Lambda, AFR and target mixtures

λ is the mixture relative to stoichiometric. It is the same number on every fuel, which is why tuners think in λ.

Lambda (λ) is the actual air-fuel ratio divided by the stoichiometric ratio of the fuel. λ = 1.00 is chemically exact; 0.85 is 15 % rich; 1.10 is 10 % lean. AFR is λ multiplied by the stoich ratio: 14.7 on gasoline, about 9.8 on E85. Say "0.85 λ" and everyone knows what you mean on any fuel; say "12.5:1" and you have to ask which fuel.

What the mixture does

• Best power is around λ 0.86–0.88: slightly rich, so every oxygen molecule finds fuel. Leaner than that loses power slowly and heats things quickly.
• Best economy and lowest emissions sit at λ 1.00: that is why idle and cruise targets are stoichiometric.
• Richer than ~0.80 makes no more power. The extra fuel evaporates and cools the chamber (good under boost) but costs torque and eventually washes the oil off the bores.
• Past ~0.68 rich or ~1.30 lean the mixture stops lighting reliably: misfire.

Typical schedules

• NA: 1.00 at light load, 0.95 around three-quarter load, 0.86–0.88 at wide-open throttle.
• Turbo: 1.00 off boost, 0.90 at atmospheric, down to 0.78–0.80 at full boost. The extra fuel is there to cool the chamber and buy knock margin; the power is in the air, not the fuel.