Fuel Injector Size Calculator
Size injectors from target horsepower, BSFC and duty cycle - in lb/hr and cc/min, for gasoline and E85, with every assumption stated.
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Injector Size Results
Required size at each duty cycle
| Duty | LB/HR each | CC/MIN each | Total LB/HR |
|---|
* Above the 80% industry-standard duty ceiling - flow goes nonlinear and the injector runs hot with no margin for fuel pressure drop.
The total column is what your fuel pump must deliver at that duty cycle. For gasoline, multiply lb/hr by 0.63 for liters per hour.
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Flow rises with the square root of the pressure across the injector. Data sheets usually rate at 43.5 psi (3 bar) - set "running at" to your base pressure to correct for it.
How the sizing works
Required flow per injector is target horsepower times BSFC, divided by the number of injectors times the duty cycle - the formula RC Engineering, the injector flow-bench standard, publishes with its recommended BSFC bands: 0.45 to 0.50 for naturally aspirated gasoline, 0.55 to 0.60 supercharged, 0.60 to 0.65 turbocharged. Multiply lb/hr by 10.5 for cc/min. The default case here is RC's own worked example - 200 hp, four cylinders, BSFC 0.50, 80% duty - which they size at 31 lb/hr or 330 cc/min. Power means crank horsepower: if you only know a wheel figure, add your assumed drivetrain loss back on first (the trap speed calculator explains that distinction).
BSFC is an assumption, not a measurement
Brake-specific fuel consumption is how many pounds of fuel the engine burns per horsepower per hour. It is the soft input in this formula: a modern efficient engine at light load can run under 0.45, while a boosted engine tuned rich for knock protection and charge cooling can exceed 0.65. That is why the table above shows the answer at every duty cycle rather than pretending one number is exact - and why a BSFC measured on your dyno beats any published band.
E85 and duty cycle
E85 carries about a third less energy per pound than gasoline, so the widely published rule is 30% more injector flow for the same power - this calculator multiplies by 1.3, and builders wanting margin go 35 to 40%. Because spec sheets flow-rate injectors on gasoline, the E85 figures here are the gasoline-rated size to shop for, best compared in cc/min. On duty cycle: 80% is the industry ceiling because injectors driven harder run hot and go nonlinear, but oversizing has a cost too - a huge injector meters poorly at the tiny pulse widths of idle, so buy the next size that fits, not the biggest one made. Fuel pressure is a real lever as well: flow rises with the square root of the pressure across the injector, so 550 cc injectors at 58 psi flow like 635s - at the cost of pump headroom and injector heat.