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AFR

Car culture term

What is AFR?

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.

Related terms

ECU tuneKnockOctane

Frequently asked questions

What does running rich mean?
Running rich means more fuel in the air-fuel mixture than stoichiometric. The result is reduced power efficiency, increased emissions (especially carbon monoxide and unburned hydrocarbons), and possible spark plug fouling. Slight rich (12.5-13:1 at WOT) is typical for performance applications.
What does running lean mean?
Running lean means less fuel than stoichiometric. The result is increased exhaust temperatures, increased detonation risk, and potential engine damage. Lean conditions under boost are particularly dangerous because they can cause detonation that breaks pistons or rods. Most tuning aims to avoid lean conditions especially under WOT.
What is stoichiometric?
Stoichiometric is the ratio at which all fuel and all oxygen react completely. For gasoline, that ratio is 14.7:1 (air to fuel by mass). At stoichiometric, the catalytic converter operates most efficiently and emissions are lowest. Most production cars run stoichiometric at idle and cruise but richen at WOT for power.
Is E85 better than gasoline for AFR?
Different consideration. E85 stoichiometric is 9.8:1. The ECU must inject more fuel for E85 than gasoline at the same load. E85 has higher knock resistance (effective octane around 105 vs 87-93 for gasoline), allowing more aggressive ignition timing. Cars tuned for E85 typically make significantly more power than the same car on gasoline.
How do I check my AFR?
Wideband oxygen sensor with display (AEM, Innovate, Zeitronix). Cars without factory wideband require installation of an aftermarket sensor in the exhaust. Many modern tuning solutions include built-in wideband logging through the OBD-II port. Logging AFR at WOT is essential for verifying tune safety.

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