Yeah. If you imagine it like a shower valve - the big tube is the water line, the little tube is the knob. Boost in the intake is the water, and vacuum in the manifold is your hand turning the knob.Originally posted by gifty74
so the smaller tube, on top of the BOV, just shows the actual pressure & vacuum in the intake in front of the throttle body (on the side towards the front of the car), and provides a vacuum source that opens the BOV to vent pressure on the opposite side (intercooler side of the throttle body) when the butterfly abruptly closes. this was the part that was confusing me. hopefully i got this right, i love learning more about the wrx...
An engine is an air pump, with explosions being set off inside. If you were to just take a motor and crank it, you'd pump air through. There's be pressure in the exhaust manifold and vacuum in the intake manifold. A normally aspirated car always has vacuum - that's how it pulls air in. Because a turbo engine has an air pump in the intake (the turbo), the vacuum doesn't do much once the throttle is open - there's almost always pressure in the intake from the turbo.
Which brings me to an interesting tangent - I believe that the power assist for the brakes is lost when on boost. The power assist is powered by manifold vacuum and there's a check valve in the line (the inspection of which is part of the 15k mile service) to keep boost out. You'd never really notice it unless you were left foot braking while on the throttle (F1 style!). Even when downshifting, you only need to crack the the throttle to rev match. Or maybe it's powered by boost from another line (hadn't thought of that 'till just now). Interesting to think about though.