5 years ago I was at a SCCA GT-1 (thundercars--big V8's) race at Portland International Raceway. On the first lap there was a Firebird or Camaro race car that came around turn 8, clearly ahead of the pack. When his car went by the grand stands, where I was sitting, his car made only a WHOOSH. No exhaust sound whatsoever! I could hear clearly the engines mechanical sounds (rods, lifters, etc.) It was pretty amazing. The exhaust was absolutely quiet. When I later asked my friend (who was in second place behind him) I was told that he had an "active muffler." Since that time I haven't heard anything thing about these mufflers. And that was 5 years ago. Anyway, if you want absolute stealth quiet, there is something out there.

The 2.8 L engine has used them since at least 1996 (maybe sooner)

It is a vacumn controlled flap.

It is for performance, as well as sound adjustment - but not for total silencing.

Cheers
Jim Cash