Race gas & E85 blend calculator
Plan your fuel blend with the math done right.
Mix race gas with pump gas, or E85 with 91/93 — and see the real stoich AFR, octane and density. Mass-weighted, not the linear average that forum spreadsheets get wrong.
Terms explained
- AFR
- Air-fuel ratio by mass. Stoichiometric AFR is the chemically ideal mix — ~14.7 for gasoline, ~9.8 for E85 (ethanol needs more fuel).
- AKI (R+M)/2
- Anti-Knock Index — the octane number on US pumps, the average of RON and MON. Higher = more resistant to detonation.
- RON / MON
- Research and Motor Octane Numbers — two lab methods for octane. US pumps show their average (AKI); much of the world shows RON.
- λ (lambda)
- Air-fuel ratio relative to stoichiometric. λ = 1.0 is stoich for any fuel; below 1.0 is rich, above is lean.
- E85 / ethanol %
- Pump 'E85' is 51–83% ethanol and varies seasonally. More ethanol = lower AFR and more knock resistance, but more fuel volume.
- BSFC
- Brake-specific fuel consumption — fuel mass per HP per hour. Used to size injectors; ethanol and boost raise it.
- Injector duty cycle (IDC)
- How much of the time an injector is open. 85% is the common safe ceiling — above it you lose fueling control.