jfnar, et.al,
I first read about the "H" pipe concept on the 1960s in one of the hot rod publications, maybe Hot Rod Magazine. It explained that Mopar engineers had discovered small performance gains as well as quieter exhaust.
Occasionally this subject also comes up on the Performance portion of the Chevy Talk forum. The moderator there "Grumpy Vette" has posted a lot of information on the details of "H" and "X" pipe construction and their placement in the exhaust system. If I remember correctly he also posted details of actual measured performance gains.
In the summer of 1967, when I was a college kid, I worked at Doug's Headers tig welding. Since then I've been really interested in exhaust system design and I've followed the balance tube and other exhaust related inventions with interest. To most of us it's fun to learn new things.
