Fuel Injector Size and Duty Cycle Calculator
Sizes fuel injectors for a target power and checks the duty cycle of the ones you have. The BSFC figure is the whole calculation, so pick it for your fuel and induction.
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The formula
lb/hr per injector = hp × BSFC ÷ (injectors × max duty) cc/min = lb/hr × 10.5 flow at pressure = rated flow × √(pressure ÷ 3 bar)
BSFC is how much fuel it takes to make a horsepower for an hour, so power times BSFC is the total fuel the engine needs. Divide that among the injectors, allow for the fraction of time they can safely be open, and you have the size. The pressure term is from Bernoulli — flow through an orifice goes with the square root of the pressure across it.
What to watch out for
Three things that change the answer:
- BSFC — 0.5 naturally aspirated, 0.6 or more on boost, and double that on E85, which carries about 70% of petrol’s energy per pound. Get the fuel wrong and the injectors are a third too small.
- Duty cycle ceiling. 80–85%. Beyond that the injector is effectively always open and the ECU can no longer trim the mixture.
- Fuel pressure. Raising base pressure from 3 to 4 bar gives about 15% more flow from the same injector, which is often the cheapest way to cover a small shortfall — within the pump’s capability.
Frequently Asked Questions
What size injectors do I need for 300 hp?
With four injectors, a BSFC of 0.5 and 80% duty: 300 × 0.5 ÷ (4 × 0.8) = 47 lb/hr each, about 490 cc/min — so 500 or 550 cc/min injectors. On E85 or a turbo, substantially more.
What is injector duty cycle?
The fraction of the available time the injector is open. 100% means it never closes. 80–85% is the practical maximum for reliable control; a duty cycle above that means the injectors are too small.
How do I convert cc/min to lb/hr?
Divide by 10.5 for petrol — 440 cc/min is about 42 lb/hr. The factor is the density of petrol; it is slightly different for E85 and methanol.