Air-fuel ratio. The ratio of air to fuel by mass in the engine combustion chamber. 14.7:1 is stoichiometric for gasoline.
AFR (air-fuel ratio) is the ratio of air to fuel by mass in the engine combustion chamber. The ideal stoichiometric ratio for gasoline is 14.7:1 (14.7 parts air to 1 part fuel by mass), where all fuel and oxygen react completely. Real engines typically operate slightly rich (more fuel than stoichiometric, producing more power and reducing detonation risk) or slightly lean (less fuel, producing better fuel economy).
AFR varies based on operating conditions. Idle and cruise typically run at or near stoichiometric (14.7:1) for emissions compliance. Wide-open throttle (WOT) typically runs richer (12.0-12.5:1) for power and detonation prevention. Boost conditions on turbocharged engines run especially rich (11.5-12.0:1) to manage cylinder temperature. Different fuels have different stoichiometric ratios; E85 stoichiometric is approximately 9.8:1.
AFR is monitored by oxygen sensors (lambda sensors) in the exhaust. The ECU adjusts fuel injection duration to achieve target AFR for the operating condition. Tuning involves modifying these targets to optimize for power, fuel economy, or emissions. Wideband oxygen sensors (used in tuning and racing) provide more precise AFR readings than the narrow-band sensors used in most production cars.