Bryans12v

Good way to tell if shes running well? Rich? Lean?

Mike O.

...0.875V is about 13.5:1

0.750V is about stoich.

Get an air/fuel gauge and that will tell you or use a VAG-COM or similar.

Mike O.

scott12v@pghaudi

Also, is there a conversion chart somewhere that tells what the O2 sensor equals in terms of ratio?

Thanks,
Scott

J. Daniel

It's short for stoichiometric which means that the proportion of fuel to air is correct to burn all the fuel and all the air with no left overs. 14.7 lbs air/lb fuel for gas.
I've seen lots of conversion curves, but never a conversion chart. In any event it isn't consistent. For a given A/F ratio the ouput voltage will change depending on the O2 sensor temperature.
Everything I've seen shows stoichiometric is approximately 450-600 mv output. The output curve is very steep and goes from 100 mv to 900 mv over a very small range of A/F ratios. So, say a specific mv output = a certain A/F ratio is a gross oversimplification.

Mike O.

We know, for example, that the target 02 voltage for a 1.8T is .875v. Power starts to decrease as the voltage gets above .890v. EGTs go way up under .840v which means things are lean.

Therefore, this value is indeed an accurate measurement of the mixture and burn quality.

Mike O.

eharleq

All your points are absolutely correct.

The voltage change due to temp has greater error/drift than the accuracy of most basic digital voltmeters. Beyond that, *any* contamination or voltage drop differentials in the wiring will make each output voltage different from car to car.

The point of inflection on the curve is very short/narrow and requires high quality equipment.
VAG can show the average, which is useful information if you are way off.

But, the ONLY way to do this with any legitimacy is a wideband O2 sensor, temp calibration, and a calibrated voltmeter with high resolution (Horiba, etc.)

Mike O.

...but we're not talking about trying to tune using a $3400 Lambda probe!

The Bosch 4 wire heated 02 sensors that the current Audi uses are, for all intents and purposes, accurate. Does bad wiring or contamination of the 02 sensor by things like coolant affect the accuracy of the reading? Sure!

For what we are trying to accomplish with data logging the 02 sensor, specifically fine tuning the WOT fuel and ignition maps, this method is fine. The temp issue is not applicable at WOT as the 02S has reached its operating temp and is functioning at or above 95% accuracy (notwithstanding contamination). I have been involved (extensively) with datalogging this parameter to provide feedback to two different chip tuners. There are other indicators (such as long and short term fuel trim) but the 02S is the best measure of an air charge.

Mike O.

eharleq

Sure, but you can't make the statement that a given voltage for you means boo to someone else. Not without fixing the parameters down with the spendy equipment (rentable).

Bryans12v

Dont worry, I do nothing to his benefit