 Re: Modded Saws
Back pressure is needed in 2-stroke engines to keep them running better. Unlike a 4-stroke where you want to get rid of all the burned gasses in the combustion chamber before a fresh intake stroke of more gas and air mixed, the 2-stroke combustion is pushing the exhaust out at the same time it is pumping gas/air mix into the chamber for the next stroke. If you allow too much air flow, you will get raw gas/air pumping out the exhaust port before the piston closes the port and burns it. You do not want that to happen. So you need a nice ballance of back pressure from the exhaust system to keep the gas/air mix inside the combustion chamber.
The way to do that with an expansion chamber in a hot saw setup is to have an open pipe that is shaped right. What happens is the hot exhaust gasses escape from the piston at high pressure. This creates an exhaust pressure wave, which expands in the exhaust pipe until they hit the far side of the pipe where it narrows down in size. Then the pressure wave bounces back toward the exhaust port of the saw, and that in turn pushes the escaping unburned air/gas mix back in through the exhaust port before it closes.
I used to race 2-stroke dirt bikes when I was a lot younger. Some exhaust flow is good, but a completely open exhaust in a 2-stoke engine is not. I think that is a lot of cases people open up their chainsaw mufflers too much when they modify them. I opened up my 290 to only about 2.5x the size of the factory openings on the muffler (which are really small). Not a whole lot louder, but enough to get a lot more air flow, without too much air flow.
__________________ So few saws, so many trees... Stihls: 210/250-/025/290/361/FS85
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