How the recircs work and why the Goose Honk occurs:

Because the diaphragm in the recirc valve is plumbed to the upper plenum, it is not only exposed to vacuum (less than 0 psi), but also boost (greater than 0 psi). The diaphragm's surface area is greater than the valve area; in other words, if the same pressure is applied to both at the same time, the diaphragm will always win out. So, the recircs are controlled by the pressure diaphragm... which means they are not a simple "pop-off" valve.

There are four operating conditions the Recirc valves operate in:

1.      idle and/or normal driving (light load)

2.      moderate load (honk)

3.      wide-open-throttle

4.      throttle-off-high-RPM/boost (shifting)

Idle/normal driving-

Pressure at the valve: 0 psi (atmospheric)
Pressure at the diaphragm: -10 psi (estimated)

The vacuum is pulling the diaphragm up and the valve is open allowing air to bypass the turbos and flow directly to the engine. It only takes around -1 psi at the diaphragm to open the recirc valve.

Moderate load (honk)-

Pressure at the valve: 0 psi (atmospheric)
Pressure at the diaphragm: approaching -1 psi

As you open the throttle a bit more, atmospheric pressure and the turbos begin to move some air and the manifold vacuum gets closer to that magic -1 psi where the recircs are begining to close but what happens is the airflow through the valve has stagnated because the pressure on each side of the valve has equalized (keep in mind the valve is still open though, exposing a rather large volume for your harmonic pleasure). Now you have air flowing past the recirc's hose nipples in the inlet pipes just after the tee-tube and you get the same effect as blowing over the top of a bottle...

HHHHHHHHHHOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOONNNNK

It's always been there, it's just that the stock air filter box also acts as a muffler, if it can restrict 20HP worth of air from getting in, it can certainly restrict the Goose Honk from getting out. With the airbox removed it's easier to hear the sound now.

Wide-open-throttle

Pressure at the valve: 15 psi
Pressure at the diaphragm: 15 psi

The valve is closed because the spring pressure + the boost pressure multiplied by the diaphragm's surface area is greater than the boost pressure multiplied by the valve's surface area. It will always be closed in this condition, it doesn't matter whether you are boosting 15 psi or 100 psi. The recircs, by their design, are not pop-off valves that will open under high boost conditions... ever, as long as the diaphragm hoses are plumbed to the upper plenum.

Throttle off, so you can shift time -

Pressure at the valve: 15 psi (or more)
Pressure at the diaphragm: -15 psi (or less)

The throttle valves are suddenly closed while the engine is spinning 7000+ RPM and it want's air, so the manifold pressure goes way negative, the diaphragm is exposed to this and pulls open the recirc valve allowing the air flowing through the turbos to vent and NOT cause as proportional of a pressure increase on the turbo's side as the decrease the engine has just developed inside the plenum.

And that's how the Recirc valves work.

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