You might think that this chemically perfect air-fuel ratio at which EGT peaks would be where we should always operate our engines, but you’d be wrong. Just because it’s chemically perfect doesn’t mean it’s operationally useful. It’s not the mixture that produces best power, and it’s not the mixture that produces best economy. So, it’s not generally the optimal mixture for flying.
For best power, you need an air-fuel ratio of about 12.5-to-1, quite a bit richer than stoichiometric, which occurs at about 100 to 125 degrees F rich of peak. This rich mixture burns somewhat dirty (due to unburned hydrocarbons), but it also burns faster which yields a bit more power. For full-power takeoffs, we use an even richer mixture for increased detonation margin.
For best economy, you need an air-fuel ratio of about 16-to-1 , quite a bit leaner than stoichiometric and so significantly lean of peak. Such a lean mixture burns very clean and reduces combustion pressure and temperature, which is great for engine longevity but at the sacrifice of some power and airspeed.