Main VE Table Question

[dbaxter_ss]

Main volumetric efficiency table.
Based on Engine RPM and manifold absolute pressure (MAP).

The values in this table represent the efficiency of the engine's ability to fill the cylinders with air.
It is used to predict the volume of air entering each cylinder under varying conditions.

So what is considered good 50,60, 100?

Does 100% mean that is predicted the amount perfectly? Should I shoot for something like 90% for some area to work with?

If its showing 100%+, like on my s/c gto, where its shown 150+ when running at the track is that bad? I assume so since the ltft were way positive?

Just trying to understand a little better.

thx

[Steve Bryant]

This is an excellent question and fundamental to understanding the control of mixture with a PCM. The Volumetric Efficiency table is related to the engine's ability to completely fill the volume of the cylinder during each intake cycle of operation. The VE table is used to calculate the expected dynamic air mass flow for Speed Density data which is the basis of all open and closed loop operation.

If you operated any engine at a slow speed of one revolution per minute (I know it's ridiculous but bear with me) and the intake valve fully closed at Bottom Dead Center, the VE would always be 100%. That would mean that if the cylinder volume was one liter, then the intake stroke would suck in one liter of air and be completely filled for combustion.

Now, let's mention the variables of the equation that make the VE Table more complex than my description above:

1. Engine RPM (as the engine runs faster, there's less time to open and close the valves and for the mixture to rush in). However, up to the point that the induction system and valve throats and valve lifts, etc start to choke off the air, faster can be better. The dynamics of things is such though that the VE for any engine will increase with RPM, peak and then decline.

2. Valve timing - If the PCM is the electronic boss of the engine, the Camshaft and all associated valve train components are the mechanical brain of the engine. Put five different cams in the same engine and you'll have five different VE Tables.

3. Manifold Absolute Pressure - affected by normally aspirated versus forced induction, throttle opening, etc.

The VE values in each cell represent a moving or dynamic mass of air that is available to combust with fuel at that MAP/RPM intersection. Now, if the Commanded Equivalence Ratio to the PCM is 1.0 (representing stoichiometric) and the IFR value is correct and the VE value is correct, the AFR should equal very close to 14.7:1 as measured with a WB AFR. If everything else is correct (IFR, temperatures, etc.) but the VE value is too large, the mixture will be too rich (fat) because the PCM thinks more air is available for combustion. If the VE value in a cell is too small, the mixture will be lean. This error will be most apparent in speed density only open loop mode, but it will cause stumbling and driveability problems in Open Loop using the MAF and even closed loop where both the narrow band primary oxygen sensors and the open loop calculations are all blended together. With any kind of significant modification, it is extremely important that the VE table have good fidelity to the actual engine in order to have good driveability even if your vehicle will run like gangbusters at wide open throttle. Just changing the IFR can improve a poorly running vehicle, but the IFR is only one part of the equation.

More to come in time.



All my best,

Steve

Saga to be continued

[TAQuickness]

The main ve table is used in conjuction with the MAF sensor (assuming you are not a SD tune).

Positive LTFT's indicate a lean condition and that the PCM is adding fuel to the mix.

In a nut shell, if the VE value = 100 that means 100% of the air entering the cylinder is used for combustion.

In the same scenario, if the value in the VE table = 100, and MAF sensor readings agree, and the LTFT is positive, it means you are running lean. You would need to increase the value in the VE table by the LTFT value to compensate for this.

However, because of the way the PCM functions with the VE table and MAF, it's not that easy.

Best thing you can do is ensure your IFR table is scaled correctly, then perform an SD tune, then rescale your MAF table to match your VE table.

let me know if you need any help with this.

[dbaxter_ss]

Ok
Its starting to sink in, I am getting help now with an SD tune.

The IFR is stock so I would assume its correct Unless GM really hosed up the table


I and blackss02 have been working on this, and adding into the VE table to get the ltft's negative.

Its slowly getting there.

Now that you brought it up, once I do have this working how do I remap this to the MAF table?

[Steve Bryant]

I finished up this brief installment. There will be more to come in the primmer that I'm working on.

Steve

[GMPX]

Don't be concerned if the VE% is over 100%, I've seen factory ones upto 105%.

When in SD mode, the VE table directly affects the final AFR.
So, lets say in your P.E table {B3618}you have an AFR value of 12.5:1 everywhere, you give the car a run on the dyno and the WBO2 is showing 13.2:1 AFR, you would need to adjust the VE table values at all those RPM points (and the values a 100 - 105Kpa) to bring the measured AFR in line with the AFR values in the P.E table. For that example above I would change it by 5% and see what happens.

The IFR table will also affect the final AFR delivered, unless you have changed the injectors it's better to leave that alone.

Cheers,
Ross

[dbaxter_ss]

Is there a specific % to try and achieve?

Should I always be trying to get 100% across the board?

[Steve Bryant]

No, you are not shooting for 100%. You are shooting for the VE cell value that makes the real AFR measured at the tail pipe via a WBO2 in the Speed Density mode equal to the commanded AFR which is a PID. For normal Stochometric operation, the commanded AFR is 14.68 approximately as long as the vehicle is not warming up or being enriched by power enrichment or any thing else.

Take a look at several of the stock VE tables in the GM tunes provided with FlashScan and on the Tune Repository that Rick set up to get an idea of the VE Tables for a stock cam and engine.

Steve

[dbaxter_ss]

thank you for the explanation. Thats what I needed to know.

Will do looking that up

[87gmc]

Lots of good info on the ve table. Now things are starting make more sense.

So the VE table can be adjusted to compenstate from a added k&n filter to headers right? Because more air is getting in and out of the cylinders right?

I know the VE table is just a estimate of what the computer thinks is getting in the cylinders but is there a way to scale it with EFI scanning in the histogram area?

Thanks for all the help guys!!!!!!!!