LTFT effect on WOT, give me understanding
10-19-2005, 07:20 PM
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LTFT effect on WOT, give me understanding Since my header install a while back, I see my LTFTs anywhere from +.8 to + 5 at highway speeds.
Let me state also I have no way to tune at this time, but would if there was someone local. Considering a mail tune.
Okay:
A; my understanding is that the pcm IS compensating for the lean condition shown by the + LTFTs at say, a steady cruise, and is probably fairly sucessful.
B; And, once I go WOT the fuel enrichment is based on the LTFTs, and the pcm locks the enrichment based on those and it's programmed look-up tables, w/o necessarily giving optimum results?
I know that my O2s average from .910 - .920 at WOT, indicating to me (based solely on pre-header readings) that I'm running rich at WOT.
Thanks to anyone that can provide some understanding.
gwj
Let me state also I have no way to tune at this time, but would if there was someone local. Considering a mail tune.
Okay:
A; my understanding is that the pcm IS compensating for the lean condition shown by the + LTFTs at say, a steady cruise, and is probably fairly sucessful.
B; And, once I go WOT the fuel enrichment is based on the LTFTs, and the pcm locks the enrichment based on those and it's programmed look-up tables, w/o necessarily giving optimum results?
I know that my O2s average from .910 - .920 at WOT, indicating to me (based solely on pre-header readings) that I'm running rich at WOT.
Thanks to anyone that can provide some understanding.
gwj
10-19-2005, 08:04 PM
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That range of LTFTs means your WOT fueling
has a 5% range of variation as well. The 5%
will put you (by commanded) 11.8:1 to 11.2:1
without a tune and with all sensors reading true.
Your O2 voltages have no known correlated AFR
and are also very heat variable. It's well richer
than stoich but relative to best HP/TQ, who knows?
has a 5% range of variation as well. The 5%
will put you (by commanded) 11.8:1 to 11.2:1
without a tune and with all sensors reading true.
Your O2 voltages have no known correlated AFR
and are also very heat variable. It's well richer
than stoich but relative to best HP/TQ, who knows?
10-20-2005, 05:58 AM
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Originally Posted by jimmyblue
That range of LTFTs means your WOT fueling
has a 5% range of variation as well. The 5%
will put you (by commanded) 11.8:1 to 11.2:1
without a tune and with all sensors reading true.
Your O2 voltages have no known correlated AFR
and are also very heat variable. It's well richer
than stoich but relative to best HP/TQ, who knows?
has a 5% range of variation as well. The 5%
will put you (by commanded) 11.8:1 to 11.2:1
without a tune and with all sensors reading true.
Your O2 voltages have no known correlated AFR
and are also very heat variable. It's well richer
than stoich but relative to best HP/TQ, who knows?
10-20-2005, 09:31 AM
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jimmyblue,
if Im wrong let me know, but if his LTFTs are +5 to +8 that means his VE table (well those cells) are off by that much and the LTFT is adjusting it that much. But on Wide open throttle only the VE is looked at. So during PE enrichment, if his LTFTs are off by say +5% then he is running lean already and during PE is still going to be lean. So if he was say 12.5:1 PE on that cell and the LTFT is +5% then wont the Commanded be closer to 12.8-13:1 AFR since the VE is not correct?
Dixit
if Im wrong let me know, but if his LTFTs are +5 to +8 that means his VE table (well those cells) are off by that much and the LTFT is adjusting it that much. But on Wide open throttle only the VE is looked at. So during PE enrichment, if his LTFTs are off by say +5% then he is running lean already and during PE is still going to be lean. So if he was say 12.5:1 PE on that cell and the LTFT is +5% then wont the Commanded be closer to 12.8-13:1 AFR since the VE is not correct?
Dixit
10-20-2005, 02:43 PM
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Originally Posted by Dixit
jimmyblue,
if Im wrong let me know, but if his LTFTs are +5 to +8 that means his VE table (well those cells) are off by that much and the LTFT is adjusting it that much. But on Wide open throttle only the VE is looked at. So during PE enrichment, if his LTFTs are off by say +5% then he is running lean already and during PE is still going to be lean. So if he was say 12.5:1 PE on that cell and the LTFT is +5% then wont the Commanded be closer to 12.8-13:1 AFR since the VE is not correct?
