Fuel Injector Size Calculator
Injector flow an engine needs for a target power, in cc/min and lb/h, from the number of injectors, the fuel consumption per horsepower and the maximum duty cycle.
Results
What this tool does
Each horsepower needs a certain mass of fuel per hour — the brake specific fuel consumption, often taken as about 0.45–0.50 lb/hp/h for naturally aspirated petrol engines and 0.55–0.65 for turbocharged ones, which run richer. Dividing by the number of injectors and by a duty-cycle limit, usually 80–85 % to leave headroom, gives the size each injector must flow. Injector ratings are measured at a specific fuel pressure, so raising the pressure raises the flow; ethanol blends need considerably larger injectors.
Formula
injector lb/h = hp × BSFC ÷ (injectors × duty) · cc/min = lb/h × 453.6 ÷ 60 ÷ density
Variables
| Symbol | Meaning | Unit |
|---|---|---|
hp | Target engine power (at the crank) | hp |
nc | Number of cylinders | — |
bs | Brake specific fuel consumption (BSFC) | lb/(hp·h) |
dc | Maximum injector duty cycle | % |
rf | Fuel density | g/cm³ |
CC | Injector flow needed | cc/min |
LB | Injector flow in lb/h | lb/h |
FF | Total fuel flow at full power | L/h |
Worked example
- Target engine power (at the crank)300 hp
- Number of cylinders4
- Brake specific fuel consumption (BSFC)0.5 lb/(hp·h)
- Maximum injector duty cycle80 %
- Fuel density0.74 g/cm³
- Injector flow needed479 cc/min
- Injector flow in lb/h46.9 lb/h
- Total fuel flow at full power91.9 L/h
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.