Fuel Injector Size Calculator
Calculate the correct fuel injector size in cc/min and lb/hr for your engine based on horsepower and fuel type.
Injector Size Required
Fuel injector sizing ensures your engine receives enough fuel to support its power output. Undersized injectors cannot deliver enough fuel at wide-open throttle, causing a dangerous lean condition. Oversized injectors reduce low-speed drivability.
Formula:
Injector size (lb/hr) = (HP × BSFC) / (Number of injectors × Duty cycle)
Conversion:
1 lb/hr = 10.5 cc/min
What each variable means:
- HP — target horsepower at the flywheel (or at the wheels with drivetrain loss factor)
- BSFC (Brake Specific Fuel Consumption) — pounds of fuel consumed per horsepower per hour
- Naturally aspirated gasoline: 0.45–0.50 lb/hp/hr
- Turbocharged gasoline: 0.55–0.60 lb/hp/hr
- Supercharged gasoline: 0.55–0.65 lb/hp/hr
- E85 ethanol: 0.65–0.70 lb/hp/hr (requires ~30% more fuel)
- Number of injectors — typically equals the number of cylinders
- Duty cycle — maximum recommended percentage of time the injector is open (typically 80–85%)
Common injector sizes:
| Size (cc/min) | Size (lb/hr) | Typical Use |
|---|---|---|
| 160 cc | 15 lb/hr | Stock 4-cylinder |
| 220 cc | 21 lb/hr | Stock V6 |
| 310 cc | 30 lb/hr | Stock V8 |
| 440 cc | 42 lb/hr | Mild turbo 4-cyl |
| 550 cc | 52 lb/hr | Turbo 4-cyl / mild V8 |
| 750 cc | 72 lb/hr | High-boost turbo |
| 1000 cc | 95 lb/hr | Big turbo / race |
| 1300+ cc | 124+ lb/hr | Extreme builds |
When to use this calculator:
- Planning a turbo or supercharger installation
- Upgrading from stock to performance injectors
- Converting to E85 fuel (requires larger injectors)
- Building a high-performance engine
Practical example: A 4-cylinder turbocharged engine targeting 350 HP with gasoline at 80% duty cycle needs: (350 × 0.58) / (4 × 0.80) = 63.4 lb/hr or about 665 cc/min per injector.
Tips:
- Always size injectors with some headroom — running at 100% duty cycle risks fuel starvation.
- E85 requires approximately 30% larger injectors than gasoline for the same power.
- Upgrading injectors often requires a retune of the engine management system.
- Higher fuel pressure increases injector flow rate (approximately by the square root of the pressure ratio).