When I have my bin file constants set to 14.50 in the 'Stoichiometeric Air Fuel Ratio' in Tunercat anytime I wasn't in PE mode and while scanning my engine with the beta version of Datamaster it almost always reports the a/f ration as 14.50.

If I change the constant to 14.69, burn another chip and scan with Datamaster it almost always reports it as 14.69. Exceptions are when i PE mode, or heavy decel. All cruise a/f ratios are reported as whatever I set in Tunercat.

I find this odd, unless the commanded a/f ration is included in the data stream somewhere, which I did't think it was.

What I find odd about this is I didn't think the O2 sensor was that accurate. Is it just a fucntion of the ECM doing the math to report a resulting number, or does it know before it calculates it what the a/f ratio is supposed to be?

Any help understanding this is appreciated! I'm actually having a hard time tying to figure out how to word this question.

BTW, I'm using a '730 SD ECM with a heated O2 sensor in the header collector.

Thanks, in advance. . .

 

It's commanded AFR.
Not actual.

Commanded is what the AFR averages out to. The ecm toggles across Stoich, it goes rich and lean, to keep the converter working properly.

 

I understand that. What I don't understand is if Datamaster is calculating that value from the data it's receiving from the ECM through the sensor, pulse widths, etc., or does it have a way of actually knowing what value is set in the PROM?

Maybe another way to ask is; Is that the value is *supposed* to average out to, or the value it actually *is* averaging out to?

Here's what I'm talking about:

 

The commanded AFR is sent out the ALDL as one of the datastream parameters. This is what is being displayed.

RBob.

 

Be happy you can see it!! I use EASE and it does not display commanded AFR. Oh well <shrugs>

What ECM are you editing that gives you the ability to set that in Tunercat?

 

That's the conclusion I came to but wanted to verify it. There are just too many variables going on for that value to remain as steady as it does. I looked through the basic code of CM's data logging program but still didn't find what I was looking for.

BTW, the beta version of Datamaster 8D shows this value. The 3.4.1 version show a number of about 470 instead of 14.??.

I don't ask too many questions since most can be answered with a *Search* but this one had me stumped.

Thanks a lot,