For a 4-stroke, what are the advantages and disadvantages of the various types of pipes such as disc or megaphone, etc? Is one better than the other for certain ranges of RPM?

 

Most of the time on 4-strokes a "disc" type set-up is good for small hp gains across the board along with better low end torque than stock. The "megaphone" set-up is more performance oriented...especially peak power. In simple terms, the principle of the megaphone is air travels from a small area (the headpipe), to a larger area (the silencer with an open end), thus creating a "suction". Usually...the trade-off is a slight loss of low end torque and hp for a large gain in the upper RPM ranges.