[175K911]

Back when I ran the B&B system with 2-out on an old 911, the only way to get rid of the resonance was to cap the right side outlet for the street.

And when I had SSI's and a Flowmaster 2-in 2-out muffler on a previous Carrera, same thing. Only this time I experimented. Maintained the 3.5" outlet on the driver's side, but welded a restrictor (essentially a washer with a 3.5" OD and a 2" ID) into the tailpipe on the right side. still looked like a 3.5" tip but was choked down internally. Resonance range narrowed and amount of resonance was significantly reduced. Ultimately I made a cap for the right side that still looked like a 3.5" tip but with the cap at the muffler end of the tip essentially the tip was a dummy. Voila, resonance gone.

In a conversation I'd had with John Daniels at SSI some 20 years ago he suggested that the 911 engines are very sensitive to outlet diameter sizing. The factory sport muffler was a pair of 2" outlets, and much more exhaust pipe volume will resonate. He's actually the person who suggested putting a washer as a restrictor in the right tailpipe. B&B likes to make the mufflers with two really big tips so I suppose that's why they have a resonance point.

I'm not an expert on sound wave nor pretend to be, just passing along my own experiences.