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AFR ⇄ Lambda Converter
A lambda of 1.00 is stoich on every fuel — the AFR behind it is not. Convert your wideband numbers between fuels and keep your targets straight when switching from pump petrol to E85 or methanol.
AFR
12.5
Lambda λ
0.85
Equivalence ratio φ
1.176
vs stoich
-15%
Rich of stoich — stoich for this fuel is 14.7. A λ of 1.00 is stoich on ANY fuel; that’s why pro tuners scale in lambda.
Common targets → AFR on this fuel
| Operating point | Lambda | AFR (Petrol (E0–E5)) |
|---|---|---|
| Idle / cruise | λ 1.00 | 14.7 |
| Light load economy | λ 1.05 | 15.44 |
| WOT naturally aspirated | λ 0.88 | 12.94 |
| WOT turbo, petrol | λ 0.82 | 12.05 |
| WOT turbo, E85 | λ 0.78 | 11.47 |
| WOT methanol | λ 0.72 | 10.58 |
Targets are guidance, not gospel — chamber design, fuel and cooling all shift what your engine actually likes. Always confirm on the dyno with a wideband you trust.
Good to know
- λ = AFR ÷ AFR_stoich. Every O2 sensor actually measures lambda; the controller just displays it as AFR for one fuel.
- Stoich references: petrol 14.7 · E10 14.2 · E85 9.8 · ethanol E100 9.0 · methanol 6.4. Actual pump fuel varies slightly by season and region.
- Targets are guidance. Richer isn’t always safer at low load — check what your engine likes on a wideband you trust, in YOUR climate.