or + values in LTFT during PE.

PCM zeros out STFTs and freezes LTFT when going into PE, and then adds fuel to that LTFT value.

I have seen rich PT but a lean PE and also other way around depending on how far LTFT is.
STFT handles -/+ 10%, if zero'd out that still leaves LTFT with a value if it was beyond the 10%, thus under 10% then LTFT will be zero at PE, if not then PE adds to that.

Question is for your engine makeup/tune, do you want PE richer or leaner and control that by what you force LTFTs to be as going into PE esp if your just using a MAFT since that is your only control point.

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Team ZR-1:
<strong>[QUOTE]Who says ?</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Virtually everything I have read on the internet says that if the LTFTs are negative when going into PE, it doesn't add nor subtract fuel from PE therefore creating a more consistant fueling, which is why most suggest going slightly negative on your LTFTs when tuning. If its positive it adds that amount to PE and may be inconsistant from run to run. That is just what I have read from just about everywhere and pretty much everybody agrees on it, with the exception of your post. Thats why I am confused. Where did you get this data from? Is there a GM documentation out there somewhere that explains how the PCM relates to LTFTs and PE?

 

Hey JR, your post says:

Quote:
--------------------------------------------------
The engine uses the STFTs at 10 minute intervals and each time engine is turned off the STFT is erase
--------------------------------------------------
But the book you recommend by Probst says it uses it many times per second. Now I am confused; which is correct? I thought the LTFT's were updated every 10 Minutes.

 

Do you happen to have a log file of negative LTFT corrections being added at WOT? I have about 100 logs on my laptop of cars that average from -2 to -8 and see LTFT of 0 at WOT. The *only* time I see values are WOT are when the LTFT are positive.

It has also been verified through dissasembly that such is the case on LT1's, L98's, etc.
I would be very interested in seing any hard evidence to the contrary - but then I am at a loss to explain all the log files I have?

Chris

 

my 02's always read ALOT richer at the track then on the street.
and my Ltrims are around -10 at idle.
think i should lean that out a bit ?

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by McRat:
<strong>So this is probably why my 02's read .89-.91 on the street at WOT and the dyno, then .92-.94 at the Dragstrip?

DOH!!! I wondered what was happening!!! I need to read more! I assumed that since my 02's were good on the street, that I wouldn't need to adjust them...</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Your A/F ratio should generally be pretty consistent between the street and track, unless you are doing something different. Be aware that if your o2 temp is different that itself can account for the difference in voltages (a/f aside).

On the dyno you will run richer since the load is reduced.

If your l-trims are positive this can definitely carry over to WOT. I still believe that negative l-trims are not carried over - based on many logs and dyno runs, and no evidence to the contrary.

Chris

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by ChrisB:
<strong>If your l-trims are positive this can definitely carry over to WOT. I still believe that negative l-trims are not carried over - based on many logs and dyno runs, and no evidence to the contrary.

Chris</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I agree.

 

I'll try and produce some log files. My thought to solve the problem was to brake-torque the car at 2000rpm (3000 stall makes this perfect, as you actually have a heavy load on the car, but arent moving). My LTFT's drop down significantly from -10.4 to -3.4 when I do this. I'd swear it pulls harder using this method. Probably not fair considering stalling the stall converter is the whole idea <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" /> Ofcourse it's going to pull harder.

 

Also, I forgot to mention. I see my LTFT's fluxuate every 10 seconds (not minutes). My STFT's seem to fluxuate every second. The LTFT's respond exactly 10 seconds later when I apply part throttle, so perhaps there are some differences with the PCM's TeamZR-1's had experience with. This is a 1999 T/A.

 

So this is probably why my 02's read .89-.91 on the street at WOT and the dyno, then .92-.94 at the Dragstrip?

DOH!!! I wondered what was happening!!! I need to read more! I assumed that since my 02's were good on the street, that I wouldn't need to adjust them...

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Vettepartz:
<strong>Hey JR, your post says:

Quote:
--------------------------------------------------
The engine uses the STFTs at 10 minute intervals and each time engine is turned off the STFT is erase
--------------------------------------------------
But the book you recommend by Probst says it uses it many times per second. Now I am confused; which is correct? I thought the LTFT's were updated every 10 Minutes.</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">What they are saying in that book is that if the STFT goes over its +/- 10% window for more then ten seconds then a value change to the LTFT register is made.

That does not mean the PCM is adjusting every 10 seconds. Read pages 34 and 35.
It also says that it takes about 1 hour for full cycle average of LONG Term averages.
Loss of power to PCM takes at least 10 minutes before partial relearn. The STFT values are in RAM, when car is off, RAM is cleared thus when engine is started again the STFT has to be rebuilt again and you would not want it to average in just a minute or 2 for then the idle trim would dictate PT from the get go.
Read page 111,
"computer (PCM) adapts every 10 minutes".

If the PCM changed real time, every hill, bump, lift of g-pedal would cause the engine to be not very smooth and not fun to ride in.

Also with a CPU of only 16.7 Mhz, a bus speed of 10.4K baud and there just is not enough cycles to do constant changes, lets not forget the PCM came about for EPA use not for assuring perfect tuning and why EPA FTP allows AFR to be 1 1/2 times over the FTP values.
Consider the I/M 240 tests alone as per EPA have to respond over the bus at least 2 times a second, so with all the other modules talking there just is not time for the PCM's CPU to haul ***.

