Quick Long Term Fuel Trim Question

**Firebirdjones** · 02-11-2007, 12:50 PM

When looking at long term fuel trim,,,and lets say for this discussion it's reading +2.
I understand this as the car running lean and the computer is trying to add fuel,,,,so the tune would need to be fattened up a bit to bring that number back to 0.
Do I understand that correctly?

**mrr23** · 02-11-2007, 05:56 PM

if you are within +/-4%, it is fine. but, yes you would need to fatten it up slightly. weather change can affect the LTFT numbers as well.

**Firebirdjones** · 02-12-2007, 02:46 AM

Cool thanks,,,I wasn't sure if I was understanding that or not. Glad to know I am on the same page. Larry.

**tblu92** · 02-28-2007, 09:48 PM

Remember---If you have a Negative LTFT number (rich) just before going to WOT the trims will lock on "0"--However if you have a Positive LTFT number(lean) just before going to WOT it will lock on that number-adding that % amount of fuel to your WOT fuel ratio.That is why it is always best to get the LTFT's either "0" or slightlt negative.

**Firebirdjones** · 03-01-2007, 12:24 PM

Wow,,,now that is the kind of information that I need. This is the kind of important tuning info that is needed for everyone.
Now if I hadn't known this before hand I would be beating my brains out wondering why I couldn't get the WOT air fuel ratio where I wanted it,,,,not knowing that the computer was adding fuel from the LTFT's and skewing the numbers.
Thanks for that bit of info. Larry.

**Doc99SS** · 03-01-2007, 04:51 PM

I found it best to tune the LTFTs to between 0% and -5%, so that, no fuel correction factor is added to the WOT fueling. In this manner, its a bit easier to fine tune the "Power Enrichment RPM" table.
Also, there are 22 Fuel trim cells (FTCs). 0 thru 15 are the cruising around cells, 17 thru 22 are specisl FTCs, with 22 being the WOT cell. FTCs 0 thru 15 are differentiated by engine RPM and MAP value, FTC 0 being low RPM (idle) and low MAP value, while FTC 15 is higher RPM and higher MAP. FTC 0 thru 15 are arranged in a 4x4 matrix, 0,1,2, and 3 are on the bottom row, while 12, 13, 14, and 15 are on the top row. Do a search, this FTC stuff is very well described on this board.
When you jump from one of the normal driving cycle FTCs 0 thru 15, you jump into the Power Enrichment mode and FTC 22.
Oh yeh, Each FTc has its own unique LTFT value and STFT value which get update when you are in each specific cell. And the LTFT values are available at startup (key on), while the STFTs are set back to 0% each time the engine is shut off.
I apologize if you already know this stuff.

**Firebirdjones** · 03-02-2007, 02:37 AM

No I don't,,,but it hardly makes sense to me without having the program in front of me to look at what you are talking about. I am sure I will have tons of questions when I get this thing up and running.
It would be nice if these programs come with a small booklet explaining what each area is for and what it affects. For what you pay for a tuning device I would think it could be included.
It's just not a simple as a carb and distributor anymore,,,with way too many variables in a program where a guy can get completely lost. Larry.

**mrr23** · 03-02-2007, 05:23 PM

with HPtuners, when you mouse over a particular box, a brief description shows up at the bottom of the table or box. there are more in the help section. but no, there are no instructions on you do this and this is what happens type thing.

**Firebirdjones** · 03-02-2007, 05:37 PM

Well it's been a week and it hasn't shown up yet.

But I can't wait to get it up and running,,I am interested in what you said about gliding the mouse over the icons and brief descriptions coming up,,,this should at the very least give me some idea which is better than nothing.
Please bear with me as soon I will bother people with questions I am sure. Larry.

**mrr23** · 03-02-2007, 05:42 PM

the descriptions tell you what it does, but you have to figure out what it means.

figure on two weeks to get it.

here's a screenshot from hptuners website. you'll notice the description at hte bottom of the box in gray.

**mrr23** · 03-02-2007, 05:56 PM

here, i did one better. this is three screenshots from my hptuners:

VE primary table when moused over the button

PE table when moused over the EQ Ratio vs RPM

Idle when moused over the Idle Airflow button

**Firebirdjones** · 03-03-2007, 04:09 AM

Thank you for that. I especially liked the description for the Power enrichment wide open throttle section,,,,it actually tells you what math is required to determin that number for your commanded AFR,,,,thats the kind of info that I need to know. This is very helpfull,,,many thanks. Larry.

**Doc99SS** · 03-04-2007, 10:56 AM

One piece of advice that I would like to give is to change only one table at a time and go out and check the results. Make only one change at a time.

Also, purchase this book, its a great resource for you and your tuning efforts:
"How to Understand, Service, and Modify Corvette Fuel Injection" by Charles O. Probst