Air–Fuel Ratio & Lambda¶
Lambda (λ) is the actual air–fuel ratio divided by the chemically ideal one. λ = 1.0 is stoichiometric; below 1 is rich, above 1 is lean.
What it is¶
Every fuel has a stoichiometric ratio — the air-to-fuel mix that burns with nothing left over. For petrol that's about 14.7:1 by mass. Rather than quote raw ratios that change with fuel, tuners use lambda:
- λ = 1.0 — stoichiometric (≈ 14.7:1 for petrol).
- λ < 1.0 — rich (extra fuel).
- λ > 1.0 — lean (extra air).
Why it matters¶
Mixture is a balance of power, temperature, efficiency, and safety, and the best value changes with load:
- Lean mixtures burn hotter and can stop making power; too lean risks misfire and high combustion temperatures.
- Rich mixtures cool the charge and resist knock, but waste fuel and can wash oil from the bores or make a diesel soot.
In Engone the optimal target shifts with operating point: around λ ≈ 0.98 at idle, λ ≈ 0.88 at wide-open throttle, and richer again at high RPM where cooling and knock margin matter most.
How to tune it¶
You don't set lambda directly while running — you set targets in the lambda table and let fuelling (plus closed-loop trim) chase them. Use this page's intuition to decide what those targets should be.
Rule of thumb
Lean where you want economy (light cruise), stoich where you want clean and steady (mid-load), rich where you want power and knock safety (full load and high RPM).
Watch out
Chasing economy with very lean full-load mixtures is how you melt pistons. Keep WOT rich.
In the game¶
The live lambda gauge shows measured λ from the O2/lambda sensor. Compare it to the target for the current cell while tuning.