Fuel Injector Calculator
Determine the exact injector flow rate in pounds-per-hour (lbs/hr) and cc/min required to support your target engine horsepower safely.
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How Fuel Injector Sizing Works
Selecting the proper fuel injector flow rate ensures your engine receives sufficient fuel under wide-open throttle (WOT) without maxing out the injector solenoid duty cycle.
Injector Flow Formula
Flow (lbs/hr) = (Horsepower × BSFC) / (Number of Injectors × Max Duty Cycle)
Flow (cc/min) = Flow (lbs/hr) × 10.5
Why 85% Duty Cycle is the Limit
Electronic fuel injectors require a few milliseconds to open and close. Running injectors beyond 85% duty cycle can cause the pintle to float "static" (stay continuously open), resulting in erratic fuel metering and catastrophic lean conditions at high RPM.
Frequently Asked Questions
Why does E85 require 30% larger fuel injectors?
E85 ethanol has a lower energy density than pure gasoline (stoichiometric ratio is 9.8:1 for E85 vs. 14.7:1 for gasoline). To produce the same horsepower, an engine must inject roughly 30% to 35% more fuel volume.
What happens if fuel injectors are too large?
Oversized injectors have short pulse widths at idle. If the minimum pulse width is below the injector non-linear dead-time limit, the engine will idle poorly, fail emissions, and run rich.
How does fuel pressure affect injector flow?
Flow rate scales with the square root of fuel pressure ratio: Flow_new = Flow_rated * sqrt(Pressure_new / 43.5 PSI). Increasing base pressure from 43.5 to 58 PSI (4 bar) provides about a 15% flow increase.