Fuel Injector Size Calculator: cc/min, lb/hr, E85, Pressure
Work out the injector flow you need in cc/min and lb/hr from power, cylinder count, fuel (gasoline or E85), duty cycle and fuel pressure.
How to calculate fuel injector size
Required injector flow equals target power times brake specific fuel consumption (BSFC), divided by the number of injectors and the maximum duty cycle. Convert lb/hr to cc/min with the fuel density, then correct the flow to the pressure at which the injector is rated. A 300 hp turbo four-cylinder fed at 4 bar needs about 495 cc/min of rated flow at 3 bar (572 cc/min if it runs at 3 bar).
The formula
Flow per injector (lb/hr) = power (hp) × BSFC ÷ (number of injectors × duty cycle). BSFC is pounds of fuel per horsepower per hour; 1 kW = 1.341 hp and 1 metric PS = 0.986 hp. Then 1 lb/hr = 7.56 g/min, or 10.16 cc/min of gasoline at 0.744 g/cm³. Flow changes with pressure as Q2 = Q1 × √(P2/P1), per the Bosch Motorsport EV 14 injector sheet (2019).
Worked example
300 hp turbo four-cylinder on gasoline, BSFC 0.60, 80% maximum duty, 4 bar operating pressure: 300 × 0.60 = 180 lb/hr in total; 180 ÷ (4 × 0.8) = 56.3 lb/hr per injector at 4 bar, or 572 cc/min; at the 3 bar catalogue pressure that is 572 × √(3/4) = 495 cc/min (48.7 lb/hr).
Starting values
Gasoline BSFC runs 0.45-0.50 naturally aspirated and 0.55-0.60 boosted (EFI University, 2017). E85 needs 30-35% more fuel, and DeatschWerks recommends 80% duty at most (2019 interview); Fuel Injector Clinic allows 90%.
Limits
The common 10.5 factor assumes 0.72 g/cm³, and datasheet flows are often measured with n-heptane (0.684 g/cm³ at Bosch). Real BSFC depends on mixture and timing, so dyno tuning has the final word. For wheels, see the tire size calculator.
Frequently Asked Questions
How do I convert cc/min to lb/hr?
Multiply cc/min by the fuel density in g/cm³ to get g/min, then divide by 7.56. With gasoline at 0.744 g/cm³, 1 lb/hr equals 10.16 cc/min, so 550 cc/min is 54.1 lb/hr. The popular 10.5 factor assumes a density of 0.72.
What maximum duty cycle should I use?
DeatschWerks recommends 80% at most, which leaves headroom if fuel pressure drops or power goes up. Fuel Injector Clinic allows 90%. Beyond that the injector is close to staying open and the ECU has nothing left to add.
How much more flow does E85 need?
DeatschWerks puts the BSFC increase from gasoline to E85 at 30 to 35%. An engine that needs 495 cc/min on gasoline needs roughly 615 to 635 cc/min on E85, at a density of 0.78.
What does raising fuel pressure do?
Flow rises with the square root of pressure (Bosch Motorsport). Going from 3 to 4 bar multiplies flow by √(4/3) = 1.155, or +15.5%: a 550 cc/min injector rated at 3 bar flows 635 cc/min at 4 bar.
Should I enter wheel or crank horsepower?
Crank horsepower, because BSFC refers to engine output. A wheel figure, reduced by drivetrain losses, understates the need and leads to undersized injectors. The formula stays the same: power × BSFC ÷ (injectors × duty cycle).
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