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Discussion Starter · #1 ·
Not my info stole it from elsewhere!! Thanks to pontisteve.

VE is volumetric efficiency. Most VE maps use numbers typically from 0 to 100 (although they really aren't limited to 100%). Some newer cars use GMVE, which have numbers like 2200 - 2600. The numbers are different, but the function is the same.

When the car is in Speed Density mode (MAF off), or when using a MAF/MAP blended model (such as idle/lower rpm/transient fueling), the car looks to the VE table for an estimation of how much air is entering the engine. Most of these calculated airflow values are based on facts, such as RPM, cubic inches, air inlet temperature etc. These facts can be used to rather accurately calculate how much air is coming into the engine, except that doesn't factor in the efficiency of the engine due to things like cam, intake manifold, compression, exhaust, etc.

So if we take this calculated airflow value (based on 100% efficiency), and then multiply it times the real efficiency of the engine, we get the number that's in the VE table. So when you see a 87 in the VE table, that means that at that speed/load point, the engine is efficient enough to fill the cylinder 87% of the way up. Typically, you'll never get near 100% at idle or lower RPMs. As the "cam band" starts to come in, you'll start to see the efficiency quickly rise up. The VE will peak at peak torque (somewhere around 4500 - 5000 on an LS engine), and will slowly taper off as the engine RPM goes higher than peak torque. At some point, VE will rapidly drop off, as the cam just runs out of steam. Usually the rev limit point is before this rapid dropoff, so you may not see it on the map too much.

Since the air/fuel ratio is Air divided by fuel, the A/F ratio is affected by two things here. The fuel (injector specs, fuel pressure, etc which should all be set in the computer to accurately reflect the parts on the car), and the air. Since fuel should be properly fixed and set, air is the only model you need to adjust. Increasing the % of cylinder fill (VE) increases the amount of fuel injected, and richens the motor. And vice versa.

To properly tune a car, you should have the car forced into open loop, with all overtemp models turned off (CAT overtemp, exhaust valve overtemp, etc), and all commanded EQ ratio tables should be set to 1.00 (stoich). Then, the proper stoich of the fuel should be set (14.68 for straight gas, or 14.08 for 10% ethanol). Then, all modifier tables to the commanded EQ ratio table should be zero'd out (set to 1.0). This is for idle/part throttle tuning only.

At this point, the car should be running at stoich all the time. Any difference in the air/fuel ratio from stoich is an error in the airflow model. There are two airflow models. MAF and MAP. You should tune one at a time, by turning the other off.
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