Air/Fuel Ratio & Lambda Calculator
Convert between air/fuel ratio and lambda for gasoline, E85, methanol, and diesel. Includes target ranges for idle, cruise, and WOT.
Reading from your wideband O2 sensor
AFR and Lambda
Air/Fuel Ratio (AFR) is the mass ratio of air to fuel in the combustion chamber. Lambda (λ) normalizes this across all fuel types: λ = measured AFR / stoichiometric AFR. Lambda 1.0 means stoichiometric — the chemically ideal ratio where all fuel and all oxygen are consumed.
Why Lambda Matters More Than AFR
Different fuels have different stoichiometric ratios. Gasoline is 14.7:1, E85 is 9.76:1, methanol is 6.45:1. A 12:1 AFR is rich on gasoline but lean on E85. Lambda removes this confusion — λ0.85 means the same thing (rich, max power) regardless of fuel type.
Wideband vs Narrowband
Factory narrowband O2 sensors only tell you "rich" or "lean" relative to stoich. A wideband sensor gives you an actual AFR reading across the full range. If you're tuning, you need a wideband.
Target Ranges
- Idle: λ0.98–1.02 — near stoich for smooth idle and emissions
- Cruise: λ1.00–1.05 — stoich to slightly lean for fuel economy
- WOT: λ0.85–0.90 — rich for maximum power and exhaust/piston temperature protection
- Cold start: λ0.82–0.88 — extra rich to compensate for poor fuel atomization
Frequently Asked Questions
What is a safe air/fuel ratio for a turbocharged engine?
For turbocharged gasoline engines at wide-open throttle, target 11.5:1 to 12.0:1 AFR (lambda 0.79-0.82) for safety. Leaner than 12.5:1 under boost risks detonation and engine damage. At cruise/part throttle, 14.7:1 (stoichiometric, lambda 1.0) is fine. E85 stoich is 9.8:1, so WOT targets are around 8.0-8.5:1.
What is the difference between AFR and lambda?
AFR (Air/Fuel Ratio) is the mass ratio of air to fuel. Lambda is AFR divided by the stoichiometric ratio for that fuel. Lambda 1.0 always means stoichiometric regardless of fuel type. Lambda 0.85 always means 15% rich. This makes lambda more universal — lambda 0.85 means the same enrichment whether you're running gasoline (AFR 12.5) or E85 (AFR 8.3).
What is stoichiometric air/fuel ratio?
Stoichiometric is the chemically perfect ratio where all fuel and all oxygen are consumed during combustion. For gasoline it's 14.7:1, for E85 it's 9.8:1, for methanol it's 6.5:1, and for diesel it's 14.6:1. Engines rarely run at exactly stoichiometric under load — they run rich for cooling and power, or lean for economy.
Related Articles
What AFR and lambda mean, how to read a wideband gauge, target AFR ranges for idle, cruise, and WOT, and common mistakes that cause lean or rich conditions.
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