AFR, lambda and stoichiometric mixture
Lambda is simply the ratio of the actual air-fuel mix to the ideal (stoichiometric) one for that fuel:
λ = AFR ÷ stoichiometric AFR · AFR = λ × stoichiometric AFR
Stoich AFR is 14.7 for petrol, ~9.8 for E85, 9.0 for ethanol, 6.4 for methanol and 14.5 for diesel. λ < 1 is rich (more fuel, cooler, more power up to a point); λ > 1 is lean (less fuel, more economy, but more heat under load). Because λ normalises for fuel, wideband tuners increasingly target lambda directly.
Typical lambda targets
Idle & cruise (closed loop): λ 1.00 · Lean cruise: λ 1.02–1.05 · NA wide-open throttle: λ 0.86–0.90 · Boosted WOT: λ 0.75–0.82. These are starting points — knock, EGT and the specific engine decide the final number.
FAQ
Why use lambda instead of AFR?
Because a lambda target is the same for every fuel. λ 0.85 means the same 15% rich whether you're on petrol (AFR ~12.5) or E85 (AFR ~8.3). Switch fuels and your AFR numbers change completely, but your lambda targets don't.
Is richer always safer?
Up to a point — extra fuel cools the charge and resists knock. But too rich washes oil off the bores, fouls plugs and actually loses power. Best power usually sits around λ 0.85–0.88 on petrol; past that you're just wasting fuel.