After doing a little research on the Arvinode mufllers here's what I've come up with-
The Arvinode's didn't use the conventional methods of exhaust silenceing like absorbtion (glasspacks) or baffles (traditional chambered muffler), but instead used what is known as a Herschel-Quincke tube. That's kind of like a 1/4 wave tuner. Or a trombone slide. That is what the little tube that runs along the top of the mufflers was for.
The way I understand it, the wavelength of the particular frequency that was to be silenced is calculated out, and the tube is one quarter the length of the wavelength. The tube is connected to the exhaust pipe at the front, but dead-ends at the back. So, the sound wave goes into the tube, travels 1/4 of its length and at the back of the tube it is reflected back to the entrance of the tube traveling another 1/4 of its length. Now, the wave has traveled a total of 1/2 of its wavelength. It is now 180 degrees out of phase with the original wave, and through a process known as "destructive interference" the sound waves cancel eachother out.
This is the same thing that happens if you wire a set of speakers, and one is wired backwords, they go out of phase, and the volume ends up quieter.
I have never actually seen an Arvinode system in person, but I have seen this type of exhaust system (Herschel-Quincke) on large, indoor, V-12 diesel generator motors. In those instances, the tube is actually perpendicular to the exhaust system. It probably works better that way, but would be totally impractical for automotive use.
I believe this is what gave the Arvinodes their distinctive sound, but I would have no idea whatsoever on how to calculate wavelength, pipe length/diameter, or how to assemble such a device onto the muffler. But I do think that the little tube is the key to the whole system.
I'm sorry if this info is too little too late for round2K's current project, but I thought I'd throw it out there anyhow.
My disclaimer: I've never designed a muffler or worked in a muffler factory. I'm not a scientist, acoustical engineer, or anything like that at all. The top end of my acoustical knowledge comes from being a music minor in college, so please keep that in mind. I've been wrong before, so I might be totally off-base with this too. But it does seem plausible to me.
