In the last post if the pipe had been cut a little further back it likely would have run a little bit better. That outside diverging angle bell shape is to be preserved whenever possible, it enhances performance.
There are a lot of different exhaust methods used to kill sound. You actually DON'T want to kill pressure, pressure is flow, you want to break the sound waves up while killing as little pressure as possible. Some methods go straight through while using little in and out pockets and some do a lot of looping back to do the same. What the smaller tubes are doing, multiples of them are taking sound and looping back to smash the waves into each other or on another baffle wall.
What works on one can be a disaster on another, take post #3 above, I did the same (removed the last few inches of muffler) to earlier DOHC Honda and the result was horrible, the exhaust sounded like engine was not running right and the worst sound I have ever heard come out of an engine, I invented a word for it, 'blatty'. A horrible sound and no one would ever like that, people put their hands on their ears when I pulled up to a light. That back chamber I opened up was a major loop-back that provided much of the noise killing on that design. It did help in that I had no excuse by then not to go header.
The only way smaller pipes will work is if they have a wall to loop back through to meet up later with the rest, the mis-timing of that is one way to kill sound. Combining all firing events in a single pathway does too, why dual exhausts on cars get louder than single and why a header capped up gets quiet too. You can also increase power if the smaller pipes cause a restriction at the right time, at a certain point the restriction causes overlap to not over-exhaust to pull out too much intake fuel being pulled in by scavenging. So, the placement of the smaller tubes can even impact your gas mileage.
You WANT the pressure (gas speed) up, it is what allows scavenging to work. Scavenging being power increase if done right. A 4 stroke engine works exactly like a 2 stroke in scavenging, just not as radical. You want to pull more intake in using exhaust pressure until the very end, then you want to send a pressure 'plug' so to speak in at the last to act as a cork to bottle up the extra intake you just pulled in from escaping out the exhaust. What the rear half of an expansion chamber does when it shrinks way down, that is the part of the assembly that does the plugging. .
