Fuel Injector Duty Cycle Calculator
Estimate injector size from horsepower, fuel type, injector count, and duty cycle.
Calculator guide
Estimating injector size and headroom
This calculator estimates the minimum flow per injector for a horsepower goal and compares it with the current injector size.
Inputs and calculation
- Target horsepower, fuel type, injector count, desired maximum duty cycle, and current injector flow in cc/min.
Required flow per injector in lb/hr = (horsepower × brake-specific fuel consumption) ÷ (injector count × duty-cycle fraction).
The estimate uses BSFC values of 0.55 for gasoline, 0.72 for E85, and 1.05 for methanol, then converts lb/hr to cc/min with a factor of 10.5.
How to interpret the result
- Positive current-injector headroom means the entered injector is larger than the estimate. Running near the selected duty limit leaves less margin for fuel-pressure loss, mixture variation, or future power.
Assumptions and limits
- BSFC and the lb/hr-to-cc/min conversion are broad planning values. Actual fuel density, base pressure, differential pressure, injector characterization, and engine efficiency matter.
- Confirm sizing and safe duty cycle with the tuner and injector manufacturer before hardware or calibration decisions.
Example: 500 hp gasoline engine
For 500 hp, four injectors, and an 80% duty target, the estimate is about 902 cc/min per injector. A 1,000 cc/min injector shows roughly 11% headroom over that estimate.
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