Estimate the minimum flow rate per fuel injector from target horsepower, brake-specific fuel consumption, injector count, and maximum duty cycle.
Calculate fuel injector size in lb/hr and cc/min from target horsepower and injector count, with typical BSFC, duty cycle and fuel density included.
How to Use This Calculator
- Enter positive target horsepower.
- Use an engine- and fuel-appropriate BSFC.
- Enter the actual whole-number injector count.
- Choose a conservative duty-cycle limit and fuel density, then calculate.
After a successful calculation, the inputs and result remain saved in this browser. Select Reset to clear the stored values and return to empty inputs.
How the Fuel Injector Size Calculator Works
The result is a minimum planning flow. Real selection also depends on differential fuel pressure, injector characterization, fuel type, voltage, ECU control, and an appropriate safety margin.
The primary relationship is Injector flow = horsepower × BSFC ÷ (injector count × duty-cycle fraction). Calculations retain full available precision internally; displayed values are rounded for readability.
Formula Variables and Input Guide
Use an engine- and fuel-appropriate BSFC, the actual injector count, a conservative duty-cycle limit, and fuel density.
| Variable or term | Meaning | Unit or note |
|---|---|---|
| HP | Target engine power | horsepower |
| BSFC | Brake-specific fuel consumption | lb/hp·hr |
| N | Injector count | whole number |
| DC | Maximum duty cycle | decimal |
| Flow | Required per injector | lb/hr |
Choosing the Right Inputs
Use the horsepower the fuel system must actually support and a conservative brake-specific fuel consumption value for the engine, aspiration, fuel, and tuning objective. Enter the number of injectors that share the modeled fuel demand, not cylinder count automatically when staged or supplemental injection changes that relationship.
Set a maximum duty cycle that leaves suitable control margin and use fuel density in grams per millilitre for the fuel being modeled. Injector catalog flow ratings depend on differential fuel pressure and test fluid, so compare the calculated requirement with a rating at matching conditions. Engine power basis, drivetrain loss, pressure regulator behavior, and any intended safety margin should be documented separately.
Practical Uses
- Selecting a preliminary injector flow range.
- Comparing injector requirements across horsepower targets.
- Testing BSFC and duty-cycle sensitivity.
- Converting lb/hr ratings into approximate cc/min.
BSFC sensitivity at 400 hp, 8 injectors, 85% duty
| BSFC | Required flow/injector |
|---|---|
| 0.45 | 26.47 lb/hr |
| 0.50 | 29.41 lb/hr |
| 0.60 | 35.29 lb/hr |
Understanding Your Results
Required flow is shown per injector in lb/hr and approximately cc/min.
Choose the next suitable rated injector rather than a rating below the result. Compare ratings at the same differential fuel pressure and fuel type.
Worked Example
At 400 hp, 0.50 BSFC, eight injectors, and 85% duty cycle, each injector needs about 29.41 lb/hr.
At 400 hp and 0.50 BSFC, total operating fuel flow is 200 lb/hr. Dividing by eight injectors and 0.85 duty gives 29.41 lb/hr per injector before an additional selection margin.
Checking the Result by Hand
First compute total fuel mass flow as horsepower × BSFC. Divide that value by injector count and the duty-cycle fraction to reproduce required lb/hr per injector. For 400 hp at 0.50 lb/hp·hr, total flow is 200 lb/hr; eight injectors at 85% duty require 200 ÷ 6.8 = 29.4118 lb/hr each.
Multiply the per-injector answer by injector count and duty fraction to recover total operating flow. When checking cc/min, convert lb/hr to grams per minute and divide by fuel density in g/mL; a lower-density fuel requires more volume for the same mass flow.
Common Mistakes to Avoid
- Entering a fractional injector count.
- Using 100% duty cycle as a normal design target.
- Comparing injector ratings at different pressures.
- Using gasoline density or BSFC assumptions for another fuel.
Assumptions and Limitations
This is a sizing estimate. Fuel pressure, injector rating pressure, dynamic flow, voltage, fuel type, regulator behavior, and safety margin affect real selection. Confirm with the injector and ECU manufacturer or a qualified tuner.
Frequently Asked Questions
What BSFC should I use?
Use a conservative value from engine data, a qualified tuner, or a reputable technical reference.
Why not use 100% duty cycle?
The injector needs control margin and may not behave predictably when held fully open.
Does fuel pressure affect flow?
Yes. Injector flow ratings depend on differential pressure.
Does E85 require different assumptions?
Usually. Fuel density and required mass flow differ, so use fuel-specific values.
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