Any given engine has it's own performance curves. Some engines are lowend and some are highend. Some are in between there somewhere. Generally a 3-4 port engine will be a lowend, high torque engine. An 8-11 port engine will generally be a highend low torque engine. "Torque" always referring to the bottom end. Where the problems come in is most rookies believe that if they are suffering lowend performance with their engine then putting a "lowend" pipe on will fix it. Wrong. Putting a lowend pipe on a highend engine just chokes the highend range of the engines powerband.

Proper way of matching a pipe/engine is to choose a pipe that makes the most of what the engine has to offer. Highend engine/ highend pipe. Lowend engine/lowend pipe. Match the pipe to the engine so that the engine makes the greatest power in it's natural power band. Once you've have done that then adjust the clutches and gearing to take full advantage of the full power of the engine.
Don't try to MAKE the engine fit the clutch/gearing of the vehicle.

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Since micro 2 stoke engines don't have any valves they use tuned pipes to simulate a valve. Each time the piston uncovers the exhaust port the exhaust pressure leaves the combustion chamber travels down the header into the pipe, through the pipe to the end of the pipe. Since the stinger is much smaller than the engine's exhaust port then not all of the exhaust pressure is relieved. This remaining exhaust"pulse" then rebounds backward up through the pipe to the exhaust port opening.

Timing is critical for max performance but in effect if everything is correct then that rebounding exhaust pulse should reach the exhaust port right about the same time as the piston is on it's way back up on it's compression stroke. When that happens then for a moment the rebound pressure is equal to the intake pressure so the exhaust port simulates being closed. Now the critical part is if the engine is designed to have earlier or later compression timing then that exhaust pulse will either be early or late. If it's early then the incoming fresh fuel/air charge will be short and some of the exhaust gases will bounce back into the chamber. You will get a hotter less powerful power stroke. If the rebound pulse is late then some of the incoming charge will pass right through the engine and be wasted and the combustion chamber will be overly cooled by too much new fuel and no preburned fuel to help preheat the next charge.

Again, timing is critical to get maximum performance. Any micro engine will run with any pipe or muffler. Or even without one. But they will NOT run correctly or make power well.

So, if you have an engine that is designed to make it's max power in the topend range and you stick a lowend pipe on it then the rebounding pressure pulse is going to be late and out of time when the engine gets up into the high rpm range. Yet since the engine is designed for topend performance then a lowend pipe will not change the natural design of the engine. The engine will tend to always run rich on the topend so you lean it out. That improves the bottom end which gives the impression of improved performance. But the engine will not be making it's peak power ever.

As for the pipe shape. Yes the shape of the "convergence cone and the far end and the "divergence" cone and the near end (the header outlet is the reference point) has a marked affect on the amount of pressure that the return pulse generates. But again you really want to try to enhance the strongest points of the engine and then gear to the engine. Often if you are unlucky you may have to try 4-5 different pipes to find the one that makes the most of the engine. If you are real lucky you run into someone that has already found the optimum combo for your brand of engine and they save you a lot of trial and error. Or you pick a brand that has a matched combo already like OS or RB.