Figuring out A/F ratio by looking at 02 sensor output?

This is what wideband O2 sensors were developed for.

The output of a typical narrow band sensor is like this:



The size of the voltage swings occurring right around 14.7:1 and the plateaus on either side mean the sensor is really only good for rich/lean approximations. You can see that the sensor output could, in theory, change almost 600mV for little change in AFR. This is not usable for real AFR monitoring.

My suggestion: Consider a wideband setup like that from Innovate. Their LC-1 can do two very important things:

1) It can drive a proper AFR meter giving you the actual ratio.

2) It can provide a "simulator" output to drive the vehicle's PCM. This output looks like a narrow band sensor to the PCM so the fuel control using the factory PCM remains. See page 17 of the LC-1 manual:

http://www.innovatemotorsports.com/support/manual/LC-1_Manual.pdf

The nice thing about this setup is that the WB sensor can screw into the downpipe at the factory S1 location and give both WB and NB functionality. There's no need to weld in an additional bung, drill holes in the DP etc...

Real, proper tuning and monitoring can't be properly done with a NB setup.