The BSFC Calculator converts engine power output and fuel flow rate into brake specific fuel consumption, then translates that figure into brake thermal efficiency for the selected fuel type.
Calculate Engine Efficiency with the BSFC Calculator
The BSFC Calculator solves brake specific fuel consumption, required fuel flow, or power output, whichever two you don’t already know. Engine builders and tuners use it to translate a dyno’s fuel-flow log into an efficiency number, or to size a fuel system for a horsepower target.
Solving for Consumption Rate, Fuel Flow, or Power Output
Pick which value to solve for, then enter engine power and fuel flow rate, or power and a known BSFC, or fuel flow and a known BSFC.
Select a fuel type first, since gasoline, E85, methanol, and diesel each carry a different heating value used in the efficiency calculation.
The Brake Thermal Efficiency Math Behind the BSFC Calculator
BSFC is fuel mass flow divided by brake power, confirmed directly by Wikipedia’s brake-specific-fuel-consumption reference and independently by x-engineer.org’s engine-efficiency documentation.
$$ BSFC = \dfrac{\dot{m}_{fuel}}{P_{brake}} $$
Brake Thermal Efficiency inverts that relationship against the fuel’s lower heating value (LHV), and Wikipedia states this exact formula for gasoline: efficiency = 1 / (BSFC × 0.0122225).
$$ BTE = \dfrac{1}{BSFC \times LHV} $$
Wikipedia’s own worked example uses this to convert a gasoline engine’s cycle-average BSFC of 322 g/(kWh) into 25.4% efficiency, which this calculator reproduces exactly at that input.
A common mistake is applying gasoline’s LHV constant to a different fuel; methanol’s heating value is roughly half of gasoline’s, so the same BSFC number implies a completely different efficiency depending on which fuel is selected.
Fuel type matters even within one fuel choice: per the U.S. Department of Energy‘s Alternative Fuels Data Center, E85’s actual heating value varies by season and region, since federal spec allows its ethanol content to range from 51% to 83%.
Reading the Volumetric, Mass, and Injector-Unit Cards
The second card converts the same fuel mass flow into gallons or liters per hour, using the selected fuel’s specific gravity — gasoline at 0.720, methanol at 0.792, and diesel at 0.832, figures consistent with published fuel-property tables.
The third card restates that same flow in lbs/min and kg/hr and kg/min, useful for cross-checking against a fuel pump’s rated capacity.
The fourth card converts to cc/min and g/s, the units injector and fuel-pump datasheets are usually quoted in, along with whichever density factor was used to get there.
Real Engine BSFC Figures From Wikipedia’s Own Data
These are actual measured engines from Wikipedia’s BSFC reference table, not estimates.
| Gasoline cycle-average (reference point) | 322 g/kWh, 25.4% efficient | Wikipedia |
| GM Saturn I4 (production gasoline) | 250 g/kWh, 32.8% efficient | Wikipedia |
| Toyota Prius 1NZ-FXE (2003) | 225 g/kWh, 36.4% efficient | Wikipedia |
| Mazda R26B rotary (787B Le Mans) | 286 g/kWh, 28.7% efficient | Wikipedia |
The spread between these shows why a single “good BSFC” number doesn’t exist independent of the engine and fuel involved.
Common Mistakes When Comparing BSFC Across Engines
Comparing BSFC values across different fuels without converting through each fuel’s own LHV first, which produces a meaningless side-by-side comparison.
Using an engine’s rated or peak-efficiency BSFC as if it applied everywhere in the operating range, when BSFC actually varies significantly with RPM and load, per x-engineer.org’s BSFC mapping.
Treating fuel flow measured at idle or light cruise as representative of the number that matters for sizing a fuel system at wide-open throttle.
BSFC and Engine Efficiency Questions People Ask
What is BSFC?
Brake specific fuel consumption is the rate of fuel consumption divided by the power an engine produces, used to compare fuel efficiency across different engines.
What is a good BSFC value?
Per Wikipedia’s own data, a gasoline engine’s cycle-average BSFC is about 322 g/kWh (25% efficient), while the best production gasoline engines reach roughly 225 g/kWh (36% efficient).
How is BSFC different from fuel economy?
BSFC measures the engine alone, independent of vehicle weight and aerodynamics, while fuel economy reflects the whole vehicle; a lower BSFC generally helps fuel economy but doesn’t guarantee it.
Does diesel have a lower BSFC than gasoline?
Yes, typically, due to higher compression ratios and leaner combustion, even though diesel’s own energy density (about 42.7 MJ/kg) is close to gasoline’s (43.9 MJ/kg).
What’s the difference between BSFC and ISFC?
BSFC uses brake power measured at the crankshaft or dyno, while Indicated Specific Fuel Consumption (ISFC) uses cylinder pressure-based power and excludes mechanical losses like friction.