| Target HP | Per Injector |
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Sizing Fuel Injectors
Injectors must flow enough fuel to feed your power goal without running flat out. Size them from three things: the horsepower you want, how efficiently the engine uses fuel (BSFC), and a safe maximum duty cycle. This calculator turns those into the per-injector flow you need and points you to the next common injector size.
BSFC and Duty Cycle
Brake-specific fuel consumption is how much fuel the engine burns per horsepower per hour. Naturally aspirated gasoline sits near 0.50; boost and high cylinder pressure push it higher, so forced-induction builds use 0.55 to 0.65. Duty cycle is how much of the time the injector is open at peak; keeping it under about 85 percent ensures the injector never goes static and loses fine control.
Bigger Is Not Always Better
Oversized injectors give power headroom but can idle and cruise poorly because they must pulse very briefly, which hurts low-speed metering. Modern high-impedance injectors with good dynamic range tolerate this better, but the goal is still to size for your target with sensible margin, not to fit the largest injector you can find.
Frequently Asked Questions
What BSFC should I use?
Around 0.50 for naturally aspirated gasoline and 0.55 to 0.65 for boosted engines. E85 needs roughly a third more fuel, so size up accordingly.
Why cap duty cycle at 85 percent?
Past that, the injector approaches static (fully open) and can no longer add fuel for more load, leaving you lean at the worst moment. The margin is a safety buffer.
How do lb/hr and cc/min relate?
For gasoline, one pound per hour is roughly 10.5 cc per minute. The exact factor shifts slightly with fuel density and pressure.