What hose is this?

[XrSiSt]

I'm aware of how a spring bov works, but what does the hose

connecting to the top of the BOV do? I've notice that if you

block this off, the turbo flutters and the idle is fine!

[Doctor]

That hose is what operates the BOV. It is connected to the plenum and pulls the spring up when you release the throttle and hit vacuum.

The reason your turbo flutters and the car idles properly is becuase the BOV is not working.

Doc

[XrSiSt]

On idle/vacuum it's drawing in air, so wouldn't that be pulling the bov up and leaking air?

[turbomad]

The hose off the plenum is used to keep the BOV closed under boost, same boost level either side=no opening of BOV, shut the throttle and it's the actual boost pressure that forces the BOV to open and vent the air, the spring is quite weak and you can phsically push it open by hand.

[TTT]

The hose on top is for vacuum.

when you get an aftermarket bov, when you plumb it in, you need to adjust it so that it is closed during idle.

During idle, there is a certain amount of vacuum in the manifold.. (for this example lets say 10 units - whatever it is measured in)

when you accelerate to get boost and release the accelerator, there is more vacuum (say 15units).

This extra vacuum along with the compressed air that is in the pipework just below the bov will open up the bov and make it vent with a big psshhh..

When you don't connect that vacuum line, the BOV doesn't open and the air goes back out through the turbo blades getting chopped up by the blades causing the fluttering sound..

I hope this helps.

[XrSiSt]

Everythings all starting to come together now.. Thanks guys.