Fuel Injector Size Calculator (lb/hr and cc/min)
What size fuel injectors do I need for my horsepower target? Enter your power at the crank, the brake specific fuel consumption (BSFC), the number of injectors and the highest duty cycle you want to run. The result is the flow rating each injector needs, in pounds per hour and cubic centimetres per minute, for gasoline, E85 or diesel.
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Formula
total fuel flow (lb/hr) = horsepower x BSFC
injector flow (lb/hr) = total fuel flow / (injectors x duty cycle)
injector flow (cc/min) = lb/hr x 453.59237 / 60 / density (g/cc)
Assumptions: the power is crank horsepower; every injector flows equally; densities are typical values (gasoline 0.74, E85 0.789, diesel 0.83 g/cc); the rating is at the injector's own test pressure. Port-injected sizing only; direct-injection systems differ.
Worked example
400 hp, BSFC 0.5 lb/hp/hr, 8 injectors, 80% duty cycle, gasoline:
- Total fuel flow: 400 x 0.5 = 200 lb/hr
- Per injector: 200 / (8 x 0.8) = 31.25 lb/hr
- In cc/min: 31.25 x 453.59237 / 60 / 0.74 = 319.3 cc/min
With E85 (0.789 g/cc) the same mass flow is 299.4 cc/min.
Use this when
You are planning an engine build or a power upgrade and need to know whether the stock injectors have headroom. Typical BSFC is roughly 0.45 to 0.55 for naturally aspirated gasoline engines and roughly 0.60 to 0.70 for boosted ones, and many tuners keep duty cycle at or below 80 to 85 percent. Those are typical rules of thumb, not limits for your engine.
Related calculators
The horsepower calculator gives the power figure to plan around. The engine displacement and compression ratio calculators describe the engine itself, but neither sizes the fuel system.
Common mistakes
- Using wheel horsepower. Use crank horsepower here.
- Dividing the total flow by the number of injectors but forgetting the duty cycle, which sizes the injectors too small.
- Mixing lb/hr with cc/min when comparing against a data sheet.
- Ignoring fuel pressure. Flow changes with the square root of the pressure ratio.
- Using the gasoline density for E85 or diesel, which shifts cc/min.
Frequently asked questions
How do you size fuel injectors?
Injector flow (lb/hr) = horsepower x BSFC / (injectors x duty cycle). For 400 hp, BSFC 0.5, 8 injectors and 80% duty, that is 400 x 0.5 / (8 x 0.8) = 31.25 lb/hr per injector.
How do I convert lb/hr to cc/min?
Multiply lb/hr by 453.59237 g/lb, divide by 60 for grams per minute, then divide by the fuel density in g/cc. For gasoline at 0.74 g/cc, 31.25 lb/hr is 31.25 x 453.59 / 60 / 0.74 = 319.3 cc/min. The common shortcut of 10.5 cc/min per lb/hr assumes a slightly lighter fuel.
What BSFC and duty cycle should I use?
Typical BSFC is about 0.45 to 0.55 for naturally aspirated gasoline engines and 0.60 to 0.70 for boosted ones. Many tuners cap injectors at 80 to 85% duty to leave headroom. These are typical rules of thumb, not limits for your engine.
Does injector pressure or fuel type change the result?
Yes. Ratings are quoted at a test pressure, and flow rises with the square root of the pressure ratio. E85 needs roughly 50% more fuel by mass (about 40% more by volume) for the same power, which you account for by entering a higher BSFC. Confirm with the injector data sheet.
What size injectors do I need for 500 hp on a V8?
With BSFC 0.5, 8 injectors and 80 percent duty cycle, 500 hp needs 500 x 0.5 / (8 x 0.8) = 39.06 lb/hr per injector, which is 399.1 cc/min on gasoline. Round up to the next available injector size.
What injector size do I need for 300 hp on a turbo 4-cylinder?
Boosted engines usually have a higher BSFC. At 0.65, 4 injectors and 80 percent duty, 300 hp needs 60.94 lb/hr per injector (622.5 cc/min on gasoline). That is a typical planning figure, so check it against the tuner's advice.
How big should injectors be for E85?
Use a higher BSFC, because E85 needs more fuel by mass. Setting BSFC to 0.65 for 600 hp across 6 injectors at 85 percent duty gives 76.47 lb/hr, or 732.7 cc/min at the E85 density of 0.789 g/cc.