Here are two files to help with the what and whys of proportional changes.
Open the Proportional.cfg file first, then the idle prop good.hpl file.

http://www.hptuners.com/pub/idlepropgood.zip

Everything is in high resolution so you can see the data better.



Proportional fuel’s entire job is to move the AFR up and down slightly. Just enough to go above 14.7 and below 14.7

Proportional fuel is added and subtracted to the base pulse width. It's not an engine tuning value, it’s the beginning of the closed loop learning process. Unlike most PID (proportional integral derivative) controllers, the closed loop fuel is PI only.


Here is a good PID link http://www.expertune.com/tutor.html


The VCM makes the fuel go rich with proportional fuel and looks for the change on the O2 sensors. It counts the time it took to go rich.

Then after that it does the same thing for lean. It’s always switching the fuel back and for to try and “center” it around 14.7

An expired time in the AFR switching results in an STFT correction. Persistent errors result in an LTFT correction


There are three values in the third chart. Left O2, Right O2 and AFR as measured with a Dynojet Wideband Commander.


You can see that the Stock O2 sensors always report around 800 MV for rich and 50 MV for lean.

They Dynojet shows the AFR switching from around 14.6 to 15.2

The stock sensors will report 800mv or 50mv on just about anything above or below 14.7, so they are not a good tool for tuning this.


If your proportional fuel tables were too low, they would not move the fuel far enough to go above or below 14.7

If your proportional fuel tables are too high, they will move the fuel much higher and lower than need be to cross the 14.7 point. This will saturate the O2 sensor signal.

This over correction causes the engine RPM to oscillate slightly as it goes very rich to very lean.


While the proportional tables are a % Base Pulse Width modifier, you need to know the actual Airflow & BPW the engine is using in order to calculate it.

With varying load conditions it’s almost impossible to do by hand. Just log the data and see where things may need a small shift.

You normally don't need to touch these tables unless you have increased your injector size. Be fore warned that increasing the injector size without the use of a vacuum referenced fuel regulator starts to cause problems on some engine combinations.

Guys running large blower cams and forced induction will notice this very quickly. A lot of aftermarket injectors are flow rated at 43 psi while you are running close to 60 psi.

Now when you get into an area where the pulse width is very small, and the injectors cannot fire you wind up with a false lean spot.

The VCM will try to correct for this by making the engine rich. Now the engine is to rich and it will make it lean again to the point where the injectors don’t work. This cycle will repeat resulting in large engine RPM changes and weird looking STFT values.

On extreme cases it feels like a hesitation in the motor during cruise and deceleration.



Good luck and happy tuning,

Ken