Injectors must deliver enough fuel at full power without being held open all the time – 80% duty cycle is a safe maximum for street engines. Turbocharged engines run richer and have a higher brake specific fuel consumption, and ethanol fuels need roughly 30–40% more flow than petrol.
How to use the Fuel Injector Calculator
- Target horsepower
- Horsepower measured at
- Number of cylinders
- Fuel
- Induction
- Maximum duty cycle
- Fuel pressure
- Every result field is editable: type into Required injector flow and Target horsepower plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the target horsepower and whether it is measured at the crank or the wheels, the cylinders, fuel and induction (which set the BSFC), the maximum duty cycle and your fuel pressure.
Tips
- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field’s menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.
Common mistakes to avoid
- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.
Inputs explained
| Field | What to enter |
|---|---|
| Target horsepower | In hp |
| Horsepower measured at | Choose one: Crankshaft (flywheel), Wheels |
| Number of cylinders | A number |
| Fuel | Choose one: Petrol / race gas, E85, Methanol |
| Induction | Choose one: Naturally aspirated (BSFC 0.50), Supercharged (BSFC 0.55), Turbocharged (BSFC 0.60) |
| Maximum duty cycle | In % |
| Fuel pressure | Units: psi, bar, kPa |
What each result means
- Required injector flow = [Target horsepower]×[Induction]×[Fuel]×[Horsepower measured at]÷([Number of cylinders]×[Maximum duty cycle]÷100)
- Required flow in cc/min = [Required injector flow]×10.5
- Injector rating needed at 43.5 psi (3 bar) = [Required injector flow]×sqrt(43.5×6894.757293168÷[Fuel pressure])
Example values
How the main result changes when Target horsepower changes and everything else stays at the example values above:
| Target horsepower (hp) | Required injector flow (lb/hr) |
|---|---|
| 200 | 15.6 |
| 300 | 23.4 |
| 400 | 31.3 |
| 500 | 39.1 |
| 600 | 46.9 |
| 800 | 62.5 |
Formula
Injector flow (lb/hr) = hp × BSFC × k ÷ (cylinders × duty cycle)k = 1 for crank hp, 1.15 for wheel hpRated flow at 43.5 psi = flow × √(43.5 ÷ fuel pressure)Worked example
Inputs — Target horsepower: 400 hp; Horsepower measured at: Crankshaft (flywheel); Number of cylinders: 8; Fuel: Petrol / race gas; Induction: Naturally aspirated (BSFC 0.50); Maximum duty cycle: 80 %; Fuel pressure: 43.5 psi.
Results — Required injector flow = 31.3 lb/hr; Required flow in cc/min = 328 cc/min; Injector rating needed at 43.5 psi (3 bar) = 31.3 lb/hr.
Frequently asked questions
What size injectors for 400 hp?
For a naturally aspirated V8 at 80% duty: 400 × 0.5 ÷ (8 × 0.8) ≈ 31 lb/hr each.
How do I convert lb/hr to cc/min?
Multiply by about 10.5 for petrol.
Why does fuel pressure matter?
Flow rises with the square root of pressure, so raising pressure from 43.5 to 58 psi increases flow about 15%.
What formula does the Fuel Injector Calculator use?
Injector flow (lb/hr) = hp × BSFC × k ÷ (cylinders × duty cycle).