Built my own exhaust from header flange to turn-out. 1-7/8 to 3-1/2 collector, w/tapered transition into a 3" x 18" perforated core straight through muffler, and 3-1/2" turn-out. I wouldn't do it again (too much work
)... next time I would have somebody else build it, only with merge collectors.
Motor drives and responds to tuning same as with open exhaust. This is on a 430" motor with a very large solid roller, a single DP 4 barrel, and locked timing (670hp). Original sidepipes were Lobak spiral cores. Although quiet, throttle response was terrible. In fact, I ran more timing to compensate for the diluted intake charge caused by the excessive (for my combination) backpressure.
Re: "flapping noise" on decel; I dont have any with this system. But, FWIW, I have louvered baffles in a 2-1/4" pipe on my Harley and it "flaps" like crazy. Remove the baffles and it goes away??