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A blow-off valve can be hooked up to work just like a compressor bypass valve by putting a hose from the blowoff output to a 'T' piece behind the AMM sensor so the air is brought back to the intake.
The advantage of a blowoff really comes when you push the boost to 20psi or so. The bypass valve is a very simple spring-loaded vacuum operated actuator mechanism and when the pressure difference between the boost pressure and the intake becomes bigger it will force the valve open and starts to bleed off pressure actually requiring the turbo to spin (much) faster to keep a sustained boost level (efficiency drop).
Blowoff valves usually have a diffferent construction using pistons or diaphraghms which can't be forced open by the boost pressure unless it reaches a certain pre-set limit (overboost protection) or a vacuum 'trigger' is applied (eg. when lifting the throttle). Then they open very rapidly and vent the excess air to prevent compressor stall.
Blowing off to the outside can indeed cause problems on air-mass systems, so a 'feedback' hose is usually a good idea.. (less 'cool' as it becomes pretty quiet again, but who cares)
Bye, Arno.
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