Cheat sheet
AFR & lambda targets
Lambda targets do not move when the fuel does — that is why they are the safer number to tune to. The AFR column assumes petrol; for any other fuel, multiply lambda by that fuel's stoichiometric ratio from the second table.
| Operating point | λ | AFR on petrol | Notes |
|---|---|---|---|
| Idle | 0.98 – 1.00 | 14.4 – 14.7 | stable and clean; richer only if it will not hold |
| Cruise, closed loop | 1.00 | 14.7 | the ECU is trimming to this anyway |
| Light load | 0.95 – 1.00 | 14.0 – 14.7 | economy zone |
| NA wide-open throttle | 0.86 – 0.90 | 12.6 – 13.2 | best torque sits near 0.88 |
| Boosted, low boost | 0.80 – 0.85 | 11.8 – 12.5 | richer than NA to control charge temperature |
| Boosted, high boost | 0.72 – 0.78 | 10.6 – 11.5 | the fuel is doing cooling work, not just burning |
| Overrun | fuel cut | — | no meaningful ratio while injectors are closed |
Ranges, not settings. Knock margin, charge temperature and fuel quality decide where in a range you belong — and a wideband is the only way to know you got there.
| Fuel | Stoich AFR | λ 0.85 equals |
|---|---|---|
| Petrol (E0) | 14.7 | 12.5 |
| Pump petrol (E10) | 14.1 | 12.0 |
| E30 blend | 13.0 | 11.1 |
| E85 | 9.8 | 8.3 |
| E100 ethanol | 9.0 | 7.7 |
| Methanol | 6.4 | 5.4 |
| LPG | 15.7 | 13.3 |
| Diesel | 14.5 | — |
Ethanol blends interpolate almost linearly between E0 and E100, which is why a blend's stoich ratio can be read straight off the ethanol percentage.