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An exhaust pipe on a pocketbike works nothing like an exhaust pipe on a two stroke. If you thought they were similar and you knew everything then think again, because I'm about to burst your bubble. I can't get too lengthy here but I will explain a number of things which should answer most questions people have about pipes.

An exhaust pipe (pipe from now on) consists of a number of parts. For simplicity we'll just list them as the header, the expansion chamber and the stinger. The header is the part that comes off the motor, the expansion chamber is the barrel part of the pipe and the stinger is the butt end of the pipe (right under your butt).

Pipe length is determined by RPM, exhaust timing and speed of the sound in the exhaust system (which should stay pretty much the same regardless of RPM). Believe it or not, if you increase the RPM then the pipe has to get shorter! If you increase port timing the pipe has be get longer.

The speed of sound within the exhaust system depends on the exhaust gas temperature. The higher the temp, the longer the pipe length must be for a given rpm. When tuning a pipe the average (mean) operating temperature must be known, and also the RPM where you want your power band to be.

Each time the piston uncovers the exhaust port , the pulse of exhaust gases rushing out the port creates a positive pressure wave which radiates from the exhaust port. The sound will be be the same frequency as the engine is turning, that is, an engine turning at 24,000 rpms generates an exhaust sound at 24,000 rpms or 399 cycles a second--hence, an expansion chamber's total length is decided by the rpm the engine will reach, not displacement.

The exhaust opens on the down stroke and a pressure wave emanates from the exhaust port into the header pipe. This pressure wave travels through the exhaust gases that are in the pipe at the speed of sound. It’s the pressure wave that travels at this speed, not the exhaust gases themselves. The wave reaches the front divergent cone of the expansion chamber and a weak negative wave (negative pressure or ‘suck‘) is sent back to the exhaust port which reaches the exhaust port while the transfers are open helping to remove exhaust gases from the cylinder which in turn helps fresh mixture from the crankcase up through the transfers into the cylinder. ( some of which will enter the front part of the header)

So how can a pipe increase performance in your bike? Well the information above tells us that the pressure wave helps to bring fresh mixture into the cylinder. This push/pull action can actually allow a bike to have a larger charge than your cylinder size. A 47cc cylinder could have a 55-60cc charge of air/fuel or more!

How is this possible? Through the wonders of the tuned pipe and the function of compression, the cylinder can become "super charged" and give you a bigger bang over stock. This isn't cheating, this is why tuned pipes exist and why two stroke motors can build such superior power than a comparably size 4 stoke motor.

One item that I have left out on purpose is port timing, as this will be covered in a later article.

This article was written by Bruce Peresky for pocketbikeforum.com. Copying is permitted provided you give credit to http://www.pocketbikeforum.com

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