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FWIW (probably not much since I'm pretty new around here) I can't see where that is much better than the stock system - other than possibly the way it sounds.
It looks like it has the same size down tubes. It has the same lousy-flowing T-intersection of the two downtubes (instead of a smooth flowing Y design) and they go into the same 2.5" pipe. It is only after a 2'-3' section of the 2.5" pipe that it flares out to the larger 4" pipe just ahead of the muffler.
$320 seems like a lot to spend for a better sound but very little (if any) improvement in exhaust flow.
Looks to me like you could just put a 4" muffler and tail pipe on the stock system and have the same thing for about 1/4 (or less) of the cost.
I just checked out their web site and it looks as though the 2.5" down tubes flow into a 3" pipe which goes into a 4" pipe prior to the muffler and 4" out the back of the muffler. heres the link to the pdf page 130 http://www.dynomax.com/ecat/Dynomax2006.pdf
Well, that sure isn't what it looks like in the picture. The size difference between the long tube from the Y pipe and the 4" tail pipe sure looks like more than a 1" difference to me.
You're right though, the spec sheet shows 2.5" - 3" - 4" PLUS being mandrel bent has got to help some. Still don't like the looks of the way the PS pipe and DS pipe form a T instead of a Y, but other than that it looks like an improvement over stock.
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