As said the 'flutter' noise is the boosted air passing back through the turbo compressor blades when throttle off (butterfly closed) as the boosted air can only escape this way (no blow off valve). The concern with damage to the turbo is the pressure it puts on the turbos shaft/bearings as the air faces the compressor blades in the opposing direction. This causes a shearing affect on the shaft and therefore more pressure on the bearings. Its of course better not to have it then to have it, but really would be more of an issue higher rpm, with big boost and or larger turbos. Surge at low rpm I don't think there is much concern at all.
Most BOV's can act very similar as you've described. ie flutter at low rpm and no flutter and total blow off at higher rpm where boost is higher. It just comes down to the reaction time of the BOV. Most BOV's can be adjusted by the spring, so you can play around with it and see what suits best. As said though, it probably won't solve you early flutter issue, but again as said above, flutter at low rpm is not an issue. Its common for most diaphram/piston traditional BOVS act this way.
A BOV does not work just on boost or just on vacuum alone. It works BOTH on boost AND vacuum.
Explain:
Theres 2 'opposing' sides to a BOV. The boost side where the bov is connected to. Which is your intake pipe (before Throttle body). And theres the vacuum side usually connected to the top of the BOV, this is connected to the Intake Manifold (after the Throttle body). Both these connections work on a push/pull basis and oppose each other.
When your Accelerating in gear, foot down on the gass Butterfly is open right? The intake pipe side of BOV sees boost, lets say 8psi. What does the Vacuum side see? 8psi as well. Why? Because your foot is down on the gas and Butterfly is open. So you have equal pressure on both sides of the Throttle body. The BOV stays closed (which is what you want). Its because the 8psi in the intake piping is trying to push open the BOV (pre throttle body), but it can't because you have 8psi trying to keep it closed from the intake manifold (after throttle body) and also the spring. To sum up: In gear > on the gas > bov remains closed.
When your off the gas changing gears. Now that your off the gas, butterfly is now fully closed and you want it to blow off. But whats happening? The turbo is still spinning. Intake side is still boosted as the butterfly is now closed, so theres 8psi trying to push open the BOV. But whats happening on the other side of the BOV? now that the butterfly is closed it cannot see boost, in fact it now sees vacuum / negative psi, because the engine is still trying to suck in, but the butterfly is fully closed. So we have 8psi one side of the BOV and vacuum on the opposing side of the BOV. You have a push/pull affect. Pressure (push) on one side of the BOV and sucking vacuum pulling the BOV open. So the BOV fully opens and blows off.
If your interested Synapse makes a fast action BOV which does away with the delay of the traditional diaphragm design of every other BOV. See here:
http://www.phantasmusa.com/synapse-blow-off-valves.html
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Originally Posted by senna
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