Dixit
if Im wrong let me know, but if his LTFTs are +5 to +8 that means his VE table (well those cells) are off by that much and the LTFT is adjusting it that much. But on Wide open throttle only the VE is looked at. So during PE enrichment, if his LTFTs are off by say +5% then he is running lean already and during PE is still going to be lean. So if he was say 12.5:1 PE on that cell and the LTFT is +5% then wont the Commanded be closer to 12.8-13:1 AFR since the VE is not correct?
Dixit
Remember, part throttle is the only time the PCM has feedback from the O2's. Therefore, the PCM is guessing that it's lean up top since it thinks it's lean down low (whether it's really lean or it's a false reading because the O2's aren't hot enough). The only real way to tell if he's lean up top is with a wideband. Either way, because of this guessing game, the PCM wants to play it safe. So, it will dump in extra fuel based on the positive fuel trim cells that are close to WOT (which I believe are 8, 9, 12, 13, etc.). In theory, he's probably slightly leaner than stock at WOT since he added the headers. But, more than 5% leaner? Probably not. Adding in that extra fuel more than compensates for his part-throttle, lean condition. That's my understanding of why people run a little richer at WOT after header installs...TIFWIW.
10-20-2005, 03:03 PM
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WOT above 4000RPM only sees the MAF if MAP
is steady (and it should be).
The issue is that low-RPM errors (VE table, cold
O2s) have nothing much to do with the accuracy
up top (MAF, fuel fade). Sure, the PCM tries as
best it's able, by retaining the last known trim as
a safety measure (if positive). But the validity of
it, is questionable. WOT is WOT, so which is the
right LTFT to overlay, given that different pre-WOT
FTCs can give you variously 0, +5, +8%?
Trick question
- the right answer is zero, and
tune WOT by WOT results, not with some randomly
selected residue.
A stock PCM PE table commands 1.25 fuel air mult.
If he tuned away from that then maybe he'd get 12.5
(a good safe, maybe conservative number). But throw
+5% on top of 11.8:1 and it's only worse. Fuel air
multiplier is what's scaled by the +LTFT and that is
14.7/AFR. +LTFT makes AFR go down.
is steady (and it should be).
The issue is that low-RPM errors (VE table, cold
O2s) have nothing much to do with the accuracy
up top (MAF, fuel fade). Sure, the PCM tries as
best it's able, by retaining the last known trim as
a safety measure (if positive). But the validity of
it, is questionable. WOT is WOT, so which is the
right LTFT to overlay, given that different pre-WOT
FTCs can give you variously 0, +5, +8%?
Trick question
- the right answer is zero, andtune WOT by WOT results, not with some randomly
selected residue.
A stock PCM PE table commands 1.25 fuel air mult.
If he tuned away from that then maybe he'd get 12.5
(a good safe, maybe conservative number). But throw
+5% on top of 11.8:1 and it's only worse. Fuel air
multiplier is what's scaled by the +LTFT and that is
14.7/AFR. +LTFT makes AFR go down.
10-21-2005, 06:54 PM
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Speaking only of my + LTFT at cruise condition......
So, if I'm reading this right, the algorithm /look up tables programmed into the software will play it safe and dump extra fuel in
at WOT. Which with any set of variables affecting LTFT
and any other factors the pcm sees that are programmed
as "relevant" ,at the time WOT begins there will always be
some variabilty for the A/F multiplier.
....which may or may not have a gross effect on HP.
JB, to clarify, you're saying that for the MAF to have any effect on the pcm "decision" as to the A/F mix -above 4K rpms- it looks for a steady MAP? If not it falls back to SD?
Thanks, guys.
gwj
So, if I'm reading this right, the algorithm /look up tables programmed into the software will play it safe and dump extra fuel in
at WOT. Which with any set of variables affecting LTFT
and any other factors the pcm sees that are programmed
as "relevant" ,at the time WOT begins there will always be
some variabilty for the A/F multiplier.
....which may or may not have a gross effect on HP.
JB, to clarify, you're saying that for the MAF to have any effect on the pcm "decision" as to the A/F mix -above 4K rpms- it looks for a steady MAP? If not it falls back to SD?
Thanks, guys.
gwj
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10-21-2005, 07:14 PM
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...after reading some other threads today, I feel somewhat confused. (maybe a normal condition). I obviously do not understand how sensor components -beside the MAF- allow the pcm to make an adjustment to percieved air flow. It would seem to my dumb *ss that the MAF output would give the pcm what it needs to adjust A/F ratio, especially at a steady low rpm cruise. WOT I can see as an entirely separate condition. Please point out the fallacy in my thinking.
gwj
gwj
