Fuel Injector Size Calculator

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. If you only know a wheel figure, add back the drivetrain loss, roughly 15% for a manual and 20% for an automatic.
• BSFC (Brake Specific Fuel Consumption): pounds of fuel burned 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 (needs about 30% more fuel by mass)
• Number of injectors: normally the same as the cylinder count
• Duty cycle: the highest percentage of the time an injector should be held open, usually 80–85%

Common injector sizes:

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 on gasoline at 80% duty cycle needs (350 × 0.58) / (4 × 0.80) = 63.44 lb/hr, which is 666 cc/min per injector. Nothing is sold at 666, so the calculator points you at the next standard size up, 750 cc/min, where the same fuel demand lands at a comfortable 71% duty.

Tips:

• Always size injectors with headroom. An injector held open 100% of the time has nothing left to give, and the first hot day or the first tank of poor fuel turns that into a lean condition at full boost.
• 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).