Animation of Two Stroke expansion chamber:
As the pressure wave reflects back, it has a similar effect to turbocharging or supercharging a 4 stroke- it rams fuel and air that leaked into the pipe back into the cylinder under higher pressure, causing the motor to have more power (more fuel & air = bigger bang). That's a very loose explanation, but there you go.
I've always wanted to build an expansion chamber... This is where P.O.J. (pronounced Podge) comes in. This Piece Of Junk was (and is) my first motorcycle... found it in a grain bin around '95, back when I wasn't allowed to have bikes. Somehow, this one snuck home. I dug it out of storage a few years ago and began this process, so follow along as I make mistakes and learn a thing or two about building your own expansion chamber!
Please note, this is NOT the only way to do this. I think there is an easier way- done by cutting out two sheets in the correct shape, welding the edges, and the pumping ultra high pressure water in to "expand" the pipe, but I didn't have the tools at the time to do it like that. This is just how I did it...