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I may be wrong, but from what I can find, blow-off valves are installed between the turbo and the throttle butterflies, and are actuated by engine vacuum or throttle position to avoid excess turbo pressure and keep the turbo spooled up (and make lots of noise) when the throttle is closed suddenly. Pop-off valves are actuated by direct pressure on the disc, and are installed in the plenum by the sanctioning body in order to control cheating.
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