ram-it

Can someone explain the importance of having the LTFT's at negative during idle/cruise and 0 at WOT? I currently run at 4-6 at idle, do not know what cruise is nor if I am at 0 at WOT. My HPT arrives tomorrow so I can get that other info over the weekend. I just believe that on a modified car that being negative is not all that important. Am I wrong?

ram-it

Has anyone ever wondered about this or am I the first? 25 reads and not even a can't help you. It seems that anytime I ask a question on here no one responds. And to think I even took a shower before posting. confused:

monodax

Just a quick FYI, Everybody shoots for Neg LTFT's to keep the PCM from Adding additional fuel when the car goes into WOT mode.

Imagine you are seeing +5 LTFT and you instantly stomp the accelerator. The PCM will remember the last fuel cell you where in and keep that LTFT adder and add that additional amount to the WOT fuel mix. So when you are tuning with a wideband you do not want the computer adding any additional fuel for WOT mode. This could be +5 in one cell and +10 in another depending on what point you were at when you went into WOT Mode.

ram-it

Okay, I think I am following you. So if it's reading + and you go to WOT what would happen? I would think you would go towards 0 but still be slightly positive like +1 or something. If that is what happens does that mean your still lean? Now for second question, what parameters would help correct this? Thanks monodax!

monodax

The best way to determine what fuel cell you where in before going into WOT mode is to have your scanner logging.

Here is a matrix of the FTC you will be looking for:

Fuel Trim Cell Definitions:

0 Coast
1 Non-Load
2 Non-Load
3 Non-Load
4 Non-Load
5 Non-Load
6 Load
7 Load
8 Load
9 Load
10 Load
12 Load
13 Load
14 Load
15 WOT Engine Load = 100%
16 ??
17 Idle
18 Idle
19 Idle
20 Idle
21 Coast
22 WOT Engine Load < 100%
23 ??

These FTC's generally map out to a location on your VE table and you need to correct your VE table in that area to make the Fuel Trims negative. Their is someone on EFILive's site that did this mapping as I am not too sure of exactly how accurate it is but it may help you in determing what area of the VE Table and other tables to modify when you are trying to correct this condition.
This is just one thread that maps the logical view (MAP\kPa to RPM) to the actual FTC. Their are others out there but this may get you going.
http://www.efilive.com/forum/viewtopic.php?t=474

Basically what this comes down to is trying to keep your WOT A/F ratio as stable as possible. If your PCM is modifying the fuel mixture when you go into Cell 22 it can throw your A/F ratio off depending on how much it is changing it. It is not as critical as alot of people say it is in my opinion but it is one of those things a lot of tuners shoot for when tuning. I try to get it as close to 0 as possible but with a Large cam it is pretty difficult if it has alot of overlap.