Helmholtz Resonance and Expansion Chambers
That is WAYYY cool!
What does the large exhaust pipe do? Does it have really low back pressure but still muffle sound?
Proper length headers on a 4 stroke, expansion pipes on a 2 stroke, and intake runner length on either engine can greatly improve the flow of incoming air and exhaust gases through the engine. In a 2 stroke as the piston covers the transfer port and the intake pulse of air is stopped, there is a pressure wave that travels back to the venturi and then bounces back towards the crankcase. This pressure wave travels at a fixed speed based on air pressure and temperature, close to the speed of sound. If you time those pressure waves bouncing back and forth to be traving towards the engine as the piston uncovers the transfer port by adjusting the length of the runner(or when the valve opens on a 4 stroke) you get a surge of pressure into the crankcase large that the vaccum would provide on its own. This helps the engine take a larger breath of air during the intake cycle.
On the exaust side it is easy for a 4 stroke, the runner length from the head to the collector for a header can be calculated. Again this can take advantage of the pressure wave by having the correct length exhaust runner you can have the pressure wave moving away from the engine as the exhaust valve opens, scavenging more exhaust out of the cylinder. On a 2 stroke it is more complex due to the overlap of the intake and exaust port opening times. That is why the expansion pipe is used. The length of the pipe to the expansion chamber, the angles of the chamber inlet and outlet, and the length of the chamber all have different affects on the power band of a 2 stroke. Correctly designed the pipe will help pull the exhaust away from the engine as the port is opened and then the pressure wave will bounce back to stop the flow of fresh intake gases from going out with the exhast to maximize the fresh mix in the cylinder.
Using this type of tuning theory can greatly increase the power of and engine, if done properly I belive you can exceed 100% volumetric efficiency in a very narrow rpm range. It can also make an engine that has a VERY narrow powerband. But done correctly it can increase horsepower and adjust the torque band of and engine.
There are formulas to calculate runner size and length for the intake as well as expansion chamber dimensions available online and I believe in the 2-stroker tuners handbook. I bet some of the guys in the Hotsaw forum are up to speed on the designs and can tell you how narrow the power band can be along with what kind of peak power they can produce in that narrow range.
Why not use an expansion pipe on an everyday saw? Size, weight, and narrowing of the power band would all be concerns.
If you want to see some saws with big pipes and v-8 engines you should head on out to Deming, Washington for the Deming Log Show this weekend. You will find some cool saws there!
opcorn: 5 miles from my front door.
http://www.demingloggingshow.com/:greenchainsaw: opcorn:
