exhaust system idea

[Dave Williams]

-> Its a great idea.  A few years ago I designed a muffler, like a turbo
-> muffler, that had a butterfly in it.  When closed, the exhaust passed
-> through lots of perforated holes and was damped by 2 Helmholz
-> resonators. When open, the exhaust went right through with minimal
-> silencing.

 Pontiac had mufflers like that, optional on various late '60s models.
They had vacuum actuators and many yards of vacuum line.