Injector Sizing Calculator
Calculate required fuel injector size from target horsepower, BSFC, number of injectors, and maximum duty cycle. Results in lb/hr and cc/min.
Fuel consumption rate per horsepower. Use presets or enter custom.
80% is a safe max for most applications. 85% for well-tuned setups.
Why Injector Sizing Matters
Undersized injectors can't deliver enough fuel at high load, causing a dangerous lean condition. Oversized injectors make low-load tuning difficult because they operate at very low duty cycles where fuel delivery becomes inconsistent.
Understanding Duty Cycle
Duty cycle is the percentage of time the injector is open during each engine cycle. At 100% duty cycle, the injector is open continuously — it can't deliver any more fuel. The standard safe maximum is 80%, which leaves headroom for:
- Fuel pressure variations
- Voltage drop under electrical load
- Hot fuel (lower density) conditions
- Injector aging and wear
BSFC Explained
Brake Specific Fuel Consumption (BSFC) measures how much fuel an engine burns per horsepower per hour. Lower is more efficient. Typical values:
- NA gasoline: 0.45–0.55 lb/hr/hp
- Forced induction gasoline: 0.50–0.60 lb/hr/hp
- E85: 0.55–0.65 lb/hr/hp (E85 has ~30% less energy per gallon)
- Diesel: 0.30–0.40 lb/hr/hp
The Formula
Required Flow (lb/hr) = (Target HP × BSFC) / (# Injectors × Max Duty Cycle)
To convert lb/hr to cc/min: multiply by 10.5 (for gasoline-density fuels).