Your exhaust systems main job is to work in harmony with your engine by helping gas out of, and therefore also in to, the combustion chamber as quickly as possible – and the tuned length is critical to this. Most exhaust systems have three main sections: header pipe, intermediary section and silencer, each of which has an effect on the working of the exhaust system and the tuned length.
In lay terms, the 'bang' from the combustion process causes a positive wave that flows down the pipe. When it encounters an edge, change in volume or significant change in velocity, an inverse reflection is send back up the pipe. This can be put to good effect if it arrives in the exhaust port as the valve opens because it increases the difference between the positive chamber and port pressure – giving the gas even more reason to flow into the exhaust.
The general layout of an exhaust system,, balance pipes and power valves affect the tuned length at different rpm, and help that reflected wave to arrive at the correct time over the widest possible rev-range. As a rule of thumb, a four-into-two-into-one exhaust system, will work to enhance mid-range performance, where a 4 into 1 system will work best at higher revs. Interconnecting 'balance' pipes also help fool the engine into thinking the tuned length is different and will give a 4 into 1 system a little increase in mid-range. As well as the tuned length, the exhaust systems cross-section area is important as it affects the gas velocity. If the bore is too big the velocity will be too low, too small and the pipe will be too restrictive for the volume of gas. Shape is relatively unimportant. An oval pipe with the same area will work just as well as a round one, and can often improve ground clearance. The optimum cross-sectional area is normally arrived at through modeling, historical data and trial and error....
The silencer is perhaps the most famous part of the exhaust system and keeps a whole after-market industry alive. Its job is to slow and dissipate the energy in the gas to reduce noise. In road trim, it does this by forcing the gas through several chambers and changes of direction before venting to atmosphere. This does however cause some restriction (and therefore loss of power), as well as adding considerably to the motorcycle's weight.
Header pipes have two main jobs. To increase the gas velocity and funnel it downwards to join the rest of the exhaust. The biggest problem is avoiding sharp turns, which slow the gas and causes hot-sports. By gradually reducing the cross-sectional area of the pipe the exhaust gas speed is increased – these are called tapered headers. Small joining pipes between headers are called balance pipes, and change the exhaust's perceived tuned length. They normally bolster mid-range performance. The gas temperature at the header pipes is typically 700 to 800 degree Celsius at the exit.
Or mid-pipes, as they're called. This is the area in which the feeding pipes merge down into one or two bigger pipes, creating something like a four-into-one layout. Changing the length of this section has a big effect on where and how power is made. Catalytic converters are normally found in this area. While they do reduce gas velocity a bit, the large change is cross-sectional area causes more problems.
Whether it's road legal or not, the silencer's job is to reduce noise. On a road legal exhaust pipe it does this by directing the sound waves through a series of chambers designed to reduce the wave's energy. On a race exhaust pipe, the lack of chambers is less restrictive, 'allowing the engine to breath out more easily and increasing its power-making potential. Energy is still bled off through perforations in the inner tube. Removable baffles are a quick fix but reduce power as they restrict the flow of exhaust gas.