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Originally Posted by rwhite692
Troy thanks for the "real-world" info regarding the noise issues on the various types of bypass valves, I will be running two (twin turbo application) and I will definately consider the hose-connect type and some sort of breather arrangement...-Rob
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If it's turbo they're blow-off valves and not bypass valves. They are similar in form & function but the opening/closing conditions are different.
For a bypass valve (supercharger) they are open as long as there is some manifold vacuum, usually 2-3 in/hg. That way as long as you are at light load it is open, dumping the air moved by the supercharger and taking a load off the supercharger and saving a few HP. When you step on the throttle and vacuum drops, manifold vacuum (running to the diaphragm on the valve actuator) is no longer enough to hold the valve open against the spring and it closes and boost starts building. The boost is also fed to the diaphragm and gives it additional holding power. Once you let off the throttle and manifold vacuum returns the valve re-opens and stays open as long as vacuum is sufficient to overcome the spring pressure.
For a turbo, the blowoff valves aren't open constantly under cruise like a centrifugal. They are normally closed. They only open when they see a pressure differential between the turbo plumbing and manifold vacuum so they only open when you close the throttle under boost (preventing compressor surge) and then they close back up. They're normally poppet-type with an adjustable spring and a line from the diaphragm that runs back to the manifold. The spring holds them closed under manifold vacuum only as manifold vacuum alone isn't enough to overcome the force of the spring. Step on the throttle and build boost and you now have pressure in the intake piping trying to push the valve open against the spring, however you also have manifold pressure now fed to the diaphragm trying to push the valve closed plus the pressure of the spring. Thus the valve stays closed and it builds boost. However, when you're on boost and let off the throttle the combination of boost in the piping pressing against the poppet plus manifold vacuum now on the diaphragm side (from closing the throttle) is enough to open it to vent the intake pipes to prevent compressor surge. Once the differential across the valve drops (because the boost in the intake piping has decreased back to a cruise level) the valve closes back up again. Adjusting the spring on the blowoff changes how early and how much it will open, adjust it too much to the "weak" side and the spring pressure is no longer enough to hold it closed under manifold vacuum and it will stay open all the time like a bypass valve.
Basically, a turbo blowoff will only be noisy when you transition from boost to vacuum (i.e. letting off the throttle when under boost, either to slow down or shift.) They won't be loud like a centrifugal bypass because they aren't letting air out all the time, only during transitions from boost to vacuum... and turbos don't flow enough air under cruise conditions to need a bypass or even then make a lot of noise out of them (unlike a centrifugal supercharger.) The more CFM you are flowing and more boost you're making, the louder they get during shifts/letoffs. If you want to quiet them down some you can recirculate them back into the airfilter/inlet side of the turbo.
You can recirculate blowoff valves too, it makes them quieter. It's also pretty much mandatory on a MAF based computer as the computer has already accounted for all that air. Dump it to atmosphere and the car will go really rich on shifts... to the point you can get fireballs out the back. Pretty commonly seen on turbo imports where guys have "upgraded' to a dump-to-atmosphere blowoff valve, removed the catalytic converters, and didn't retune for it. The car goes pig-rich on the shifts because of the blowoff dumping to atmosphere and the unburned fuel ignites once it gets more oxygen when it leaves the tailpipe.