Where does this hose go?

[ChrisD]

Bit of a random question but I just want to understand something...

I've got an AVO turbo with a blow off valve that does 50% recirculating and 50% dump to atmosphere. So as far as I understood, there should be two connections to this - the connection to the throttle body where it picks up the pressurised air from when you come off the gas, and then the connection to the turbo inlet for it to recirculate some of the air back to.

But there is another connection, which has a small vacuum hose going round to join another hose which looks like it goes in to the back of the engine.





The blue hose in these pics is what I'm referring to. Can't really see very well in the second one but it goes in to a T piece that connects it to that black hose coming out of the engine cover.

On the AVO site it says "AVO blow-off valves retain the return tube, keeping the stock computer system happy"

So I assume that's what this is? But "return tube" is a bit vague and I don't really understand what it is returning to where, or why.

Can anyone explain this to me?

Thanks
Chris

[Spartarus]

That's a vacuum hose. That is t'ed up off the pcv line, that's why it looks like it goes into the engine. The other end of the black line connects to the intake manifold. The bov/recirculation valve needs a vacuum source downstream of the throttle. That's how it 'knows' when you let off the throttle.

It is actuated by differential pressure, that's how it stays closed under boost. When you let off, the pressure in the plastic intake manifold drops and becomes a vacuum. That blue line creates vacuum on the back side of the BOV, and the difference in pressure on both sides of the BOV/rv causes it to open. When you're on the throttle, the pressure in the intake manifold is positive, and that blue hose is applying positive pressure to the back of the BOV. This plus the spring force keeps it closed.

At idle, you have vacuum in the manifold and ambient at the BOV, but the spring holds it closed.

Who installed it? Usually you are supposed to use the brake booster line to T off of because there's better vacuum there and no crankcase vapors.

[ChrisD]

Originally Posted by Spartarus

That's a vacuum hose. That is t'ed up off the pcv line, that's why it looks like it goes into the engine. The other end of the black line connects to the intake manifold. The bov/recirculation valve needs a vacuum source downstream of the throttle. That's how it 'knows' when you let off the throttle.
...

Ahh ok thanks for the detailed explanation, that makes sense. I always thought it was just the pressurised air having nowhere to go once the throttle is closed that was enough to open it.

Fensport installed it, and they've done a lot of these turbo installs on GT86s so I guess there's a reason they used that PCV line instead of the brake booster.