chainsaw exaust

[Ted Jenkins]

DRF has described quite well the whole system of tuning exhaust for a two stroke. Yes the muffler box is just that a box that baffles sound. More flow will be less restrictive thus an increase in power. The expansion chamber allows burning exhaust to escape exhaust port and get to the larger part of the chamber and some what explode pushing some of the fresh fueled gases back into the combustion chamber. This process is like supercharging for two strokes. In general a narrow expansion chamber will perform a more narrow power band with a fatter chamber will be a wider band with some limitations. Thanks

 

[Yotaismygame]

Here's a few

 

[MacAttack]

Here's a few

Do you have a ratio of exhaust opening to exhaust port % that you follow?

I need to finish a homemade exhaust, i was going to go about 80% of exhaust port but I've read some folks sticking to about 5/8.

It's for an old large displacement piston port saw.

 

[ericm979]

Even the chain saw muffler box is tuned. It's just not very effective. In the 90s when I was into this stuff there was software to model the exhaust tuning of chainsaw type mufflers (and of course motorcycle individual expansion chambers and snowmobile/watercraft multi cylinder exhausts)

The chainsaw muffler is a small box because the saw needs to be light and small. Saw engines make up for not having much of a tuning effect using displacement. Saw engines have a low hp/cc ratio compared to other two strokes like motorcycles. A 250cc MX bike is about .18 HP/CC where a MS460 is .08HP/CC. A 125cc road race bike would be around .36.

Motorcycle style expansion chambers do not have to result in a narrow powerband. They often do because the temptation to make a lot of power is great, and it's a lot easier for the tuner to make a lot of power if they forget about a broad powerband. And of course it's application dependent. For some engines like the 125 road racer "broad" powerband means from 9,000 to 12,000. A saw's got to make enough mid range power to keep pulling when the operator lets the load slow the engine.

The expansion chamber makes power using the sound waves from the exhaust port. The opening cone reflects a negative (suction) wave back to the port. The closing cone reflects a positive wave. The head tube between the port and the opening cone, and the belly section between the cones, determine when the waves hit the port. When the engine is in the rpm range the pipe is tuned for, the waves align with the port so that the negative wave sucks out exhaust and air/fuel mix, and then right as the port closes the positive wave stuffs the fuel/air mix back in. So there's more fuel/air mix than would have normally gotten in there. It's free power from sound! I've greatly simplified and left out a lot of details but that's the summary.

Further reading:
A. Graham Bell's Two Stroke Tuning book
Gordon Blair's two stroke tuning book
Gordon Jennings also had a book

All this stuff is old now but it's advanced compared to production saws, and they cover the basics well.

 

[BGE541]

A few

 

[Huskybill]

I just seen a new saw on amazon that had triple exhaust pipes out the top of the muffler facing the front. Looks like the Chinese are making some performance upfront changes.

 

[huskihl]