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).
Frequently Asked Questions
How do I calculate the right fuel injector size?
Injector size (lb/hr) = (target HP × BSFC) ÷ (number of injectors × max duty cycle). BSFC is brake-specific fuel consumption — typically 0.45-0.50 for naturally aspirated gasoline, 0.55-0.65 for turbocharged. Max duty cycle should be 80-85% to leave headroom. For example, a 500 HP turbo V8: (500 × 0.60) ÷ (8 × 0.80) = 46.9 lb/hr — so 50 lb/hr injectors would work.
What is injector duty cycle and why does it matter?
Duty cycle is the percentage of time the injector is open during each engine cycle. At 100% duty cycle, the injector is always open and can't deliver more fuel. Running above 85% risks fuel starvation under varying conditions (hot fuel, voltage drops). Sizing injectors to 80% max duty cycle provides a safe operating margin.
What is BSFC and how do I choose the right value?
BSFC (Brake Specific Fuel Consumption) measures how much fuel an engine uses per unit of power. It's expressed in lb/hp/hr. Stock naturally aspirated engines are typically 0.45-0.50. Turbocharged engines run 0.55-0.65 because they run richer for safety. E85 uses about 1.3× more fuel by volume, so multiply BSFC by 1.3.
How do I convert between lb/hr and cc/min for injectors?
1 lb/hr = 10.5 cc/min (for gasoline). So a 42 lb/hr injector flows about 440 cc/min. This conversion factor is specific to gasoline's density — it differs slightly for E85 or methanol.
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