Why bother dialing in the VE table?

[pauly24]

Ok stupid question, but why bother? LTFT dial it in for you?

When the car first injects the fuel and its mixture is off, the cars drive ability will be slightly off because of the wrong mixture.
But once the LTFT have been learnt, the second time round it injects fuel it will be the correct amount. So there is no stumbling.

So whats the difference if you have it dialed in with the VE or you leave the LTFTs on?

[mr.prick]

I guess if you don't mind cold start fueling being overly rich or lean then yeah, why bother.
High LTFTs cause problems too.

[swingtan]

Because LTFT's are simply an "average" of all the STFT's and then might be applied to the PE fuel. Also remember that there are no STFT's during PE mode, so there is no "learned data" for LTFT's there. The fueling in "guessed" from the trims learned in completely different load cells. Think of LTFT's like the "Octane Scalar" of fuel correction, it works, but not particularly well.

Simon.

[WeathermanShawn]

Even though I think the concept of Trims can be advantageous for a car driven on the street, remember the corrective fueling is initially applied after the fact (STFT). In other words you are driving, O2's say lean..then the fuel correction is applied.

Radically large swings in LTFT's can make for a very poor driving vehicle. It will buck, hesitate, then bog. Your PE and WOT will be hosed.

Personally I like to get Trims (LTFT's) as close to zero as possible. If your VE and/or MAF is dialed in..thats called a tuned vehicle.

Enjoy..

[swingtan]

The other option is to simply try it out. Do a full VE ( & MAF is running one ) tune up and don't touch any spark tables. Then once finished, save the tune and flash the stock tune back in and for good measure do a PCM reset and ensure all trim data is cleared out. Then go for a test drive noting how the car feels. Pull over, flash in the "fixed fueling" tune, reset and clear trims again and do the same drive. That should give a pretty clear reason why it's good to dial in the VE.

Simon.

[pauly24]

So what is the best indication of a properly tuned vehicle, when you turn the LTFT on and they stay close to zero? Or do you guys leave them off completely and just turn the STFT on?

Also is there any difference if you use the LTFT to zero in the VE table or the BEN AFR correction?

[swingtan]

One indication of correctly dialed in fueling is that the trims remain within +/- 5%, this goes for both STFT's and LTFT's. In the past I have turned off the LTFT's and only use the STFT's, lots of people do this as if the STFT's are very close, the LTFT's will not make much of a difference. In fact ATM I'm running full OL so don't use any trims at all. I do have a permanent WB gauge though so I always can see what is going on.

LTFT's should not be used in any correction work as they will not give a fine enough result to accurately adjust anything. When adjusting the VE or MAF, turn off all trims and us a WB sensor.

Simon

[pauly24]

I logged my WB signal and used that to create the BEN factor.
Once I dialed in my VE, I got all the BEN factors to around +-1-2%

But when I turned on my fuel trims, it got all the BENs to 0, doesnt this prove that the fuel trims are trimming each load point to the EXACT fueling thats required?
Hence they would make a good offset for the VE wouldn't they?
I guess I can experiment, ill dial in the VE using the fuel trims, then ill turn them off and log the WB BEN factor, if the BEN factor is the same results I get dialing in the VE using the BENs, then using the fuel trims is just as good.

[WeathermanShawn]

I have had more success apply Trims (STFT or LTFT) to MAF. Less success with applying them directly to the VE Table.

If you stick with closed-loop you can do either STFT or LTFT. Again if you are not running MAF, applying the full LTFT correction to the VE Table may take some more mastery..

[swingtan]

Pauly, if you turn on the trim and then attempt to do a BEN, the BEN will "always" be "0".

read up on exactly what the BEN is and you will understand. The WB-BEN is a "Fueling correction factor" that is averaged over many data points. It is based on the commanded AFR and the measured WB AFR, the result being a correction factor. If you have STFT's turned on, then they will always adjust the fueling to be stoichiometric and if you are commanding a stoichiometric AFR, then the WB-BEN MUST always be "0". This proves nothing in terms of how accurate the fuel mapping is, just that the trims can correct ( hence why CL fueling is used for a generic tune ). The only time you can really use a WB-BEN factor is when no other "fueling correction" is taking place.

You can however set up a "STFT-BEN" that uses the STFT averages to create a BEN factor. It can be used to massage the VE and/or MAF where the NB-O2's are in use but cannot be used for correction in PE mode or if OL fueling is used.

Simon