When exhaust is applied thru a long straight pipe, the engine's cylinders push exhaust pulses into the pipe. As each new exhaust-pulse starts travelling down the pipe, the pulses ahead of it help pull the new pulse along. A bit like up-ending a piece of tube with water in it. If you put your hand on top of the pipe you'd feel a suction force as the water travels down the pipe out the bottom-end. Same thing [more or less] happens with a straight length of exhaust pipe. I think the word "scavenge" applies here.
If the exhaust pulse pass thru a muffler, the scavenging becomes less as it loses some force due the larger dimensions of the muffler. Worse, is if it has to go thru muffler's baffles. If it was me, I use as much straight pipe as I could, preferabley using "Hotdog" mufflers which have the same diameter and no baffles.
JJ
