How does Autotap figure AFR off the PCM

I just hooked up Auto tap on a 97 TA, it showed the AFR at WOT was 12.9. I think 13.0 and 13.1 is ideal for PCM for less tuning.

My question is how does if calculate AFR off the pcm.

Does anyone know of a formula to calculate AFR off any parameter off the pcm??

AutoTap can not tell you "actual" A/F ratio. What you get out of AutoTap is the "target" A/F ratio.... the ratio that the computer is using for its calculations.

The only feedback to the PCM on actual A/F ratio is the O2 sensor, and that is only accurate at 14.7:1 stoichiometric A/F ratio - what the engine is designed to operate at in closed loop/part load/part throttle use. When you go to the enriched A/F ratios needed to produce maximum torque or HP, the O2 sensors are no longer accurate. That's why the PCM no longer uses them to make corrections to the calculations.

What you are seeing from AutoTap is the "target" A/F ratio that the PCM calculated for full throttle operation... or power enrichment (PE) mode. This is based on two tables that generate PE values as a function of coolant temperature and RPM.

The only way to measure A/F ratio is with a wideband O2 sensor. These are available as kits for low $$$ and there is a commercial product of very high quality for about $350.

This really belongs on the "Computer......" forum. Moving it there.

So Autotap only calculates the Target AFR for you.
Andrew
Target AFR=(1-((#in the temp table)-(#in the rpm table))/100)*14.7
Shapiro
Target AFR=14.7/(1+# in cool temp table/100+#in rpm table/100)

This is what the pcm thinks the AFR should be right?


How accurate is this to a real wideband??

The MAF measures the air flow. The PCM uses the "target" A/F ratio to calculate the fuel required to match that air flow. In closed loop, the O2 sensors tell the PCM if it is making the correct calculations. If it "made a mistake", the PCM can use the "long term fuel corrections" (also known as BLM's) to get the exact results it wants.

When you are operating at WOT, the PCM uses the formulas to pick out what A/F ratio it "wants". If it says the target A/F ratio is 12:1, it calculates the injector pulse width required to supply 1 pound of fuel for each 12 pounds of air. That is all it knows..... it TRIED to set an A/F ratio of 12:1. But there is no feedback to tell it if it got the results it wanted.

When nyou measure the ACTUAL A/F ratio with a wide-band sensor, there is no certainty that what the wide-band measures = the PCM's "target". That's why you need a wide-band sensor to "tune", and you can't rely on simply altering the PE table values to give you the "target" A/F ratio. You adjust the PE table values to give you the ACTUAL A/F ratio as measured by the wide-band.