How this instrument works
Brake specific fuel consumption measures how much fuel an engine burns to produce a given amount of power at the crankshaft. 'Brake' refers to the dynamometer brake used to load and measure the engine's actual output power, as opposed to the theoretical indicated power computed from in-cylinder pressure. BSFC is simply fuel mass flow rate divided by that measured brake power, typically expressed in kilograms of fuel per kilowatt-hour (kg/kWh) or the equivalent grams per kilowatt-hour.
A lower BSFC means the engine extracts more usable power from every unit of fuel burned — it is a direct efficiency figure, not a power figure. Well-tuned gasoline engines at their most efficient operating point typically land around 0.2 to 0.3 kg/kWh, while modern diesel engines, which run leaner and at higher compression, often achieve a noticeably lower (better) BSFC in the 0.19 to 0.22 kg/kWh range at their sweet spot.
BSFC is not a single fixed number for an engine — it changes across the rpm and load range, which is why engine tuners plot a full BSFC map or 'island plot' across the operating envelope rather than quoting one figure. This calculator gives you the BSFC at a single measured operating point: whatever fuel flow rate and brake power a dyno pull records at that moment.
- Enter Fuel mass flow rate as measured by the dyno's fuel flow meter, in whatever mass-per-hour unit your test log uses (e.g. kg/h).
- Enter Brake power output — the power the dynamometer actually measured at that operating point, in the matching unit (e.g. kW).
- Read BSFC directly beneath the inputs — it is fuel flow divided by power, so keep both figures in consistent units.
- Compare BSFC at several rpm points from the same dyno run to see where the engine is most fuel-efficient, not just where it makes the most power.
- A lower BSFC value is better; it means less fuel was needed to produce each unit of power at that test point.
Worked example — 20 kg/h fuel flow at 100 kW brake power
A dyno pull records a fuel mass flow rate of 20 kg/h while the engine holds a steady 100 kW of brake power at that point. Enter 20 into Fuel mass flow rate and 100 into Brake power output. BSFC = 20 / 100 = 0.2 kg/kWh.
That 0.2 kg/kWh sits at the efficient end of the typical gasoline-engine range, meaning the engine needs only 0.2 kilograms of fuel to sustain each kilowatt of brake power for an hour. A second engine burning 45 kg/h at 150 kW would instead compute to 0.3 kg/kWh — a noticeably less efficient point on its own map, even though it is making more absolute power.
Questions
What does 'brake' mean in brake specific fuel consumption?
'Brake' refers to the dynamometer brake used to apply a measured load to the engine and read its actual delivered power at the crankshaft or flywheel. That measured figure is called brake power, distinct from indicated power (computed from in-cylinder pressure traces) or friction power (lost to internal engine friction). BSFC always uses the brake — i.e. the real, usable — power figure in its denominator, which is why it reflects what the engine actually delivers to a load.
What counts as a good BSFC value?
It depends on engine type and operating point. Naturally aspirated gasoline engines commonly show a best-point BSFC around 0.2–0.3 kg/kWh (200–300 g/kWh), while efficient diesel engines often reach 0.19–0.22 kg/kWh at their optimum load and rpm because they run leaner and at higher compression ratios. Any single BSFC number is only meaningful at the specific rpm and load it was measured at — an engine's BSFC varies considerably across its full operating range.
Why is BSFC reported instead of just fuel consumption in gallons per hour?
Raw fuel flow (gallons or kilograms per hour) tells you how much fuel is being burned, but not how efficiently that fuel is being converted into power — a big engine at high load and a small engine at low load can burn the same fuel flow while making very different power. Dividing fuel flow by brake power normalizes for output, so BSFC lets you compare efficiency fairly across engines of different sizes or across different operating points of the same engine.
Does a lower BSFC always mean a better engine overall?
It means better fuel efficiency at that specific measured point, not necessarily a better engine overall. An engine can have an excellent BSFC at its efficiency sweet spot but a much worse BSFC elsewhere in its rpm range, or it might trade some efficiency for higher peak power, lower emissions, or better drivability. Tuners typically look at the full BSFC map across rpm and load, not a single number, before judging an engine's overall efficiency character.
What units should I use for fuel flow and power?
Any consistent pair works because BSFC is just a ratio — kilograms per hour and kilowatts give kg/kWh, while pounds per hour and horsepower give lb/hp·h, a common form in US dyno sheets. This calculator does not convert units for you, so make sure the fuel-flow unit and power unit you enter match the units you want in the BSFC result, and stay consistent if you are comparing BSFC figures pulled from different dyno software.
How is BSFC different from specific fuel consumption (SFC) in general?
Specific fuel consumption is the general family of metrics relating fuel flow to power output, and it comes in a few variants depending on which power figure is used in the denominator — brake (delivered) power, indicated (in-cylinder) power, or effective power in other contexts. BSFC specifically uses brake power, the measured, real-world output figure from a dynamometer, which is why it is the version most commonly quoted in engine test reports and tuning discussions.