What is max injector duty cycle?

Max injector duty cycle is the percentage of time an injector is open delivering fuel. You should never exceed 80-85% duty cycle to prevent the injector from locking open or overheating.

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Fuel & Efficiency

Injector Size

Size injectors for target horsepower and duty cycle.

Injector Flow Rate Estimator

Size (cc/min) = (HP × BSFC × 10.5) ÷ (Cylinders × Duty %)

Determine the required injector size based on horsepower target, fuel type, fuel system load, and boost options.

Engine Aspiration:

Fuel Type:

E85

Target HP:

Cylinders:

Max Duty (%):

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About this tool

The BuildLockr fuel injector size calculator helps builders determine exact fueling needs. Use this tool to calculate your required injector CC/Min based on horsepower, boost, and fuel type. Whether you are building an E85 turbo setup or an N/A track car, this injector sizing tool handles the complex BSFC math. This helps tuners safely size their injectors so they never run lean or max out duty cycles.

Understanding Injector Sizing

Selecting the correct fuel injector flow rate is critical to matching your engine's power target without running out of fuel supply or overloading the injector's solenoid.

Brake Specific Fuel Consumption (BSFC)

BSFC represents the pounds of fuel consumed per hour per horsepower. Boosted setups require higher BSFC due to richer safety targets, and alternative fuels like E85 require significantly higher volume to produce equivalent power.

Safety Limits:

• **Duty Cycle:** Never exceed **85%** duty cycle under peak load to prevent the injector from locking open or overheating.
• **Fuel Pressure:** Lower effective pressure under boost in fixed-pressure systems reduces flow, requiring larger injectors.

How do you calculate injector size?

To calculate injector size, you multiply target horsepower by Brake Specific Fuel Consumption (BSFC) and a conversion factor, then divide by the number of cylinders and the max safe duty cycle (usually 80-85%). Our calculator handles all this math automatically.

What injector size do I need for E85?

E85 requires roughly 30% more fuel volume than gasoline to produce the same horsepower. You must factor this higher BSFC into your calculator to ensure your injectors are large enough. Use the fuel type dropdown in our tool to instantly see the size difference.