I'm running about 17-17.5 no bucking or popping but who knows could be bad?

Is there any real argument against running cars so lean? It seems odd that maufacturers wouldn't program stock cars to run so rich if they could safely get significantly better gas mileage with a leaner tune. Just wondering what ill effects there are from doing this, aside from the bucking previously mentioned.

My car will also cruise at 19-19.5:1 without problem. Leaner than 19.6:1 can cause bucking from time to time.

Split, what you'll want to do is *advance* timing, not retard it. The super lean mix burns slow. And for the record, I've been consistently breaking 30 mpg according to my 4.08 rear and 4.30 speedo drive, in mixed traffic. Hard top on, A/C on 50-75%.

mmmmmmmmmm A/C..... i need an idler pulley.

Yeah, EGT goes up to a point that isn't good if timing is not advanced. Power goes down. Emissions goes way up; your cat is designed to work effectively at stoich A/F. The EPA will hate you.

Positives: Way less fuel consumption. Due to (literally) less fuel squirted and wider throttle angle for lower pumping losses.

Some of the newer hondas go into a "supercruise" mode on the highway, where the computer keeps the mix around 18:1 in closed loop with a wideband--other cars do as well. It was an uphill battle for them re: the EPA.

part of the reason they won the uphill battle is better NOx catalyzation than the standard catalytic converters use.

and yes, EGT goes up with a leaner mixture because of the slow speed of burn as Ben said. when it exits the exh valves, it's still burning and hence EGT goes up.

so you advance timing (ignite it sooner) and it burns in the chamber more.