Learn How to Tune Your ECU.

An AFR of 14.7:1 is stoichiometric — the air-fuel ratio at which a gasoline engine, in theory, combusts every molecule of fuel completely. In practice, no production engine cruises at exactly 14.7. The ECU targets it under light load because that's where the catalytic converter is most efficient, not because it makes the most power.

Under high load — boost, full throttle, climbing — the ECU enriches the mixture deliberately. Not because the engine "needs more fuel to make power", which is the usual hand-wave, but because extra fuel pulls heat out of the combustion chamber. Cooler combustion means less likelihood of detonation, less stress on the pistons and exhaust valves, and a wider safety margin if the conditions drift.

The typical full-load target for a forced-induction engine sits between 11.0 and 11.8 — rich enough to cool, lean enough to still burn cleanly. Going richer doesn't make more power; it dilutes the charge, fouls plugs, and washes oil off the cylinder walls. Going leaner doesn't either; it just runs hotter, and the safety margin disappears.