I am confused by something that is happening with the car, and wanted to check with the Advanced Tech forum to see if it could be caused by the timing.

Running tune #1, at WOT I got o2 readings in the 950-960 range.
Loaded bolt-on tune from a mail order place (tune #2) and got readings in the 880-900 range.

Split the difference (PE vs RPM) between the two tunes (now tune #3) and got readings in the 920-930 range.

Increased the PE vs RPM table of tune #3 to match that of tune #1 and retarded the timing for 95 and 100 kpa from 2800 RPM and up by two degrees. Now I get readings of 890-910.

I've been attempting to get my nitrous system tuned, but I'm talking about motor only readings here.

My question for the Advanced Tech people is this: Would pulling 2 degrees of timing from the tune, with the same PE fueling, cause the mixture to appear so much leaner than in the original tune?

Can adjusting the timing have that kind of an effect on the air/fuel ratio?

 

any signs of the computer retarding for knock? A possibility is that the previous tune may have caused some knock that the computer dealt with by retarding the timing past the 2 degrees you took out. Retarding the timing may cause a richer running condition as the combustion isn't as efficient. You can see this a bit better on a wideband. Lets say taking out 2 put you out of a knock retard situation, thus causing the tune to look leaner as you are able to maintian your timing settings. This is only an idea. You may not have signs of knock.

 

Looking at the tune, the new tune has about the same timing from 2800-4000 RPM, and 2 degrees LESS timing from 4000 RPM up than the old tune.