Fuel Injector Size Calculator

Fuel injectors need to be sized correctly for your engine's power output — too small and they'll run at maximum capacity, starving the engine of fuel. Enter your target power, power unit, number of cylinders, induction type, fuel type, fuel pressure, and maximum duty cycle into the Fuel Injector Size Calculator to get the required injector size in cc/min and lb/hr. Secondary outputs include total fuel flow required and the BSFC (brake-specific fuel consumption) value used in the calculation.

psi

Base fuel pressure at idle

%

Conservative limit for injector operation

Results

Required Injector Size

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Required Injector Size

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Total Fuel Flow Required

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BSFC Used

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Frequently Asked Questions

What is BSFC and how does it affect injector sizing?

BSFC (Brake Specific Fuel Consumption) measures how much fuel an engine consumes per horsepower per hour. Turbocharged engines typically have higher BSFC values (0.55-0.65) compared to naturally aspirated engines (0.45-0.55), requiring larger injectors for the same power output.

What duty cycle should I use for my injectors?

For reliable operation, keep injector duty cycle between 80-85% maximum. This provides safety margin for fuel pump variations and ensures injectors can maintain proper spray patterns. Racing applications may run up to 90-95% but require careful monitoring.

How does fuel pressure affect injector size calculations?

Higher fuel pressure increases injector flow rate. The calculator assumes your base fuel pressure at idle. If using a rising rate fuel pressure regulator with forced induction, the base pressure should be your idle pressure, not boost-referenced pressure.

Should I calculate based on crankshaft or wheel horsepower?

Use crankshaft horsepower for engine fuel requirements since that's where combustion occurs. Wheel horsepower is lower due to drivetrain losses (typically 15-25%). If you only know wheel horsepower, add 20-25% to estimate crankshaft power.

Do I need different sized injectors for E85 vs gasoline?

Yes, E85 requires approximately 30-40% more fuel flow than gasoline for the same power output due to its lower energy density. The calculator automatically adjusts for different fuel types to give you the correct injector size.

What happens if I use injectors that are too small?

Undersized injectors will hit 100% duty cycle before reaching target power, causing lean conditions that can damage your engine. Always size injectors with proper headroom for safety and to maintain good fuel atomization.

Can I use larger injectors than calculated?

Yes, but oversized injectors can cause idle quality issues and poor low-RPM fuel economy due to difficulty controlling very small pulse widths. Stay within 20-30% larger than calculated for best drivability.

How do I convert between cc/min and lb/hr flow rates?

The conversion depends on fuel type: For gasoline, multiply cc/min by 0.0875 to get lb/hr. For E85, multiply by 0.09. The calculator shows both units automatically based on your fuel selection.