After reading a research paper revealing how straight exhaust pipe lengths affect delivery ratio (% the cylinder fills with air/fuel mixture) I experimented with pipe extensions on my 48cc piston port engine with a 6.9" intake length (measured from piston to carb slide). The research paper had specified this intake track length so I duplicated it. Here's my results:
Pipe street 1 street 2 flat
21.5" X 18.5mph 26.0mph (11.5" more than stock)
18.5" X 20.0mph 25.0mph
16" 13.5mph 20.0mph 25.5mph (best)
13" X 20.5mph 25.0mph
10" X 18.0mph 25.0mph (standard pipe length)
"Pipe" length is actually the exhaust length from piston to muffler. (On mine the pipe extends 5.75" inside the muffler also.) So 16" is the standard 10" with an additional 6".
The "X" means I couldn't climb the street without pedaling.
Street 1 is steep for this engine.
Street 2 is a moderate incline.
"Flat" is just a flat street for testing max mph speed.
16" gave better low and high rpm power (evident by the higher speeds).
I used automotive hose to test with but found out the heat is too much for it after a while. You can test it that way but then weld in the same diameter metal pipe for a permanent setup.
Here's mine after welding in an extra 5" long section. It gave more low rpm power and 8% more top speed. If the pipe is upswept then it is easier to have a section longer than 5".
The next test was cutting an opening in the exhaust pipe right after it enters the muffler, and removing the baffles. (see first drawing below) The total added pipe length before the muffler that gave the best results was 13". I attached a can to the end of the muffler with twelve 5/32" dia holes in the end of it as a silencer. This caused a gain of 4mph more going up the steepest hill. This is definitely the way to go if you are climbing hills (in addition to the intake extension). And it didn't rob speed on the other two streets. If you have a muffler with an internal stinger pipe (see second drawing below) then you may not need a silencer. But then again it may not be as good climbing hills. I didn't test it.