Power to Win

KB1027 - Fuel Injector selection guide


When a fuel injector is energized by the ECU its internal parts physically move to let the pressurized fuel flow through. The amount of fuel delivered is controlled by how long the injector is energized by the ECU driver circuit. This time is usually specified in milliseconds (mS). An injector is "pulsed" open for short periods of time at idle, and held open longer as the engine's airflow increases, which requires a corresponding increase in injector pulse.

Injector Sizing

To correctly size the injectors we need the following basic information about the engine and installation.

Note : BSFC typically ranges from 0.4 to 0.6 for gasoline engines. A number of 0.5 is a reasonable estimate for most gasoline applications. To avoid operating the injectors at their maximum we will use a duty cycle of 80%.

Injector Flow = (Peak Power x BSFC) / (Number of Injectors x 0.8)

Using this formula for a 650HP engine with 8 injectors, a BFSC of 0.5 and a duty cycle of 80% we calculate the required fuel flow per injector to be 50.78 lb/hr

Injector Drivers

The two most common styles of fuel injector are differentiated by their solenoid design. The soleniod can be either high or low impedence and each requires a different driver circuit. Failure to use the correct driver may result in injector failure.

The saturated circuit driver is used for high impedence injectors (typically 12 to 16 ohms) and is the simplest design. The injector is supplied with 12 volts and then grounded by the ECU circuit to energize the soleniod. The current draw is relatively low which leads to a slow opening time and reduced dynamic flow range. Opening times of 2mS are typical for this type of injector.

The peak and hold driver is used for low impedence injectors (typically 2 to 4 ohms) and is a more complex design because it varies the current to the injector during its on-time. Initially the the injector is supplied with 12 volts and then grounded by the ECU circuit which supplies a large amount of current to open the injector. After the injector has opened the current is reduced to a lower holding value for the duration of the on-time. Opening times of 0.5mS are typical for this type of injector.

Fuel Pressure

Fuel Injectors are typically flow rated at 43.5 psi (3 bar) but they will also perform and flow more fuel, with higher fuel system pressure. Increasing the fuel pressure also better atomizes the injector's fuel spray and improves its uniformity. If you make slight changes to your engine combination you may be able to compensate without having to replace all of the injectors simply by increasing the fuel pressure.

Note : Increasing the fuel pressure will require more injector drive current.

If you know an injector's static fuel flow at one pressure you can derive its static flow at another pressure using the following formula.

Flow Rate = Sqrt(P2 / P1) x Q1

Using this formula for a 50 lb/hr injector rated at 43.5 psi and a new pressure of 72 psi (5 bar) the flow rate would increase to 64.3 lb/hr.

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