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Injector Sizing Calculator
Size fuel injectors for horsepower, fuel and duty cycle
Injector sizing calculates the fuel injector flow rate an engine needs, from its horsepower, fuel type and a safe duty cycle.
Formula: Flow per injector (lb/hr) = HP × BSFC ÷ (injectors × duty cycle) · cc/min = lb/hr × 10.5
What this number means
This calculates the injector flow rate needed for a target horsepower, number of cylinders, fuel type and maximum safe duty cycle. Fuels like E85 need roughly 30% more flow for the same power.
What to change next
Choose the next injector size above the result so duty cycle stays at or below about 80% at peak power. If your current injectors are already near that limit, upgrade them before raising power or switching fuels.
Worked example
500 hp, 8 cylinders, gasoline, 0.50 BSFC at 80% duty cycle needs ≈ 39 lb/hr injectors — so choose at least 42 lb/hr.