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Correcting your AFR - understanding a VE fuel table

EFI Tuning Fundamentals

Discussion and questions related to the course EFI Tuning Fundamentals


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Hi Andre

After watching this module I'm still a bit confused about VE-based fuel model, as I find the previous modules did not explain how fuel delivery affects VE.

By adjusting numbers in a VE table cell, are you effectively controlling the amount of fuel injected, i.e. injector pulsewidth?

It would seem obvious since it's a fuel table, but still it kinda conflicted with what you explained in 'Volumetric Efficiency' module:

VE = actual airflow / theoretical airflow (piston swept volume)

This equation did not take fuel into account (or it wasn't clearly explained in that module). If that were the case, VE would be a set value table calculated from the MAF input and cylinder volume, and fuel delivery would have very little impact on VE.

But that's clearly not the case with an actual tuning software where you can modify the VE value in any particular load/rpm cell. So it leaves me to one possible explaination of how a VE-based fuel table works:

VE = mass of air+fuel mixture / mass of air of piston swept volume at standard temp/pressure (or current ambient temp/press?)

If this stands then everything would make sense. VE table % value is just another way to represent the air/fuel mixture.

Please tell me if this is the right way to understand a VE fuel model.

On pulsewidth based fuel tables, you control the AFR by changing the table (bigger number = more fuel). If you want to run richer at WOT, then you add fuel to that area of the table.

In a VE system, there are two components to the fuel pulse width -- the VE table (how much air will be used, still bigger number = more fuel), and the target AFR -- the pulse width is calculated from this and the injector characterization. By tuning the VE so that the target AFR is actually reached, you can then change the target AFR table, and the engine will reach this without touching the VE table.

So, using a VE model, you can tune the engine with just a single AFR/Lambda target, and once that has been done, then you can try different targets on the dyno to see which makes the most power for your combination.

Thanks David! That seemed to clear things up a bit for me.

So, tuning the VE table is essentially just a calibration process before you move on to tune the AFR table?