STI and Standard Injectors
When increasing the performance of a car, it is sometimes necessary to replace the engine's fuel injectors. Improving the flow of the induction & exhaust systems and increasing boost pressure all contribute to increases in air flow to the engine. Since the ratio of air to fuel must be kept within a tight range, any increases in air flow must go hand in hand with an increase in fuel.
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STI and Standard Injectors |
The sophisticated Subaru ECU continuously monitors air flow and calculates how much fuel must be injected on each induction stroke in order to maintain the desired air fuel ratio. Based on the flow rate of the engine's injectors, the ECU calculates for how long the injectors must remain open in order for the correct quantity of fuel to be delivered. The time that the injectors must remain open is based on two characteristics - injector flow rate and injector opening time.
As the volume of fuel required increases (with air flow), a point is reached where the injector is open continuously - the injector pulse for each induction stroke has become so long that the train of pulses have joined up. At this point, the injector and has reached its maximum flow and any increase in air flow will causes the mixture to become leaner - more air, but the same amount of fuel.
In order to increase maximum fuel flow, injectors with a higher flow rate may be substituted for the originals. In general, an injector with a higher flow rate will flow a larger volume of fuel in a given period of time. However, be careful when comparing injector flow rates specified by different manufacturers, since the flow may not have been measured at the same injector duty or fuel pressure.
The standard 2002 WRX injector is specified as 380cc, whilst the STI part is 500cc flow rate. Since the parts are from the same manufacturer, it can be assumed that the STI part will flow around 30% more fuel for a given opening time and fuel pressure. As well as just swapping the mechanical parts, the ECU must be recalibrated to account for the mechanical change. The ECU must be set to calculate injector opening times based on the higher flow rate injectors. For example, if the new injectors are of twice the flow rate of the originals, then the ECU need only open the injectors for approximately half the period, in order to deliver the same volume of fuel.
Injector Opening Time for Standard WRX Injectors |
As well as a flow rate, injectors have an opening time characteristic. This characteristic defines how quickly the injector opens after being electrically energised. Until this period has elapsed, little fuel will flow. To further complicate matters, this opening time will depend on the car battery voltage, since the injector is operated by an electro-magnetic solenoid. The graph above shows the injector opening characteristic for standard WRX Impreza injectors. The higher the battery voltage, the quicker the injector opens. It is also interesting to note that the 500cc STI injectors use the same opening time calibration, suggesting that there is little mechanical difference between the two parts.
When changing injectors, it is important to recalibrate for injector opening time (as well as flow rate), since larger injectors may take longer to open. This can cause unstable idle, since the ECU cannot accurately meter very small quantities of fuel.
When deciding on replacement injectors, biggest is not necessarily best. Choose an injector that is capable of flowing the required amount of fuel at approximately 80% duty. This gives sufficient headroom for boost spikes, cold weather and possible fault conditions such as leaking boost control pipework. Choosing an injector that is larger than this may unnecessarily compromise cold start and idle capabilities. This is because it is difficult to meter very small quantities of fuel from large injectors, since the required injector opening times are so small.
Also, don't forget to consider the capabilities of other parts of the fuel delivery system. Whilst fitting larger injectors will increase fuel flow, the increase may not be as large as expected if the increase in flow reduces fuel rail pressure.
MoTeC Professional Lambda Meter |
When altering fuelling configuration on an engine, always check the result with a wideband lambda device such as the excellent MoTeC PLM. Furthermore, the wideband fuelling data may be logged alongside other engine parameters using the external data logging ability of DeltaDash.