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The stock muffler on the GM HD-fullsize trucks create an enormous amount of backpressure.  As you can see one of the outlets on the muffler is not even connected.  The exhaust exits the two inlets against the solid flat plate on the back of the muffler.  It then changes direction toward the front of the muffler through the small holes in the metal plates. The exhaust then goes down to the bottom of the muffler and enters the single pipe where it exits the muffler and enters the tailpipe.