Does anyone know the relationship between the stock O2 sensor output and the A/F reading? It seems like it should be pretty easy to ghetto rig a voltmeter to work as an a/f guage, and it cant really be less accurate than an autometer gauge.

 

sure you can. it goes up and down around .45 V (above is richer, under is leaner).

But it's difficult to drive and look at a voltmeter at the same time, it's easier to look at a leds. Thats why most AF gauge use some kind of leds or bargraph.

You can build your own AF gauge easily. There are plenty of circuits described on the web.

Hugues -

 

There are alot of pages on how to build a/f guages, but I haven't been able to find one which relates voltages to actual a/f ratios. The closest was this page,
http://www.pelicanparts.com/techarti...el_monitor.htm but its still doesnt say much. And I assume the .45v you mentioned was for a stoichometric ratio, but is that 14.5/1 or 12.5/1 or something else?

 

The stoich AFR depends on the fuel. It is about 14.7 AFR for unleaded pump gas. The sensor voltage will change with temperature, especially as the AFR moves higher or lower than stoich.

AFR Gauge comparison with voltages as tested:
http://www.alltrac.net/tuning/afgauge.html

DIY AFR Gauge sites:
http://www.techedge.com.au/
http://www.diy-wb.com/

 

Standard O2 sensors are only accurate around 14.7 (stoic). At that AFR they put out .5V.
Once the sensor is up to operating temp it should not really be affected by changes in exhaust gas temperature.
I also don't believe it is affected by the type (octane) of gas you run... unless you've converted the car to run on Methanol or CNG, it'll be fine.
If you want to tune for max power (12.5) then you will have to get a wideband. Stock O2 sensors are crap outside of 14.7. As you can see by the graph at either end you can have a large change in AFR with no change in output voltage.

Good luck

 

The centerline on the chart you posted will stay pretty much unchanged, but the upper and lower curve and the associated ends of the line will warp with temperature changes.

Pump unleaded gas has about the same stoich AFR regardless of the octane rating. However, leaded pump gas, racing fuel, AVGAS, etc., will have a different stoich AFR. The important thing to remember is that an O2 sensor is a LAMBDA SENSOR. Lambda and AFR are not the same thing. Lambda is relative to the fuel used, while AFR is a straight ratio. Most AFR meters are calibrated to convert the Lambda voltage into AFR based on unleaded pump gas ratios.

You can still tune for max power (which is not necessarily 12.5:1) with a narrowband O2 sensor. It is not as accurate as a wideband outside of the stoich range, but you can still use it, especially as a safety monitor.

There is a lot of BS floating around the internet that makes people think that a widband O2 sensor will let any idiot tune an engine. This is totally false. An O2 sensor, whether narrowband or wideband, is simply a tool.