This is not to control boost nor does it affect it, directly. When you change gears or slow down the turbo is still spinning, when you take your foot off the throttle it closes a flap stopping the flow of air into the engine. this is commonly called the "butterfly" The turbo (still spinning) tries to keep pumping air in and can't. So it rapidly slows down with the restricted flow. When you put your foot down again, opening the butterfly and reducing the restriction the turbo has to "spool " up or gain rpm's again to develop boost. (This is called turbo lag)

The difference in time from foot down to full boost (at a suitable RPM) is the lag time.

NOTE remember full boost can not be gained when load is not correct as turbo RPM is based on a "thermal load" NOT rpm as some people think.

Back to the blow off valve....

What this does is detect a vacuum on the engine side of the butterfly and opens a valve to "dump" excess pressure, (back to the inlet filter) and hence reduces the restriction on the turbo allowing it to almost free spin, and NOT slow down as quickly. As a result when you put your foot down again its likely to be still spinning fast and takes LESS time to get to full boost and this creates LESS lag.

Obviously the valve is CLOSED on full boost.

Remember it does not control boost, BUT it can leak causing lack of boost and also "creep" this is it can theoretically open at low boost levels if there is enough vacuum on the engine side of the butterfly to start it to open.

This venting of pressure is the whoosh you hear on gear change especially noticeable on early model WRX's where the vent pipe was close to the air filter assembly.

The whirr noise problem on current model WRX's like a blowing across a bottle is the vibration of the valve beginning to open.


Advice

If you wish to fit an externally venting "BOV" then read below:

• Spend your money wisely, fit one last if you want power, (most BOV's especially Subaru's are ok up to 16 PSI) if you want that great noise then sure, fit one now!
• A blow off valve does NOT control the boost LEVEL
• The main benefits of an after market one is adjustability to remedy low boost leakage.
• Capacity (as in early models) [the late model one is simply huge!