LTFT's and timing modification

[Wheelz]

Quick and I hope simple question.. Do the long term fuel trims effect the ECM's estimation of how much air is entering each cylinder (which effects the spark advance), or does it simply modify the amount of fuel to use?

[joecar]

STFT/LTFT modifies fueling.

Cylinder airmass is determined solely by MAF and/or VE tables.

[Wheelz]

Thanks for confirming that. I was about to reach that conclusion on the basis fuel trims adjusted each bank and there was no provision in the air mass calc's for specific bank adjustment.

So air mass needs to be correct so that proper timing is commanded when using the timing table from one tune to the other.

[joecar]

If the LTFT's differ significantly across banks, then there is a physical problem (air/fuel/compression/spark leak, O2 sensor variation, etc...).

[joecar]

Yes, airmass needs to be correct in order to achieve proper timing... and also to achieve correct torque modelling so that transmission line pressure can be correct.

[Wheelz]

Thanks joe. That helps.