Exhaust boost?

Many of the aftermarket exhausts for four stroke MX bikes had a add on cylinder reasonably close to the engine that appears to be a boost bottle (like the old two strokes of the early 198s) that I am a familiar with. I was wondering why none of the aftermarket exhausts for the RC390 do not have this add on?? If it works to smooth out power (or whatever) on a 4 stroke single on a dirt bike...why isn't anyone putting them on the RC exhausts?:confused: Any thoughts?:confused: parillaguy
 
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