Ask The Editors: What’s That Canister On The Pipe?
Making sound waves work for you.
Close! The expansion chamber was a staple of the 2-stroke exhaust system; a place to store the hot gases escaping the engine in every stroke so as to control the amount of back-pressure making its way back to the cylinder.
What you are observing on 4-stroke pipes is a resonance chamber. Surprisingly the psychics behind the two chambers aren’t all that different, but they make use of different approaches to achieve similar results.
A resonating chamber is, in essence, nothing more than an empty space. Remember that in any engine, a high-pressure exhaust note is released then followed immediately by an opposing low-pressure note. We detect this as thumps.
These exhaust notes enter the resonance chamber through small holes and that empty space it offers creates an interference in the exhaust flow. If timed right, the positive and negative sound waves will actually cancel each other out because of this disruption rather than continue to force the flow in either direction.
Lastly these chambers provide a bit more tuning control because they allow for regulation of the amount of back-pressure at the moment all of the valves are open.
Whereas once the resonance chambers made use of a piggyback canister design, more modern iterations manage to integrate it as a bulge within the pipe itself.
