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 it off the pcm. Is it off the O2 readings or what.

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

 

I prob can't help you but just for future reference what is AFR? (sorry dont know all the tech jargon)

 

AIR FUEL RATIO

 

Yeah, it takes the AFR through the O2 sensors just like the PCM does.

 

You have to think of what Autotap tells you as, at best, an estimate. The stock O2 sensors are very sensitive at an AF ratio of ~14.7:1. Much above or much below that they are nearly useless. So, do not try to tune with the AFR numbers you get from Autotap other than to think of them as "rich" if >14.7:1, "lean" if less, and "stoich" if right around 14.7:1.

Rich Krause

 

I thought the cars were set at 11.7, which is why we can make up power by going leaner. Like around 13.1

I thought 14.7 was only used in the mathimatical equation.

AFR=(1-((#in temp table)+(#in rpm table))/100)*14.7

This is what the computer thinks the AFR of the car will be

Correct me if i am wrong

 

14.7.1 AFR would be too lean at WOT. Too much of that and you will fry some pistons. These cars are factory programmed to run at 11.7 at WOT only, which is too rich for optimum power, that is why leaning out to 12.8-13.4 nets out more power.

Under normal driving conditions, the ECM tries to hit a 14.7.1 ratio, until you hit a certain pre programmed rpm range or throttle position only then will it shoot for a 11.7.1. AFR (WOT).

 

I answered this on the "Computer....." forum, so let me repost it here:

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. This is what the PC M "wants" it to be. There is no feedback to tell the PCM if it made the correct calculations and got the correct results.

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.