Long Term means just that, over time what should it correct for engine lives of STFT values to function and LTFT makes larger AFR over time changes.

<small>[ August 22, 2002, 01:10 AM: Message edited by: Team ZR-1 ]</small>

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Nic00Z28M6:
<strong> </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Team ZR-1:
<strong>[QUOTE]Who says ?</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Virtually everything I have read on the internet says that if the LTFTs are negative when going into PE, it doesn't add nor subtract fuel from PE therefore creating a more consistant fueling, which is why most suggest going slightly negative on your LTFTs when tuning. If its positive it adds that amount to PE and may be inconsistant from run to run. That is just what I have read from just about everywhere and pretty much everybody agrees on it, with the exception of your post. Thats why I am confused. Where did you get this data from? Is there a GM documentation out there somewhere that explains how the PCM relates to LTFTs and PE?</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">IMHO, don't believe everything you read in cyberspace, look back a while ago when everyone was saying to buy a aircleaner setup that hung a few inches off the ground, was not until engines were sucking water in and trashing engine that fad went bye bye or the new fad of cranking torque management to the roof and a guy totalled his C5 out because TM did not kick in and car went into a wall.

Look at the PE (WOT) within the PCM calibration, it simply does what EPA agreed to, WOT is seldom used thus smog rules do not apply so simple process, add fuel to go richer, there is no decision if LTFTs are rich or lean since engine lives on STFT which is a 10% window, not 22-25% LTFT is.

The PE scaling is narrow, thus if LTFT is -20, PCM is taking 20% fuel out, now go into PE and the value added may not be enough and you still end up too lean at WOT.

I know I forced/relearned LTFT way rich, at the drag strip and my O2s went from .890 to .850's.

I also have forced LTFTs lean and ended up with too rich at WOT.

<small>[ August 22, 2002, 01:34 AM: Message edited by: Team ZR-1 ]</small>

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by ChrisB:
<strong>Do you happen to have a log file of negative LTFT corrections being added at WOT? I have about 100 logs on my laptop of cars that average from -2 to -8 and see LTFT of 0 at WOT. The *only* time I see values are WOT are when the LTFT are positive.

It has also been verified through dissasembly that such is the case on LT1's, L98's, etc.
I would be very interested in seing any hard evidence to the contrary - but then I am at a loss to explain all the log files I have?

Chris</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I have analyzed close to 1,800 PCM recording, at least 30 minutes per within the last 2 years.
I do this 7 days a week, several a day and finding LTFT at zero only ocurrs if the LTFT is within the -/+ 10% of the STFT's window. If STFT is zero'd out going into PE then of course LTFT will be reduced up to that amount can can be frozen at zero but if outside that 10% then what is left over is frozen during PE and if you add a value to a negative value, well the traces below show results of too lean or too rich LTFTs during the locking of STFTs entering PE process.

Review the LTFTs while in WOT :

<img src="http://teamzr1.com/pcmecm/tlean.gif" alt=" - " />

<img src="http://teamzr1.com/pcmecm/trich.gif" alt=" - " />

I have seen this countless times with a ill tuned PCM calibration and again shows that the condition of part throttle has a great effect as to how WOT performs, so again a lean PT will be adding fuel, enter PE adding more fuel and you end up with a lean PE but a rich WOT or visa versus.

As to ECM based engines, the cpu was even slower and only had half the bus width so I doubt ECM had much conditional conditions.
L98s was worse only a 8 bit bus and far less control.

<small>[ August 22, 2002, 02:30 AM: Message edited by: Team ZR-1 ]</small>

 

So after all this what settings are we shooting for here ? <img border="0" title="" alt="[Confused]" src="images/icons/confused.gif" />

 

Do we want to be on the lean side at PT or on the rich side ? <img border="0" title="" alt="[Confused]" src="images/icons/confused.gif" />

 

That log file is very interesting! Could you post it up for download?

I went and check every log file I had though, and can still say I have *never* seen a negative l-trim at WOT. Even on logfiles with things like 40lb/hr injectors on stock calibrations.

I even went out and tried to duplicate it - achieving a l-trim that varied by more than 10% from the s-trim, but still wasn't able to?

Has anyone else here seen a negative l-trim at WOT? Regardless I still think the best methodology is to tune for -2 to -8 or so on the l-trims (stabilized) at part throttle.

Chris

<small>[ August 26, 2002, 10:09 PM: Message edited by: ChrisB ]</small>

 

Since this thread was about drag racing using a MAFT,
as I mentioned most of the time the car is sitting with a hot idle and IAT and you may find that your fuel trims are too lean, thus when you launch, your PCM is trying to cure the lean trims by going rich.
PCM then enters PE and you end up with a too rich WOT.

If that is the case then tune for a bit richer idle (base on MAFT) which forces trims rich which commands PCM to take out fuel, enters PE and you now have a leaner WOT.

Use knock values to know if your too lean, and look at timing, and if all is well best case is about 26-28 degrees during WOT.

You may also find that when you do a burnout after sitting idle it will effect then the STFTs which again effects once you go PE, so record PCM during your runs and tune from them.