Originally posted by MRGSL-SE
The stoicometric ratio is based on what's the best ratio the CAT can convert those bad gasses into H20 and CO2 I think the ratio was 14.7 parts air and 1 part fuel... some say 15 to 1 but whose counting...
No, this is wrong, the stoicometric ratio isn't influenced at all by the catalytic converter, my previous post explains exactly what it is.
For gasoline, it's 14.7:1 air:fuel on a mass basis. This doesn't have anything to do with the aspiration of the car, tubo or natural. It's dependent on the hydrocarbon fuel, so gasoline is different from propane or hexane or diesel.
Most engines and especialy rotories run rich. The combustion of a rich mixture is cooler than a lean mixture so it's safer for the engine to be rich. Also, the maximum power of an engine is rich of stoic. For these reasons you will see that people tune to a/f ratios in the 11-13 range.
Under cruising, where the engine is at a constant speed and a fairly low load is the only time when you want your car to be running near stoicometric.