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Bypass valve
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Posted by BillJ (more from BillJ) on Fri, 27 Jul 2001 09:31:38 Share Post by Email
In Reply to: Re: Blow off valve, Jon Liddle, Fri, 27 Jul 2001 09:00:51
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The bypass valve works as follows:

During acceleration, the turbo pressurises air in the inlet manifold.
On releasing the throttle, the throttle plate closes the inlet. Without the bypass valve, pressure will continue to build in the inlet and a shock wave travels back from the throttle plate to the turbo, stalling it (in extreme circumstances, probably not on a standard road car, this has been known to shatter the turbo compressor blades). A stalled turbo will take much longer to spool back up again when you start accelerating.

The bypass valve is opened by vacuum in the inlet manifold, which exists as soon as the throttle is closed. It then allows pressurised air from the pressurised side of the turbo to escape back to the non-pressurised side, removing the condition that would otherwise cause the turbo to stall. This causes the hiss you hear.

Problems occur when:

1. The valve doesn't open due perhaps to a cracked vacuum line, causing turbo stall.
2. The valve takes too long to close again, leading to slow build-up of boost pressure on subsequent acceleration.
3. The valve blows open under boost, due to a weak or broken spring or a split diaphragm, causing low or fluctuating boost pressure.

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