Calculating Injector Size

Calculating Injector Size

Fuel injectors play a crucial role in delivering the precise amount of fuel to an engine's combustion chambers. Determining the correct injector size is vital for optimizing engine performance, ensuring proper fuel delivery, and achieving optimal fuel efficiency. This video will delve into the process of calculating injector size, providing you with the necessary information to make informed decisions when upgrading or replacing injectors.


Understanding Injector Flow Rate:

The flow rate of an injector is typically measured in pounds per hour (lb/hr) or cubic centimeters per minute (cc/min). It indicates the amount of fuel an injector can deliver within a given time frame. The injector size you need will depend on the engine's requirements, specifically its power output and the desired air-to-fuel ratio.


Step 1: Determine Engine Requirements:

Before calculating the injector size, you must assess your engine's requirements. Factors to consider include the desired horsepower, the type of fuel system (naturally aspirated or forced induction), the fuel pressure, and the desired air-to-fuel ratio. These parameters provide the foundation for calculating the injector size accurately.


Step 2: Calculate the Base Injector Flow Rate:

To calculate the base injector flow rate, you need to determine the engine's approximate fuel requirements. The general guideline is to multiply the desired horsepower by the Brake Specific Fuel Consumption (BSFC) rating for your engine.      

BSFC is a measure of the fuel efficiency of an engine and is typically expressed in lb/hp/hr or g/kW/hr. Different types of engines have varying BSFC values, and factors such as fuel type and engine technology also influence it. For example, naturally aspirated gasoline engines generally have a BSFC of around 0.50 to 0.60 lb/hp/hr, while turbocharged engines may range from 0.60 to 0.65 lb/hp/hr.


Step 3: Consider Additional Factors:

The duty cycle is usually expressed as a percentage and represents the injector's on-time relative to its total cycle time. It is crucial to select an injector that can provide enough fuel flow at your desired duty cycle to avoid performance issues or premature injector failure.  The maximum safe injector duty cycle is generally somewhere between 70-85%.  Most tuners will settle around 80%.  This would be at it’s maximum duty cycle, so we suggest up sizing slightly to avoid running constantly at the maximum duty cycle. 

Fuel pressure affects the flow rate of injectors. Higher fuel pressure generally results in increased flow rates, while lower pressure reduces them. It is important to account for the actual fuel pressure at which the injectors will operate and adjust the calculations accordingly.

Injector latency, or dead time, refers to the time delay between the injector receiving an electrical signal and actually opening. This delay affects fuel delivery accuracy. You may need to consult manufacturer data or conduct tests to determine the injector latency for precise calculations.


Step 5: Select the Appropriate Injector:

Once you have the adjusted injector flow rate, you can select an injector with a flow rate closest to that value. Manufacturers provide injector flow rate data, allowing you to choose the right size from their product offerings.


Calculating the correct injector size is essential for optimizing engine performance, ensuring proper fuel delivery, and achieving the desired air-to-fuel ratio. By understanding your engine's requirements and following the step-by-step process outlined in this guide, you can accurately determine the injector size needed for your specific application. Remember to consider additional factors such as duty cycle, fuel pressure, and injector latency to make an informed decision.

Calculate Injector Size Based On Desired Crank HP


[(Crank HP Desired ÷ Maximum Injector Duty Cycle) x BSFC] ÷ Number of Injectors = Injector Size in #/hr

Example: [(500 ÷ 0.80) x 0.65] ÷ 8 = 50.78

Calculate the Potential Crank HP Based on Injector Size


[(Injector Size in #/hr x Number of Injectors) ÷ BSFC] x Maximum Injector Duty Cycle = Estimated Crank HP

Example: [(50.78 x 8) ÷ 0.65] x 0.80 = 499.98

 Injectors that The Tuning School Recommends LINK HERE

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