| Either the vacuum control line isn't connected to the intake manifold, has a split in it, or the diaphragm is ruptured. When letting out of the pedal from a boosted condition, you have both a high degree of vacuum "pulling" the valve open as well as the pressure within the charge pipes "pushing" the valve open. These two combined forces will result in the valve very rapidly opening and venting pressure. If there is something wrong, such as what I mentioned above, the "pull" on the valve will be compromised and only the "push" of the pressure within the charge pipes will force the valve open, which can cause it to instantly flutter from the moment the throttle is closed. The thing one needs to take into consideration though is the difference between surge resulting from an improperly sized/configured turbocharger vs. the surge that is experienced as a result of the bov/recirc system during de-spooling of the turbocharger. If the compressor is in surge while it is producing boost pressure it will cause the most damage to the rotorbearing system. When boosting, the pressure being produced by the compressor wheel also acts on the backside of the compressor wheel. This inter-stage pressure (pressure of the air at the periphery of the compressor wheel BEFORE it passes through the diffuser section) will produce a large forward axial load on the rotating group. This load is carried by the thrust bearing in journal bearing turbos and by the front ball bearing in a dual BB cartridge. In a GT2860RS, the comp wheel is 60mm, 1.181" radius, which has an area of 4.381 in^2. At 18psi of boost, the interstage pressure is roughly half, so figure 9psi acting on the back of the comp wheel. 9psi working on 4.381in^2 = a forward axial load of 39.429 lbs of force. Think about that for a second. =) As that rotating group is spinning some 150KRPM, the bearings are not only having to survive the punishment of the speed element, but they are also having to bear nearly 40lbs of thrust load at the same time... and that's only at pumpfuel boost pressures. Additionally, when you have a double overhung rotor assembly such as a turbocharger rotating group, and it is spinning over a range of RPM up to 2500 times per second, the shaft is experiencing rotational dynamics that cause it to flex and "ring" like a tuning fork, if you will. This creates various whirl motions of the entire rotating group. During the development of these turbochargers, a rotor dynamic study is performed to analyze how the rotating group behaves over its range of operation - what RPMs the critical speeds are at, shaft orbits, whirl modes, etc, and they use this information to refine the design so that it does not experience any damaging rotational patterns. The turbos are happy when they operate within their envelope, but if you push them into surge at high pressures and rotational speeds, the effect of surge will destabilize the rotating group in such a way that forces it into rotational orbits and whirl modes that the rotorbearing system cannot endure. The case I described above is not what any of this conversation is about. Our vehicles and the proven upgrade components we use on them do not experience this. The very small amount of surge that is experienced at the tail end of a BOV event is negligible - the turbo is not producing any significant boost pressure and it is rotating at very low speeds where the forces experienced by the rotating group in this negligible "surge" event are incredibly small - inconsequentially small... In the case you referred to where the BOV was fluttering immediately following throttle closure, you certainly would not want to continue running it that way. In that case, the turbo is spooled to high speeds, producing significant boost pressure, and the BOV is creating a rapid oscillation of pressure and flow within the charge pipes. This oscillation is felt by the rotating group, causing a huge spikes in pressure which undoubtedly put the comp into surge, thus destabilizing the rotating group and pounding the bejeesus out of the rotorbearing system... :-/
Enthusiasts soon understand each other. --W. Irving.
Are you an enthusiast? If you are out to describe the truth, leave elegance to the
tailor.
Albert Einstein
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