The dump valve is also called a blow off valve and air by-pass valve. Replacing your OEM dump valve with an aftermarket one is a common tunning mod. Some tuners claim replacement dump valves have the following benefits:
- improves pick-up, through reduced turbo lag
- sounds good when it vents to the atmosphere (read: loud)
- helps regulate the boot pressure better than OEM valve
How it works
What a dump valve does is allow the pressurised air to vent, either to atmosphere (aftermarket values), or back through the intake plenum (OEM is a recirculating system - as per Impreza). This reduces the restriction on the turbo impeller allowing it to spin almost freely and therefore retain momentum. If you then re-apply the accelerator soon after (such as during a gear change) the butterfly opens and dump valve closes and the turbo still spinning at quite a high speed. It takes much less time to spin up to full boost and thus creates less lag. If the dump valve should leak it will not allow full boost to be applied to the engine.
Fitting Instructions for Classic Impreza's
- Fittment is very simple, your kit should have obviously a dump valve (DV) in it, a blanking plug and sometimes a new silicone hose to go from the DV to the intake manifold.
- The idea is to remove your old DV (RED DOT ON PIC). Make sure you note the way it goes on, there should be an arrow on your factory DV, this will come in handy if you ever wanted to go back to standard.
- Once removed you will see that the DV has 2 pipe connections x1 to the intercooler (BLUE DOT ON PIC) and x1 to the intake (GREEN DOT ON PIC).
- Your new DV goes to the Intercooler pipe (BLUE DOT ON PIC). Make sure that it is clamped to the pipe or jubilee clipped.
- The blanking plug in your kit blanks off the hole in the intake return pipe (GREEN DOT ON PIC). Make sure that it is clamped or jubilee clipped.
- Then if you have a new silicone hose just swap that for the one that went from the intake manifold to the DV on your factory DV (YELLOW DOT ON PIC).
- Make sure everything is tight and secure then go for a run and see what you think.
Information Source - SIDC forum thread, WRX Classic Dump Valve Diagram Help