The pneumatic flappers do move, they loose pressure and oscillate. I have high po ones from CMI, never lost one but I don't run much with them closed either except at slower speeds. They are not to be used at higher Rpms for extended periods.

Randy

Most of my time is spent crusing at 3000 to 3500 rpm. and believe me the exhaust stays closed because I can hear it when the flappers arn't closed all the way. This boat is LOUD.