Lambda, AFR, and what the gauge says
Three ways of writing one mixture, and why a car on E85 at a perfectly good lambda reads 12.1 on a wideband — both numbers correct, only one an air/fuel ratio.
A mixture can be written three ways, and mixing them up is routine.
Lambda is how far the mixture is from stoichiometric, as a ratio. λ = 1.00 is exactly enough air to burn the fuel, λ < 1 is rich, λ > 1 is lean. It means the same thing on every fuel there is, which is why tuners talk in it.
Air/fuel ratio is kilograms of air per kilogram of fuel. It only means something once you say which fuel: 12.5:1 is rich on petrol and catastrophically lean on methanol.
What the gauge shows is the one that catches people.
The wideband is lying to you, correctly
A wideband sensor measures oxygen. It computes lambda, and then — to show an AFR — multiplies by 14.7, because that is petrol’s stoichiometric ratio. It does this whatever is in the tank, unless you have told it otherwise.
So a car on E85 running a perfectly sensible λ 0.82 displays 12.1:1 on a gauge scaled to petrol. Its actual air/fuel ratio is 8.0:1, because E85 is stoichiometric at 9.8. Both numbers are correct. Only one of them is an air/fuel ratio, and the one on the screen is not it.
This is why “what AFR should I run on E85” gets contradictory answers: half the people answering mean the gauge number and half mean the real one, and neither says which.
Talk in lambda and the problem disappears.
Stoichiometric ratios
| Fuel | Stoichiometric | λ 0.85 is |
|---|---|---|
| Petrol | 14.7:1 | 12.5:1 |
| E85 | 9.8:1 | 8.3:1 |
| E100 ethanol | 9.0:1 | 7.7:1 |
| Methanol | 6.4:1 | 5.4:1 |
Note that changing fuel does not change the lambda you want. It changes how much fuel that lambda takes — which is a fuel system problem, covered in sizing fuel injectors.
Roughly where things sit
Best torque on most petrol engines is somewhere around λ 0.88 to 0.95 — slightly rich of stoichiometric, because the extra fuel makes sure every molecule of oxygen finds something to burn. Under boost people run richer still, 0.75 to 0.82, and the extra fuel is doing a second job there: it is evaporating in the chamber and cooling the charge, which is what buys back some detonation margin.
Light-load cruise runs at or slightly lean of stoichiometric, because that is what a catalyst needs and what uses least fuel.
Those are neighbourhoods, not targets. What is right depends on the engine, the load, the fuel and the timing, and any page that tells you a number without knowing those is guessing.
One of 7 calculators here. The formula it uses is written out above.
Where this came from
- Stoichiometric ratios by fuel; lambda as the normalised form