Fuel Injector Selection

Obtaining the correct amount of fuel flow from a group of injectors in an EFI system is similar to determining the right size jets for a carbureted engine. The fuel flow of both systems must be matched to the airflow requirements of the engine over a broad rpm operating range.

A carburetor may have three or more separate control circuits such as idle, mid-range, and power, to deliver a fuel curve over the engine’s operating range. An injector also must be able to supply a small amount of fuel to support engine idle, a large amount at wide-open throttle to prevent high-rpm lean-out, and transitional amounts to cover all of the operating conditions in between. These wide operating boundaries demand that an injector have a wide ”dynamic range” of operation.

The amount of fuel delivered by an electronic injector is controlled by how long it is held open by the Engine Control Unit (ECU). It is “pulsed or energized” open for short periods of time at idle, and held open longer as rpm and airflow increase. The time the injector is held open is referred to as duty cycle and expressed as a percentage. In other words, if an injector is maxed out and being held open to meet the engine’s fuel demands, it is at 100% duty cycle. Typically, you should strive to keep maximum duty cycle at less than 80%.

Just as the wrong-sized jets in a carb can cause driveability problems such as rough idle, surging, poor throttle response or even high-rpm lean-out, so can incorrect injectors. Following is a guideline equation for approximating fuel flow per injector based on estimates on engine Horsepower (HP) and Brake Specific Fuel Consumption (BSFC). Take note of these conditions for equation accuracy:

  1. Engine HP must be realistic estimate of engine output.
  2. BSFC is determined from engine dynamometer measurements. It typically ranges from 0.4 - 0.6 for gasoline powered engines. A BSFC of 0.5 is a reasonable initial estimate for most engines.
  3. The 0.8 multiplier of the "Number of Injectors" helps us derive a practical, maximum "Injector Flow Rate" for each injector based on an effective real world injector operat-ing pulse time and fuel flow. It's unrealistic to establish the fuel flow to the engine based on an injector operating pulse time of 100% (wide open all the time). This formula uses an injector operating cycle based on 80%. Some full race engine management systems may operate at 85 - 95% duty cycle, but doing so for some time increases the likelihood of overheating the injectors which may cause irregular fuel rates or a decrease in low rpm operation.

For example, to calculate the individual injector size for a 650 HP V8 using eight injectors and assuming a BSFC of 0.5:

Use MSD Competition Fuel Injector PN 2013, rated at 50 lb/hr static flow at 43.5 psi (3 barometric) fuel system pressure.

If you have a known injector fuel flow rate you can solve the above equation for a rough estimate of fuel system capacity like this:



NOTE: Keep in mind your application and other mechanical modifications that have been made to your engine. The number of cylinders or extremely high rpm engines (such as rotary engines) may require larger injectors due to on/off times.

 

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