LTFR why so important, am I wrong?

[dsmlights]

Why do we care for LTFT if they are not used when the car is WOT from what I read on a post. Is this correct or am I just messing up..

From what I have aquired the LTFT are for Partial throttle right? So how do they adjust to WOT … can someone please explain .

Thanks,

Ed

[TAQuickness]

Good question Ed.

During CL, the PCM has several cells for various conditions with LTFT values stored in each cell. In short, this let's the PCM adjust the fueling for various driving condions....

Now, when you transition from CL cruising to OL WOT, a few things happen:

1) Being OL WOT, the PCM is no longer adjusting fueling based on NB feedback. It just commands an AFR and assumes the airflow and fuel flow tables are correct to achieve the commanded AFR (CAFR).

2) The WOT CAFR is determined by OLFA and PE tables - commanded will = the richer of the two.

3) When you transition to WOT, if the LTFT was positive, the PCM assumes the tune is leand and will add additional fuel to the WOT CAFR

3a) When you transition to WOT, if the LTFT was negative, the PCM will derive WOT CAFR based on #2 above. No additional fueling for this condition.

Does that help?

[TAQuickness]

Also - don't hesitate to search the board a bit. I'm not trying to discourage you from asking questions. There is a lot of really good info on PCM functionality floating around here.

[dsmlights]

believe me I been searching but I read all those already :)...

Ok so if my car has -7LTFT then i am fine as long as it is not positive?

what are the advantage of having 0 LTFT over -7LTFT

[joecar]

what are the advantage of having 0 LTFT over -7LTFTLTFT -7% means that one or more of the IFR, VE, MAF tables is in error by some amount(s) (adding up to -7%).

LTFT 0% means all the tables are correct.

All this is with respect to the NBO2 sensors functioning correctly.

During thottle transitions, the PCM ignores the MAF and uses the VE table...
if the VE table is correct (0%), then engine's throttle response is very quick (assuming you have a suitable commanded AFR)...
if the VE table is not correct (-7%), then transient throttle response is poor (too rich or too lean during the transition), engine responds [comparatively] more slowly.

Cheers
Joe
:cheers:

[dsmlights]

LTFT -7% means that one or more of the IFR, VE, MAF tables is in error by some amount(s) (adding up to -7%).

LTFT 0% means all the tables are correct.

All this is with respect to the NBO2 sensors functioning correctly.

During thottle transitions, the PCM ignores the MAF and uses the VE table...
if the VE table is correct (0%), then engine's throttle response is very quick (assuming you have a suitable commanded AFR)...
if the VE table is not correct (-7%), then transient throttle response is poor (too rich or too lean during the transition), engine responds [comparatively] more slowly.

Cheers
Joe
:cheers:


k thianks for this info. so Now everyone tried to get to 0 so they can get the fastest response avail. IE no bogging or sputtering ???

thannks for the help.

Ed

[SSpdDmon]

I think of it this way...

1) We know positive fuel trims carry over to WOT.

2) Our cars run richer in colder weather (at least this is what I have noticed).

3) A -7% LTFT means the PCM will have to see a +7% change in part throttle fueling before it'll start adding fuel to WOT.

With those three facts/theories, I would say tuning a car to 12.7:1 at WOT when there are -7% fuel trims is rather risky. If fueling does change back to 0% LTFTs (or higher), it's very possible you'll be running 7% leaner at WOT too....which puts you at 13.6~13.7:1. Definitely not good. You follow?

[dsmlights]

but that is initially when i go full Throtle or during the hole power band.

[TAQuickness]

if you transition from a +LTFT to WOT, extra fuel will be added thru the duration of WOT.

[dsmlights]

I think of it this way...

1) We know positive fuel trims carry over to WOT.

2) Our cars run richer in colder weather (at least this is what I have noticed).

3) A -7% LTFT means the PCM will have to see a +7% change in part throttle fueling before it'll start adding fuel to WOT.

With those three facts/theories, I would say tuning a car to 12.7:1 at WOT when there are -7% fuel trims is rather risky. If fueling does change back to 0% LTFTs (or higher), it's very possible you'll be running 7% leaner at WOT too....which puts you at 13.6~13.7:1. Definitely not good. You follow?


but I though -LTFT do not carry over to WOT?

[SSpdDmon]

Assume when you tune....you dial in AFR across the board. In other words...you get idle, part/moderate/heavy throttle, and WOT dialed in.

Then, assume the weather changes say from winter to summer. You notice fuel trims have increased during closed loop by let's say 3%. There's a good chance your WOT fueling has leaned out 3% as well. If you tuned the car with -4% fuel trims, they're now reading -1% on average in closed loop. But because they're still negative, nothing is changed about WOT. However, WOT was dialed in at 13:1 at the time of tuning. Now, that weather change has potentially shifted your WOT AFR to 13.35:1 since no extra fuel was being added. If the weather changes more, your WOT AFR may lean out another 1% or so and then stay there as the positive fuel trims are added in.

The thing with positive vs. negative fuel trims is....closed loop fueling will always be accurate assuming everything is working right. Tune the car with too negative fuel trims and the learning ability to compensate dissipates. On the other hand, tune the car with slightly positive fuel trims at WOT and the PCM can pull/add fuel to WOT bases on observed changes at part throttle.

I believe this whole time, the INTERWEB :rolleyes: has had it wrong....and nobody realized it. http://www.motownmuscle.com/forums/images/smilies/lol.gif

[dsmlights]

but the AFR will only change if the LTFR go positive....

[SSpdDmon]

but the AFR will only change if the LTFR go positive....
No...

Tell you what. You force a car in open loop (so fuel trims are off & nothing changes the AFR in the tune) and then tune WOT in the winter time when it's 30~40*F outside. Then, park the car until the summer time when it's 80~90*F outside. I'll bet you $100 it'll be leaner come summer time. ;)

[dsmlights]

how the hell is that, it should go richer cuz there is less air going into cyl? im so confused. send links so I can read againnnn cuz i am lost.

[SSpdDmon]

how the hell is that, it should go richer cuz there is less air going into cyl? im so confused. send links so I can read againnnn cuz i am lost.
I know...it seems backwards. It's not so much in any links that I can send....just experience. I think it has to do with how accurate the physics-based theoretical calculations work with the sensors in the car. I didn't notice it so much until I tuned one of my cars in the winter-ish weather. By summer, I had to redo my VE to get the fuel trims back to where they were.

[dsmlights]

so then pretty much I have to be tuning 2 times a year.