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AFR & Lambda Calculator
Convert between air-fuel ratio and lambda (λ) for any fuel type. Understand whether your tune is running rich, lean, or at stoichiometry.
Stoichiometric AFR for selected fuel:
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AFR and Lambda Explained
Air-fuel ratio (AFR) is the mass ratio of air to fuel in a combustion mixture. Lambda (λ) is a normalized version — it expresses AFR as a fraction of the stoichiometric ratio for that fuel, making it universal across fuel types. Lambda = 1.0 always means stoichiometry, regardless of whether you're burning gasoline, E85, or methanol.
Lambda (λ) = Measured AFR / Stoichiometric AFR
AFR = Lambda × Stoichiometric AFR
Rich vs Lean
- λ < 1.0 (Rich) — More fuel than air can combust. Produces CO, hydrocarbons. Used under high load for cooling and detonation safety
- λ = 1.0 (Stoichiometry) — Theoretically perfect combustion. Three-way catalysts operate most efficiently here
- λ > 1.0 (Lean) — More air than fuel. Maximum efficiency, lowest fuel consumption, but detonation risk under high load
Why Lambda Matters More Than AFR
A wideband O2 sensor reports lambda — not AFR. The "AFR" number displayed by most gauge units is just lambda × 14.7 (gasoline stoich). If you're on E85 but your gauge is scaled for gasoline, every reading is wrong. Lambda is the only fuel-agnostic measurement. Always confirm your gauge or ECU is calibrated for the correct fuel type.
Target AFR Ranges (Gasoline)
- Idle: λ 0.97–1.03 (AFR 14.2–15.1)
- Cruise / economy: λ 1.00–1.05 (AFR 14.7–15.4)
- Light load acceleration: λ 0.95–1.00 (AFR 14.0–14.7)
- Full load NA: λ 0.88–0.92 (AFR 13.0–13.5)
- Full load turbo (street): λ 0.80–0.86 (AFR 11.8–12.6)
- Full load turbo (race/high boost): λ 0.74–0.78 (AFR 10.9–11.5)
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