Fuel Injector Size Calculator
Work out the injector size your power target needs, in pounds per hour and cc per minute, at a duty cycle that leaves headroom.
Your build
- Injector flow ratings are quoted at a specific fuel pressure, usually 43.5 psi or 3 bar. Higher base pressure raises flow by roughly the square root of the pressure ratio.
- Oversizing hurts idle quality, because very large injectors struggle to meter the tiny amount of fuel an idling engine needs.
- BSFC is an assumption, not a measurement. A well tuned engine may do better and a poorly tuned one much worse.
How injector sizing works
Power needs fuel, and the relationship is close to linear. Brake specific fuel consumption is how many pounds of fuel an engine burns per horsepower per hour. A naturally aspirated gasoline engine sits around 0.50, so 500 horsepower needs about 250 pounds of fuel every hour.
Divide that across the injectors and divide again by the duty cycle you are willing to run, and you have the size each one needs to be.
Why duty cycle matters
Duty cycle is the fraction of available time an injector is open. At 100% it is open constantly and cannot deliver any more. In practice injectors become unpredictable well before that, because the pintle needs time to fully close and the flow curve stops being linear.
80 to 85% is where most tuners stop. Sizing to 100% leaves nothing for a hot day, a low fuel pressure moment, or a tank of fuel with a different ethanol content than you expected.
Boost and ethanol both need more
Boosted engines run richer than naturally aspirated ones to keep combustion temperatures down and hold off detonation, so their BSFC is higher. Ethanol carries less energy per pound than gasoline, so E85 needs roughly 40% more fuel mass for the same power. Combine both and injector requirements climb fast.
Frequently asked questions
What size injectors do I need for 500 hp?
On a naturally aspirated gasoline V8 at 85% duty, around 37 lb/hr or 390 cc/min each. The same 500 hp from a boosted engine needs closer to 46 lb/hr because boosted engines run richer, and on E85 it climbs past 60.
How do I convert lb/hr to cc/min?
Multiply by about 10.5 for gasoline. So a 42 lb/hr injector flows roughly 440 cc/min. The factor comes from gasoline's density, which is why the conversion differs slightly for other fuels.
Can injectors be too big?
Yes. A very large injector has to open for an extremely short time at idle, and below a certain pulse width the flow becomes inconsistent. That shows up as a rough idle and poor low load driveability. Modern injectors handle a wider range than older designs.
Does fuel pressure change injector flow?
Yes, roughly with the square root of the pressure ratio. Raising base pressure from 43.5 to 58 psi increases flow about 15%. It is a way to gain a little headroom, not a substitute for correctly sized injectors.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.