//Nerd Mode on
Here's how I understand it.....
The exhaust generates a pressure wave where the pipe diameter changes. In a 4-1 pipe, theres a strong primary harmonic pressure wave and and a few others. Since there's only one diameter change, these harmonics don't interfere with each other. This pressure wave travels back up the pipe towards the cylinder. When the negative portion of the wave coincides with an exhaust stroke - and more particularly the valve overlap between the exhaust and intake, more exhaust is scavenged and more fuel/air mix is drawn into the cylinder. When a positive wave hits, it doesn't allow as much exhaust and the cylinder gets a short fill. This manifests as a big ugly flat spot, followed by a neck-snapping high spot. The RPM range where these harmonics hit are tuned by the length and shape of the headers.
A 4-2-1 generates several smaller waves at different frequencies. These waves act directly on the motor in the same way as the 4-1 pipe, but they also interfere with each other, both destructively and constructively. Therefore in a 4-2-1 pipe there are lots of harmonics for designers to work with. A well tuned pipe (such as the Akro by all accounts) provides a smoother effect in more spots in the RPM range.
I have a Muzzy, and in my experience (1 bike), it's tougher to tune the fuel across RPM than others I've heard of. It has a big harmonic right around the idle(which really fucks up the idle), another strong wone right at 3K and then it's pretty damped and smooth from there on up.
Anyone else have something to add, or if I've got it wrong - post away! <img src=http://www.ezboard.com/intl/aenglish/images/emoticons/smile.gif ALT=":)">