In my old Alpina B3s cabriolet this wasn't so much as a valve but a trap door that blocked off the passenger-side tail-pipe. Needed to be lubed otherwise the squeak from it was horrific!
Something about increasing back pressure at low revs said Sytners when I asked.
All I knew was that it made one hell of a noise in stop-start traffic.
God help you if the standard pipes suffer from this too.
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Madgambler
325i (E30) => Alpina B3.33 (E46) => Alpina B3s (E46) => 130i M Sport (E87) => Cayman S (987c) => Carrera S (997) => M135i (F21)
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