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With many of the BPVs having a piston area in the spring chamber of equal or greater area to the area of the effective valve seat or piston face, the pressures will balance under steady state condidions and the spring force will prevail. So a weakened spring will not reduce steeady state boost. This assumes that there is not a large pressure drop between the PBV location and the VAC line pressure tap which is usually in the TB with an OEM setup. So there is no theoretical reason to expect any problems.
If you have a real heavy spring, the valve will open later and open less. So that whats its prime task is, is it not? You want a long spring chamber a'la TurboXS and a lower spring constant.
Now that was steady state. If the turbo spins up very fast, there could be pressure at the BPV piston face that are getting high enough to lift the spring before pressure from the TB pressure tap can flow to the spring chamber and equalize. The OEM boost controllers often do not allow boost to build that fast, that is built in boost lag. So if there is an effect, it will be transient and will basically be in effect a change in the timing of the BPV closure when comming off of a closed throttle. This would not be a launch issue ever, but would be limited to gear shift events where boost recovery can be rapid.
If you worry about these things, worry about the real small orifice in the TB which limits flow to the spring cavity in the BPV, and worry about the gas volume of the BPV spring chamber. These, and the length and diameter of the VAC line will determine the time constant for the spring chamber pressurization. A larger VAC line can be self defeating as this introduces more volume to be pressurized. Best solution would be to drill and tap a large hose barb into the intake manifold. And to rework the spring chamber to reduce its gas volume. That would mean installing a solid volume to occupy the parasitic volume withing the spring 'internal diameter*. This hardware should be attached to the spring chamber cap, not the piston. You don't want to increase its mass. The G forces on the piston could be quite high when the valve shuts as well and that would subject the added hardware to shock, and you don't want to have the inertial shock of the added mass acting on the valve seat, which is often metal on metal.
* this feature is probably patentable design feature, but is now public domain. But pneumo actutated stuff is as old as the hills, and this could be a prior art. But I can't understand why with an expensive aftermarket device such as this, that they ignore such things.
As for librication, there is a lot of oil in the turbo air stream and that might keep the piston lubricated. Its not oil free air.
posted by 65.68.10...
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