Fuel Pump
The original fuel pump effectiveness varies both throughout model years and with mileage. For any horsepower goals above 320 bhp an uprated (255litres per hour rated up to 550 bhp) Walbro type pump is recommended.
Fuel Pressure Regulator
On pre MY 01 cars using an uprated fuel pump it is possible to overcome the OE fuel pressure regulator causing pressure variations. For all these models fitting an uprated fuel pressure regulator such as an SX or Fuellab is recommended.
Fuel Rails
Fuel rails varied throughout the years with Phase 1 MY93-98 , Phase 2 MY99-00 and Phase 3 MY01 onwards. Injectors are not interchangeable between phases although adaptors may be fitted to allow the installation of higher flow Phase 2 injectors into Phase 1 rails. Phase 3 are a completely different design and have top fuel feed as opposed to Phase 1 and 2 having side fuel feed. On Phase 1 and 2 fuel rails we have conducted tests and found uneven fuel delivery characterics at high outputs. This is due mainly to the bore and routing of the fuel rails. Typically number three injector at the end of the row has a lower fuel pressure than the others this can cause this cylinder to run lean, potentially causing engine damage.
There are a number of ways to overcome this – one is to rearrange the fuel feed in a parallel configuration feeding both sides of the engine simultaneously this reduces the pressure drop across the fuel rails by 50%. Optionally, using the APEXI Power FC ECU the fuelling can be adjusted for individual cylinders to suit the installation. This allows an improvement in the power, reliability and economy in comparison to what is possible with all the injectors running identical opening duration.

Most 93-98 cars have 380cc injectors which at standard pressure are rated at up to 300 bhp. Later STI and 99/00 UK cars have 440cc injectors which can run to 350 bhp assuming adequate fuel pressure.
MY 01 onwards WRX uses 380cc – MY 01 STI uses 500cc – the 500cc injectors are rated at up to 400 bhp. Upgraded injectors are available in many capacities typically
550cc for up to 440 bhp –
650cc for up to 520 bhp and
740cc up to 590 bhp.

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