I hope this article I found some time ago might help you clear your question.
Unlike four stroke engines, in which intake and exhaust valves retain fuel in the combustion chamber, a two stroke engine depends on the header and tuned pipe to retain fuel in the combustion chamber.
It has been said that the single most performance gain that one can achieve on a two stroke is made by strapping on a tuned pipe. This is very true if it is done properly. Don't just go down to your hobby shop and purchase a pipe marked ".12 Tuned Pipe" or ".21 Tuned Pipe".
How it Works:
Starting with the moment the engine fires, forcing the piston down and the exhaust port begins to open the exhaust gases are forced out of the engine and start to travel down the 'header' section of the pipe. Before long the gas reaches the expanding / divergent / cone. The effect is a pressure drop, creating a vacuum which helps 'pull' the remaining exhaust gases out of the engine.
Not only are the exhaust gases being 'sucked' out of the engine, the fresh intake mixture is being 'drawn' into the combustion chamber from the crankcase via the transfer port. Some of this new charge will follow the exhaust gas straight through the still open exhaust port.
The exhaust gas travels down the pipe, through the expanding cone till it meets the reflecting / converging cone. The converging cone forces the pressure to rise, generating a pressure wave which reflects back towards the exhaust port.
As the reflected wave approaches the exhaust port, it forces the fresh mixture (which flowed through the combustion chamber), back into the combustion chamber. As the transfer port closes before the exhaust port, this results in a pressurized charge in the combustion chamber as the exhaust port closes. The result - super-charging your engine - more power.
Sections of a Tuned Pipe
Header - Although not part of the tuned pipe, the header plays an important role in the overall tuning of your engine. The header attaches to the engine and is the straight or slightly divergent (opens up 2-3 degrees) section of the pipe. It helps to suck the exhaust gases out of the engine. The header pipe cross-sectional area should be 10-15% greater than the exhaust port window for maximum output at maximum RPM's is desired. In some cases the area of the header pipe may have a cross-sectional area 150% of the exhaust port area. The length should be 6-8 times its diameters for maximum horsepower. For a broader power curve, 11 times pipe diameter may be used. This is the part in which you trim length to tune the header.
Divergent (Diffuser) Cone - The section of the pipe that attaches to the header and opens up at an angle like a megaphone. It intensifies and lengthens the returning sound waves, thus broadening the power curve. The steeper the angle the more intense the negative wave returns, but also the shorter the duration. The lesser the angle, of course, returns a less intense wave, but for a longer period of time (duration). The outlet area should be 6.25 times the inlet area. It usually has 7-10 degree taper angle.
Belly - Located between the divergent and convergent cones, it's length determines the relative timing of the negative and positive waves. The shorter the belly the shorter the distance positive waves travel and the narrower the RPM range. This is good for operating at HIGH RPM only. The longer the belly the broader the RPM range. The diameter of the belly has little or no effect.
Convergent (Baffle) Cone - Located after the belly and before the stinger, reflects the positive waves back to the open exhaust port and forces the fresh fuel mixture back into the combustion chamber as the exhaust port closes. The steeper the angle the more intense the positive wave and the gentler the angle the less intense. It usually has 14-20 degree taper angle. The taper angle primarily influences the shape of the power curve past the point at which maximum power is obtained.
Stinger - Located at the opposite end of the pipe from the header and after the convergent cone, it is the "pressure relief valve" of the pipe where the exhaust gasses eventually leave the pipe. The back pressure in the pipe is caused by the size (diameter) or length of the stinger. A smaller stinger causes more back pressure and thus a denser medium for the sound waves to travel in. Sound waves love denser mediums and thus travel better. A draw back to a small stinger is heat build up in the pipe and engine. DO NOT USE TOO SMALL A STINGER! The stinger diameter should be .58-.62 times that of the header pipe and a length equal to 12 of it's own diameters.
Selecting a Tuned Pipe
What do we want?
1) Quick acceleration
2) Broad RPM range
3) Broad to lower power range
This means we are probably not going to turn the maximum RPM's that the engine is capable of anywhere on the course. If our engine is capable of turning 40,000 RPM's, we will probably only use up to 35,000 RPM's. Look at each section of the pipe in the above descriptions. The Header cross-sectional area should be at least 10-15% greater than the area of the exhaust port. Length at this point doesn't really matter (at least 8 diameters), but make sure it is long enough to work with. The divergent cone would be at a medium angle for a broad power curve at lower RPM's. The belly would be medium to long for a broad RPM range. The convergent cone would be at a gentle angle because we want the duration of the positive wave to be longer.
How long is the pipe? The formula for determining the length is:
Lt = (Eo x Vs) / N English OR (83.3(Eo x Vs)) / N Metric
Lt = tuned pipe length, in inches or millimeters
Eo = exhaust open period, in degrees
Vs = wave speed (1700 ft/sec or 518.16 Meters/sec at sea level)
N = crankshaft speed, in RPM
Let's say, for example, we have an engine that will turn 25,000 RPM. We calculate that we will only use 20,000 of those RPM's and our exhaust duration is 180 degrees. Then we substitute in the formula:
Lt = (180 x 1700) / 20,000 OR (83.3(180 x518.16))/ 20000
Lt = 15.3 inches OR 388.46 mm
Now this is where you need to make a personal decision. Some people say that this distance is measured from the exhaust port opening and some say that the distance is from the center of the cylinder. The choice is yours, but I take the longer distance, which is from the exhaust port opening. Remember that this is not the total length of the pipe. This is the length from the (in my choice) face of the piston at the exhaust port to the center of the convergent cone including the invisible intersection of the convergent points not just what you see.
Tuning the Pipe
Now comes the fun part! We get to go to the track, unless of course we have our very own dyno.
So we have set the pipe up so that we have an optimum length. First we want to get the right gear ratio, right fuel and right needle before we even mess with that pipe. You see this is where the "What a pipe can't do?" comes in. A pipe cannot make up for poor engine setups and crappy gear ratios. A pipe also cannot make up for bad engine timing and some engines are timed so poorly that no pipe will increase performance.
Ok, we make a few (2-3) passes. We pay close attention to what the engine is doing.
If We have the right gear ratio, and the right needle setting and the engine runs slow, something is wrong. If the mixture is correct then pipe is too long. Shorten it by 1/8" at a time until the revs start to rise (this can be done at the exhaust coupler).
If the pipe is too short the motor will run harshly and the needle setting will be unstable and critical. Add 1/8" to the length at a time (again, at the coupler).
When the pipe is at the proper length you will experience the thrill of a lifetime. You will hear the engine and pipe become one in resonance. You will see your car accelerate like you walked behind it and gave it a kick in the rear. This as known as being "on the pipe".
The good thing about today's in-line pipes is that they come almost tuned for best optimal average performance for the engines they are designed, and most offer you three different optional headers for fine tuning (short, mid and long). There is no bad pipe, just a bad choice for the application you intend for it.