As we mentioned earlier, you can increase power by flowing more air through an engine. We want plenty of air coming in, and plenty of exhaust gases going out. The problem is, our outflow has to go through an exhaust pipe (and maybe a catalytic converter too). As the exhaust gases hit different pipe diameters and temperatures, some of the pressure wave is reflected backwards. This is called back pressure . We want our engine to exhale quickly, but the exhaust system’s back pressure pushes in the opposite direction.
So, hack off the exhaust pipes and we'll eliminate back pressure, thereby increasing power. Problem solved, right? Wrong. For one, it would be ridiculously loud. Plus, a small amount of pressure is good for forcing un-burned vapor back into the cylinders. This increases fuel economy.
Besides, uncontained exhaust gases are going to melt important bits of your motorcycle. And important bits of your leg, too.
So the situation looks like this:
More back pressure = less air flow = low performance = quiet exhaust
Less back pressure = more air flow = high performance = loud exhaust
So we have a tricky relationship between performance and sound . If you want a high-performance exhaust, it has to be loud. And if you want a regular-performance exhaust that won’t require air-fuel tuning, it has to be quiet. There’s a simple way around this conundrum: the baffle .
A baffle is an insert – normally a porous tube – that goes inside your muffler. A baffle only has a slight effect on back pressure, but it has a huge effect on loudness. Short baffles with small holes can maintain normal back pressure while providing heaps of volume. On the flip side, long baffles with large holes can minimize noise without hindering the high-performance outflow. Any fibreglass-wrapped baffle will decrease volume even further.
The bottom line is this: you can have a high-performance exhaust that doesn't give ear-splitting volume. Or, you can have a loud exhaust that doesn't alter performance and, hence, doesn't require air-fuel tuning. Thanks to baffles, exhaust systems can give any combination of performance and noise.
Some models even come with removable baffles, so quiet folks can have a loud day once in a while. Or vice versa.