The following article comes from "The Essential Guide to Motorcycle Maintenance" by Mark Zimmerman:
EXHAUST SYSTEM
An exhaust system, like everything else in the world, behaves according to rules formulated a long time ago by Mother Nature. Basically, any well-designed exhaust system works like an air pump. It helps empty and refill the cylinder. When the exhaust valve, opens a blast of high-pressure, high-temperature gas bursts out of the cylinder. The gas pressure will be anywhere from three to seven times that of atmospheric. The exhaust temperature will be in the neighbor-hood of 1,700 degrees. In essence, you've just converted a fair amount of potential energy into heat and kinetic energy. Since only about one third of the fuel's potential energy does any real work, the rest being converted into heat and noise, you can imagine how much more efficiently our engines would run if we could harness the exhaust's power. That's exactly what a well-designed exhaust-system tries to do.
In this section, we'll discuss the functions of the various exhaust components. Then, we'll tell you how you can maintain and improve the whole system.
SCAVENGING POWER
Ideally, an exhaust system can harness the energy left in the hot exhaust gases to help increase engine power using a phenomenon known as scavenging. As the exhaust valve starts to open the spent gases race down the pipe at around 1,800 feet per second, depending on their temperature, As our gases rush down the pipe they eventually reach a portion of the pipe that's larger than the exhaust port. When this pulse of exhaust gases hits the expansion area of the pipe the gases slow down and expand which then creates a negative wave that flows back toward and into the exhaust port.
If the engineers did their homework correctly, the negative wave hits the exhaust valve during the valve overlap period and flows in through the open exhaust valve. Its low pressure helps evacuate any residual exhaust gases hovering around the combustion chamber, It also helps pull fresh mixture in through the opening intake valve. This Jump starting of the fresh mixture that occurs before the piston has begun to descend on the intake stroke can provide a substantial torque boost.
Now here's the rub; while engine speed constantly varies, exhaust-gas speed remains constant, Therefore our scavenging effect will only work over a fairly limited rpm range, say a few hundred at best in narrow-spectrum competition engines. It's also important to remember that these pulses, both negative and positive, resonate back and forth from the port to the exhaust's open end until they more or less run out of energy on their own.
PIPE DESIGN
Normally, the muffler of a street bike damps down exhaust pulses. It also increases the range over which the negative pulses do their thing. On a race bike, particularly one that uses individual pipes for each cylinder, a tapered megaphone performs the same function.
A megaphone allows the exhaust wave to gently transition from the round pipe to the atmosphere, because the effect of the megaphone moderates the exhaust pulses the bike becomes much easier to ride. By placing a reverse cone at the megaphone outlet we can reflect a positive wave back when we need it. A properly designed reverse-cone megaphone uses the negative wave to pull the fresh charge through the cylinder during overlap. In fact, it pulls so much charge (along with some help from inertia) that a portion of it spills into the pipe.
A nice side effect is that the fresh charge cools the exhaust valve. Before the exhaust valve closes the reverse cone reflects a positive wave back up the pipe. where it stuffs the spilled mixture back into the cylinder: the valve closes and bingo, free horsepower!
O.K. Well anyway you can now see the exhaust system is more than just a set of pipes.
I wonder, when we develop battery or solar or whatever powered cars and scooters, will we play a recording of loud exhaust pipes as we drive down the road because it's cool or is it cool to step into the future and embrace new technologies that may change the way we do things?
I enjoy a finely crafted powerhouse built to exacting specifications and sounding like it. I call it being like a sewing machine.
