Re: Technical exhaust question

Kansas John /

The most important thing in an expansion chamber exhaust is its ability to work with the pressure waves coming from the exhaust port of the cylinder.

To an extent, the denser a medium is, the easier it is for waves to be transmitted through it. That said, a higher displacement cylinder produces a larger volume of exhaust gas, especially as the RPMs begin to climb. On the one hand, this is good, because the larger volume is compressed into the same chamber, creating a denser medium. This means that it's easier for waves to be bounce around in the exhaust, doing all the good shit that expansion chambers do. Because it's easier for the pressure waves to travel through the medium, this allows the cylinder to rev up a little higher.

On the other hand, we know that as we increase pressure, we also increase temperature (if all other variables remain constant). That's why we have to let the exhaust gas OUT of the end of the exhaust, instead of just compressing it indefinitely into a bottle. As the medium in the pipe gets denser and denser, it also gets hotter and hotter, and that heat will eventually heat the cylinder to the point where the cylinder can't give off the heat as fast as it's building up, and the cylinder will seize.

That said, a pipe with a bigger chamber is usually a pipe that has larger angles on the angled sections, and extends those angles out longer. The larger chamber can result in a stronger 'pull' from the pipe. However, since the pipe is larger, a smaller displacement cylinder would not be able to fill that volume with exhaust gas as quickly as a higher displacement cylinder, resulting in a lower density in the chamber. This lower density makes it more difficult for the cylinder to rev up, because it has to work harder to transmit those pressure waves through the less-dense medium. Fortunately, the cylinder can struggle as much as it wants initially, because once the RPMs get high enough to hit the powerband of the pipe, the larger dimensions of the pipe amplify the strength of those pressure waves, "pulling" the engine into higher RPMs than it would have been able to achieve without the expansion chamber, because the pipe is amplifying the strength of those waves. At higher RPMs, even a smaller cylinder is generating a much higher volume of gas than it is at low-RPMs, and is better able to fill the volume of the expansion chamber, making wave transmission easier, and allowing the engine RPM to climb higher.

Another interesting aspect of expansion chamber exhausts is the phenomenon of 'multiple powerbands.' I don't have personal experience with this, so I'm going to outline a hypothetical explanation for how that might work.

The pressure waves we're so dependent upon in 2-stroke engine tuning are simple waves, and as such, part of the science of exhaust tuning is harmonics, as illustrated by the changing diameter of the exhaust functioning like the bell of a brass instrument to amplify the strength of the waves, which in the case of a brass instrument, is volume. An interesting aspect of harmonics is the 'doubling' of waves. Now my understanding of harmonics and wave theory is sketchy at best, but I think the basic principle is that the different angles amplify different frequencies of waves. It amplifies those waves best at a certain range of frequencies, and the frequencies outside of that range are not amplified as effectively. However, if you DOUBLE the frequency of the wave, the angles _should_ amplify them in the same way. So if you could somehow measure the frequency of exhaust pulses, and figure out the relationship between those frequencies and the RPMs of the engine... So say a pipe has a powerband at 4000-5000 RPM, if the relationship between frequency and RPM were as direct as I hope they are, it would have a second powerband at 8000-10,000 RPM. Sort of. Like I said, I don't really understand harmonics or wave theory that well, but as far as I do understand it, that's sort of how it would work.

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