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Injector Sizing Calculator

Estimate required injector flow per cylinder from horsepower, BSFC, cylinder count, duty cycle and margin.

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Calculator

Use a realistic power target.
lb fuel / hp-hour. Turbo gasoline planning often uses a conservative value around 0.55–0.65; exact engine data wins.
Number of injectors when one injector per cylinder.
Percent; leaving duty-cycle headroom improves control margin.
Percent added to calculated mass demand.
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How the calculation works

The calculator first estimates total fuel mass flow as horsepower × BSFC, adds the selected margin, then divides that mass flow across the injectors at the selected maximum duty cycle. It reports both lb/hr per injector and an approximate gasoline-reference cc/min conversion.

The cc/min conversion uses about 10.5 cc/min per lb/hr as a planning approximation. Published injector flow depends on test fluid, differential pressure and temperature; use the injector manufacturer’s actual data for selection and calibration.

Worked example

For a 400-hp four-cylinder at 0.60 BSFC, 85% duty and 10% margin, the required injector size is substantially larger than a naturally aspirated stock injector. That is expected: the margin and duty-cycle target deliberately keep the injector from living at its absolute limit.

Important limitations

Do not use injector size to infer safe horsepower by itself. Pump flow, rail pressure, regulator behavior, fuel composition, battery voltage, injector characterization and calibration all matter. Ethanol blends typically require more fuel volume for the same power than gasoline.