For a naturally aspirated car, most engines like to cruise at around 14.7:1, but run better at somewhere between 12.8:1 and 13.0:1 at full throttle. Nitrous motors should be even richer when you hit the button for safety (and because there's a good chance one or two cylinders are a little leaner than the rest). With a narrow band gauge, fully pegged can be anywhere from richer than you need to be, to richer than 14.7:1 but still lean enough to melt a piston.