They're both chambered, so its still the same way to cancel out sound waves. That's like saying Fords and Chevys are completely different. They're still cars at the end of the day and they still do the same purpose. Turbo mufflers have chambers, look where the first tube ends and that area is a chamber, then where the middle one ends there is another chamber at thee other end. You can disagree all you want but turbo mufflers are chambered.
You can say they're "chambered", but Flowmasters (& knockoffs) do not have those perforated tubes, which will completely change the sound and create more of a restriction. I like the sound of Turbos on a V-8, but I think they are slightly more restrictive than the Flowmaster design.
Same reason that all the other knockoffs don't usually rate the same in direct comparisons either. Quality control and those little attentions to detail(sharp edges and poor transitions.. not that FM's are without their own sets of flaws too though). Look at it like this.. Procomp heads are knockoffs of the Edelbrocks too.. but they don;t work quite as well either. Either way.. like I said above.. the losses are insignificant for those shopping in the sub-$40 muffler range anyways. Just tell people that they're FM's or Thrush's.. or even Summits and they'll never know the difference one way or the other. lol
Sure, you can say turbo mufflers have "chambers" at the ends but they have nothing to do with the sound, just gives the gases a place to move on to the next tube. The exhaust changes direction twice while passing through perforated tubes with sound deadening material wrapped around them in the turbos. That's like saying an over head cam engine and a pushrod engine are the same, in essence they are engines but they work totally different. We definitely aren't going to agree on this on so have it your way.
I hate flowmasters and magnaflows. I have summits on my trans am and the sound is phenomenal going down the road. http://youtu.be/bAi7cDGuckE put some mufflers on your car! unless you have a reaaally good street car the sound is bad and too loud.
Try to find a set of Hush Thrush,a little quieter & I always thought they sounded on 302s.I had a set a long time ago & was very happy w/ them,& they lasted longer than Cherry Bombs & the cops did`nt harass me.