Volumetric Efficiency


Volumetric Efficiency

Application: GM V8 based vehicles


Basic terminology

Tables and programs to use:


The VCM, Closed Loop, LTFTs and VE

The VCM learns in closed loop fuel mode. By keeping the engine in this mode, you can cover enough locations in the histogram to get an overall view of your engines tune. Make slow throttle changes and try and hit as many histogram table locations as possible.
Covering every single location is not needed, nor is it possible as the VE table covers areas the engine will never run in.
You do not want the engine to go into power enrichment mode, as this will turn off closed loop fuel.

The LTFT uses cells to “cover” the VE table. The current cell in use is called the Fuel Trim Cell (FTC). There are 16 cells (0 – 15). Cells 20,21,22 are used by the VCM when it does not want to make a permanent correction to the VE table values (examples would be Decel Fuel Cutoff, Power Enrich Fuel, Charcoal Canister Purge off).

By monitoring the current LTFT cell number (FTC) in the scanner, you can make a decision of when to ignore the LTFT correction. Do not use the histogram logged corrections for cells 20,21,22.

LTFT cells are “placed” on the VE table by using the LTFT Low, Mid High boundaries on the Closed Loop fuel tab.
The RPM boundaries on the sample are 1300 RPM, 2500 RPM, 3500 RPM
The Map boundaries on the sample are 32 kPa, 56 kPa, 80, kPa

A graphical representation of this layout is shown in the figure below.
The RPM Boundary values are shown in red, the MAP boundary values are shown in blue. The lines are for display purpose only and do not appear on the VE table.



LTFT cells are also “placed” on the Histogram LTFT table by using the LTFT Low, Mid High boundaries on the Closed Loop fuel tab. A graphical representation of this layout is shown in the figure below.
The RPM boundaries on the sample are 1300 RPM, 2500 RPM, 3500 RPM
The Map boundaries on the sample are 32 kPa, 56 kPa, 80, kPa

The RPM boundary values are shown in red, the MAP boundary values are shown in blue. The lines are for display purpose only and do not appear on the histogram.



STFT: stands for Short Term Fuel Trim. This is the VCM’s ability to make needed corrections quickly to the Air Fuel Ratio by monitoring the vehicles stock O2 sensors.
The STFT corrections are instantaneous corrections and do not employ the use of cells, nor is it a permanent correction.

The STFT Histogram keeps track of these changes so you don’t have to. The corrections displayed in the scanner are a + - % correction to the values on the VE table, zero being no change. They are to be considered a “fine tune” for the LTFT cell adjustments. Seeing as the LTFT is cell based and covers a wide area on the VE table, the STFT corrects the conditions within those cells.


Method 1 using the LTFT and STFT corrections from the scanner histogram data

After you have logged sufficient data, it is now time to look at what changes you need to make. This will require looking at both the LTFT and STFT data on a cell by cell basis.
The LTFT values are your indicator of how much error is in your Volumetric Efficiency table. If Block Learn is at 0% everything is just right. If your STFT is more than 3 % away from 0 , the LTFT value is still "learning". A motor is considered well tuned to have LTFT values between +- 4%. Not every motor can achieve this though.

There will always be a 2-4% change in learning from day to day. Weather, fuel, and engine dynamics vary quite a bit. It’s the VCM's job to learn these corrections, so don’t spend your time trying to get the LTFT numbers down to 0 and remain there forever. It will never happen.

The VE table corrections can be derived by the following formula:

(+-LTFT%) + (+-STFT%) = Final % Learned

If your LTFT histogram value @1600 RPM, 30 kPa Map is +7 and the associated STFT value is -2 your total % fuel correction is +5. Use about ½ the correction (2.5%) on the VE table cell @1600 RPM 30 kPa Map to avoid overshoot.

Repeat this process for any LTFT histogram value that is out of the +- 4% range. After you reflash your VCM with the new VE table, use the VCM Controls function to reset the fuel trims before you start recording new data. This will give you a “clean slate” to work with.

It will take you the better part of an afternoon to log the LTFT, make the corrections to the VE table, then verify the results on the histogram again.


Method 2 using the STFT corrections only from the scanner histogram data

This will require you to disable the LTFT process.
Use of this method requires your VE table to be no more than 10% off. If your VE table is beyond this, start with the LTFT method first.

Depending on your model you can either turn the LTFT enable option to “Off” or set the LTFT minimum Engine Coolant Temp (ECT) to its maximum value. When you are ready to start the scanning process to record the histogram data, use the VCM Controls function to reset the fuel trims.

This will let the VCM run the STFT correction, but no make any LTFT corrections.
The advantage to this is you only have to use the STFT table on the histogram to get your fuel % correction values. You no longer have to do the (+-LTFT %) + (+-STFT %) math or keep track of the LTFT cell number in use.

This will take you about ½ the time to log the STFT, make the corrections to the VE table, then verify the results on the histogram again.

Your corrections to the VE table cells are the +-% STFT error.
Again, a motor is considered well tuned to have STFT values between +- 4%. Not every motor can achieve this though.

Don’t forget to re-enable the LTFT when you are done.


Table Smoothness

As vehicle conditions change the cell in use of the VE table will change. A smooth VE table will help vehicle fueling transitions. A choppy VE table will cause irregular fueling.

When making changes to the VE table, try to modify surrounding cells as well to help preserve the flow of the VE table. It is unlikely for close operating ranges of your engine to vary drastically in volumetric efficiency and thus your table shouldn't either. Transitions over cliffs, ridges, spikes and other erratic data of your VE table can cause less than desirable results.


Good VE Table


Bad VE Table