Brake specific fuel consumption (BSFC) is the mass of fuel an engine burns for each unit of work it delivers at the crankshaft, normally quoted in g/kWh. Lower is better. It is the key figure for comparing engine maps, sizing injectors and understanding where power is being lost to heat. The three calculators below are linked: establish your BSFC first, then feed it into thermal efficiency or injector sizing. If you do not have measured data, the reference table at the bottom gives typical values by engine type.
BSFC from Fuel Flow & Power
The fundamental calculation. You need measured fuel flow at a known power output: a logged fuel-used channel against dyno power, a fuel flow meter, or injector flow multiplied by logged duty cycle. Enter volumetric flow (cc/min or L/h) and the calculator converts it to mass using the density you set.
Inputs
1 g/kWh = 0.001644 lb/(HP·hr). Example: 7.5 g/s at 100 kW = 270 g/kWh, about 31% thermal efficiency on petrol.
Thermal Efficiency from BSFC
Puts your BSFC in context: what percentage of the fuel's chemical energy is becoming useful work at the crankshaft. Feed in the result from Calculator 1, or enter a known BSFC directly. Because the fuels differ in energy content, the same BSFC means a different efficiency on petrol, E85 or methanol.
Inputs
LHV = Lower Heating Value of the fuel (energy content per kg, excluding the latent heat of the water vapour in the exhaust).
Result × 100 = thermal efficiency %. Values here are mass-based: ethanol's LHV is 26.8 MJ/kg (21.2 MJ per litre), which is why E85 needs roughly 45% more fuel mass than petrol for the same energy.
Injector Sizing
Given a power target and expected BSFC, calculates the total fuel flow needed and the minimum static flow per injector, assuming one injector per cylinder. Size to 80% duty cycle for most applications: above that the injector spends too much of each cycle opening and closing, its delivery becomes non-linear, and you have no headroom for a hot day, a lean cylinder or a fuel-pressure sag.
Inputs
Per injector (g/min) = total flow / (cylinders × duty cycle)
cc/min = g/min / fuel density. lb/hr = g/min × 60 / 453.6. For petrol, 1 lb/hr is roughly 10 cc/min.
Check the rating pressure before you buy
The cc/min figure above is a static flow at whatever differential pressure the injector is rated at, and manufacturers do not all use the same one. Bosch Motorsport and most aftermarket suppliers rate at 3.0 bar (43.5 psi); Ford Performance rates at 39.15 psi (2.7 bar) and publishes a 1.054 multiplier to get to 43.5 psi. Bosch also flows on n-heptane rather than petrol, so its cc/min and g/min figures do not convert with a petrol density. Compare injectors on g/min or lb/hr at a stated pressure, and if your base fuel pressure differs from the rating, correct it:
Differential means across the injector: rail pressure minus manifold pressure. With a manifold-referenced (rising-rate) regulator the differential stays constant under boost, so use your base pressure. With a fixed regulator, differential falls by the boost pressure at full load and flow falls with it. Raising base pressure is a limited tool: going from 3.0 to 4.0 bar gains only 15% flow and shortens pump life, and a bigger injector is usually the better fix.
Why "green" or "yellow" injectors is not a size
Enthusiasts often quote injectors by body colour because some manufacturers colour-code their own ranges. Those codes only mean something inside one manufacturer's family and one era. Ford's classic EV1 range used yellow for 19 lb/hr, light blue for 24, red for 30 and green for 42, yet Ford also fitted a 39 lb/hr injector in blue to the 2003 to 2004 Cobra, and the current Ford Performance catalogue lists its 24 lb/hr injector in purple and two different 47 lb/hr injectors in black with either a silver tip or a yellow stripe. Bosch, Denso and Siemens use colour for their own purposes, aftermarket bodies are recoloured freely, and a colour tells you nothing about impedance, connector, spray pattern, length or whether the unit has been remanufactured.
Identify an injector by its part number and the manufacturer's data sheet, and confirm: rated static flow and the pressure it was measured at, the test fluid, fuel compatibility (E85 in particular), coil impedance (high, roughly 12 to 16 ohms, or low, roughly 2 to 5 ohms, which needs a peak-and-hold driver), connector type (Jetronic/EV1, USCAR/EV6, Sumitomo/Denso), overall length and O-ring sizes. If any of that is unknown, have the set flow-tested before you map the engine. Colour is a starting point for a search, never a specification.
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. Efficiency figures use the LHV of the fuel in each row.
| 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 charge 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 | 31–38% | High g/kWh because of low energy density (29.2 MJ/kg), but charge cooling and knock resistance help power |
| Two-stroke petrol | 400–600 | 14–21% | Poor scavenging efficiency, fuel lost through ports |
| F1 Power Unit (2023) | ~165 | ~50% | Combined ICE + MGU-H + MGU-K. ICE alone ~45%, so not directly comparable with the rows above |
Common questions
For a naturally aspirated petrol engine at peak power, 270–320 g/kWh is normal for a road engine and a well-developed race engine gets down to 230–265 g/kWh. Turbocharged petrol engines run rich at full load for charge cooling, so 300–380 g/kWh is common even when the engine is efficient at part throttle. Modern turbodiesels are the benchmark at 185–215 g/kWh. Compare like with like: BSFC at peak power, on the same fuel, at the same lambda.
Divide 3,600,000 by (BSFC in g/kWh × the fuel's lower heating value in kJ/kg). For petrol at 43,200 kJ/kg, 270 g/kWh gives 3,600,000 / (270 × 43,200) = 0.309, or 30.9%. The same 270 g/kWh on E85 (29,200 kJ/kg) is 45.7%, which shows why BSFC only compares engines on the same fuel. Calculator 2 does this for you.
It depends on the fuel and the BSFC, which is why the question cannot be answered from power alone. Worked example for a 4-cylinder turbo petrol engine at 300 BHP (224 kW), 320 g/kWh and 80% duty: total flow = 320 × 224 / 60 = 1,195 g/min; per injector = 1,195 / (4 × 0.8) = 373 g/min, which is 494 cc/min or 49 lb/hr on race petrol. On E85 at 400 g/kWh the same engine needs 467 g/min per injector, about 595 cc/min. Round up to the next available size and check the rating pressure.
Size for 80% at peak power on most engines, 85% at most. The injector needs time to open and close on every cycle, and at high RPM that dead time is a large fraction of the available window, so delivery becomes non-linear as duty approaches 100%. Headroom also covers hot fuel, a weak pump, an injector that flows low, and the extra fuel a tuner will want to add on a hot day. Oversizing has a cost too: very large injectors idle badly and are hard to control at low pulse widths, so do not simply fit the biggest set available.
lb/hr = cc/min × density (g/cc) × 60 / 453.6. For petrol at 0.745 to 0.755 g/cc that works out at 10.0 to 10.2 cc/min per lb/hr, so the common rule of thumb of "divide cc/min by 10.5" is slightly conservative. The two figures only match at the same rating pressure, so check that before comparing a lb/hr injector with a cc/min one.
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