| at least not without destroying the entire valve. As stated earlier, the recircs all have as you say "weak" springs. They do not take set and lose their pressure, they are very low stress designs that could live literally for billions if not trillions of cycles without ever showing a loss of load. IMO Nissan set the spring pressure in the recircs this way on purpose so that at lower engine speeds the air doesn't need to flow through the entire intake system just to let the car idle (it's pretty long if you ever lay it all out on a garage floor, turbos included) . The benefit? Possibly better off-idle throttle response; who knows?
Here's my best explanation of how the recircs work and why the Goose Honk occurs:
First, you have to understand that the diaphragm in the recirc valve is not only exposed to vacuum (less than 0 psi), but also boost (greater than 0 psi). The recircs are controlled by the pressure diaphragm not the valve that's in the flow path. The diaphragm's surface area is far greater than the valve area exposed to boost pressure so if the same pressure is applied to both, the diaphragm will always win out. There are basically four operating conditions, 1) idle and/or normal driving (light load), 2) moderate load (honk), 3)wide-open-throttle, and 4)throttle-off-high-RPM/boost condition (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 flow to the engine. It only takes around -1 psi at the diaphragm to start opening the 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 vacuum gets closer to that magic -1 psi where they are almost closed 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 airbox is also a muffler, if it can restrict 20HP worth of air from getting in, it can certainly restrict the Goose Honk from getting out. When the airbox is removed it's just easier to hear the sound.
Okay, now you're at WOT boosting 15 psi -
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, I don't care 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, that is if the diaphragm hoses are hooked up properly.
Now it's off-throttle 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 closed, the engine is spinning 7000 RPM and it want's air, so the manifold pressure goes way negative, and 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 what the engine has just developed inside the plenum. Hope this helps.
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