Fuel Trims (LTFT, STFT) seem to mean NOTHING!

[Tekati]

Hope that gets some attention. ;D My dilema is this. We have been logging on my brothers LT1 6.0 liter truck which has a stock motor in it right now. Normally he has a built motor putting out 320 or so HP and has a two stage 350 hp shot of nitrous. But like any motor putting out that kind of HP it is being worked on. The last run on the motor had made some pretty impressive S's out of about 6 of the rods.

Anyway...

The computer was programmed by someone for the modifications done to the built motor. A Wide Band O2 sensor is in the truck that monitors Air Fuel Ratio.

After logging for a while with the stock motor we found that LTFT's are pretty positive around 15 or so. The Wide Band O2 sensor shows as being perfect Air Fuel Ratio with just very minor fluctuations which is completely normal.

Now my question is this, if the computer is able to make these compensations to the fuel trims and there is 0 knock and performance seems normal for the stock motor and the logging is normal other then the elevated LTFT then why is it so important to have your LTFT set to slightly negative? BTW: The IFR tables in the computer are the same as a stock 5.3 liter image we were using to compare with. So the guy who programmed the computer must be making the changes to other settings. He supposidly used to work for GM doing actual programming to the computers. He does not use HP Tuners nor LS1 Edit and he apparently uses a hex editor and some other method of applying the image. The good thing is the computer is not locked and we were able to read the image just fine.

So again I ask why is it so important to have slightly negative numbers for the LTFT? ???

[Chris@HPTuners]

when the LTFT's are positive the VCM carries this value over to WOT fueling. If you tune your WOT fueling with +ve LTFT's thats fine, but be aware than if the LTFT's drift then so will your WOT fueling.

If the LTFT's are -ve then the VCM carries over a value of 0 to WOT. Which means no matter how much your LTFT's drift on the negative side then your WOT fueling will always be consistent.

[Magic_Chicken]

Is there a reason why negative LTFT's carry over a 0 value to WOT instead of the actual negative value, and why positive LTFT's carry over the positive value instead of 0? I have an idea, but I'd like to hear it from people that know. Thanks.

[P_Mack]

Yeah it's because if it carries over positive ltft's at wot that means it's adding fuel, which may cost power but it's safe. It doesn't carry over negative ltft's because taking away fuel at wot could cause detonation.

[jimmyblue]

If your trims are near zero in the midband, and you
are running a stock fuel delivery and stock MAF, you
can bet that all your sensors and tables are pretty
much true. This means WOT fueling is likely to be
going where you set it. So there's that.

If you have nonideal fueling that makes the PCM use
big trim values, you will probably see big variation in
the trims as well. Big negatives, at least produce a
consistent WOT zero correction. Big positives, you may
see +10% if you step off from Cell 9 and +5% if you
step off from Cell 13 and +15% if you go WOT with a
sharp stomp from Cell 1. How are you going to tune
the thing to a couple of % (the difference between
12.5:1 and 13:1, maybe 10+HP) when WOT fueling
is kicked around by a random 10%? Drive you nuts.

You want truth from the sensors & actuators (like
injectors) as represented through the cal tables, upon
which you can build a tune that works consistently
over the driving envelope and environmentals. Jacked
LTFTs are telling you, somebody's a liar.

[Rotzy]

So let me know if i'm reading this correctly: -ve carry over 0% to WOT fueling. In addition, there is no real benefit to having the ltft's at, lets say, -2 instead of -6? Is there any benefit/detriment to having the ltft's more negative? I realize this is not the most "efficient" way, but it definately offers more consistent wot fueling, correct?

[furiouss99]

I think some guys fixate on fuel trim numbers (worrying if their ft's are -1 instead of -2 and so on) and then you have a "can't see the forest for the trees" situation happening. Take fuel trims for what they are-an indirect indicator of what the pcm sees as either too rich or too lean operating conditions, you're not tuning ft's-your tuning your engine. Think of it this way, if you had a huge amount of knock retard at WOT-you wouldn't just go into the timing table and pull all the advance out (problem solved, call it a day) would you? No, you would look at the variables like fuel octane, PE, etc...and make a smart tuning decision that might not involve timing at all.

As always, Chris@HPtuners shares interesting insight into the cryptic inner workings of the General's computer...-ve ft's won't be acted on in WOT fueling but +ve ones will skew your WOT-so that is a good reason to tune for -ve numbers.

[Rotzy]

Sounds good, thanks for the info. But should I be concerned with the fact that my ft's in the top left portion of the histogram chart are like -4 to -6 while the rest of the chart is 1 to -1? Or should I really only be concerned with fuel cells 6-14?

In addition, is it normal for LTFTB0 to always read higher than LTFTB1? It's consintantly 1-1.5 points higher than the other.