BSFC measures how efficiently an engine converts fuel into work — lower is better. It's the key figure for comparing engine maps, selecting injectors, and understanding where power is being lost to heat. All three calculators below are linked: figure out your BSFC first, then feed it into efficiency or injector sizing.
BSFC from Fuel Flow & Power
The fundamental calculation. You need either logged fuel flow data or a known injector duty cycle to use this.
Inputs
Equivalently: BSFC = ṁf / P · 3.6×106 · 10−3
US conversion: BSFC (g/kWh) × 0.001644 = lb/(HP·hr)
Thermal Efficiency from BSFC
Puts your BSFC in context — tells you what percentage of the fuel's energy is actually becoming useful work at the crankshaft. Feed in the result from Calculator 1, or enter a known BSFC directly.
Inputs
LHV = Lower Heating Value of the fuel (energy content per kg, not including latent heat of water vapour).
Result × 100 = thermal efficiency %.
Injector Sizing
Given a power target and expected BSFC, calculates the total fuel flow needed and the minimum injector flow rate per cylinder. Use 80% duty cycle as a safe maximum for most setups — higher than that and the injector can't keep up with heat soak between pulses.
Inputs
Per injector (g/min) = Total flow / (cylinders × duty cycle)
cc/min assumes petrol density of 0.755 g/cc. Adjust for E85 (~0.785 g/cc) or diesel (~0.850 g/cc).
Typical BSFC Values by Engine Type
At best-power (wide open throttle, peak power RPM). Part-throttle and off-peak BSFC is typically 20–40% worse.
| Engine type | BSFC (g/kWh) | Thermal eff. approx. | Notes |
|---|---|---|---|
| NA petrol — road car | 270–320 | 25–30% | Typical modern 4-cylinder road engine at peak power |
| NA petrol — optimised/race | 230–265 | 32–36% | Well-developed race engine, good cam/head work |
| Turbocharged petrol | 240–290 | 28–35% | Wide range depending on boost level and lambda |
| Turbocharged petrol — rich WOT | 300–380 | 22–28% | Running rich for cooling — efficiency deliberately sacrificed |
| Naturally aspirated diesel | 210–240 | 35–40% | Higher compression ratio, better low-end efficiency |
| Turbodiesel (modern common rail) | 185–215 | 39–45% | Best efficiency of any production IC engine type |
| E85 / Ethanol | 320–400 | 28–36% | High g/kWh due to low energy density, but charge cooling helps power |
| Two-stroke petrol | 400–600 | 15–22% | Poor scavenging efficiency, fuel lost through ports |
| F1 Power Unit (2023) | ~165 | ~50% | Combined ICE + MGU-H + MGU-K. ICE alone ~45% |