LTCHAPPA

What would be a safe A/F ratio? I am having my car tuned after the header install and the temperature will be around 40 degrees F.

Joe0121

I think your tuner would have a much better idea on that, as well as the guys over in forced induction/N02 forum.

The Wrench

Yeah, Joe is right; if your tuner doesn't know, get another tuner.
That being said, 13/1 is about as lean as I've seen anyone here go on a midly modified car, most like to go about 12.5.

DG

Mike Mercury

12.9 on a NA engine... 12.5 on a blown engine are the "usual" AFR WOT numbers.

Blow Torch

If you every run N2O, you might want to go a little richer just for a little added saftey.
You might want to drive the car a little to let the PCM adjust (learn) to the headers. There are some tuners that simply adjust the PE to get the proper WOT #s, which is not necessarly a terrible thing, but once the PCM finishes adjusting for the new mod, (usually enriching the A/F) it causes the WOT #s to be off again.