How To Calculate Injector Size

Injector Size Formula:

\[ Injector\ Size\ (lb/hr) = \frac{HP \times BSFC}{n_{injectors} \times duty\ cycle} \]

Horsepower (HP):

Brake Specific Fuel Consumption (BSFC):

lb/HP-hr

Number of Injectors:

count

Duty Cycle:

fraction

Required Injector Size:

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1. What is Injector Size Calculation?

The injector size calculation determines the fuel flow rate required for each injector to properly support an engine's horsepower output. It accounts for engine power, fuel efficiency, number of injectors, and their duty cycle.

2. How Does the Calculator Work?

The calculator uses the injector size formula:

\[ Injector\ Size\ (lb/hr) = \frac{HP \times BSFC}{n_{injectors} \times duty\ cycle} \]

Where:

\( HP \) — Engine horsepower
\( BSFC \) — Brake specific fuel consumption (typically 0.45-0.55 for NA engines, 0.55-0.65 for forced induction)
\( n_{injectors} \) — Number of fuel injectors
\( duty\ cycle \) — Maximum injector duty cycle (typically 0.8 or 80%)

Explanation: The equation calculates the fuel flow required per injector by dividing total engine fuel requirement by the number of injectors and accounting for their maximum duty cycle.

3. Importance of Proper Injector Sizing

Details: Correct injector sizing ensures adequate fuel delivery without overworking the injectors. Undersized injectors can cause lean conditions, while oversized injectors may affect idle quality and low-speed drivability.

4. Using the Calculator

Tips: Enter engine horsepower, typical BSFC for your engine type (use 0.5 if unsure), number of injectors (usually equal to cylinder count), and recommended duty cycle (0.8 for street applications).

5. Frequently Asked Questions (FAQ)

Q1: What is a typical BSFC value?
A: Naturally aspirated engines: 0.45-0.50, Turbocharged engines: 0.55-0.65, Supercharged engines: 0.55-0.60.

Q2: What duty cycle should I use?
A: For street applications, 80% (0.8) is recommended. Race applications may use up to 90% (0.9).

Q3: Should I account for future power upgrades?
A: Yes, it's common to calculate for 10-20% more horsepower than current to allow for future modifications.

Q4: How does fuel pressure affect injector size?
A: Higher fuel pressure increases flow rate. This calculator assumes standard pressure (usually 43.5 psi). For different pressures, flow rates must be adjusted.

Q5: What about ethanol blends?
A: Ethanol requires more fuel flow (about 30% more for E85). Multiply the result by 1.3 if using E85.