buttercup
Addicted to ArboristSite
I was initially curious about muffler mods and found several threads mostly on this site, and it's interesting to read about people's experiences but there have been very little calculated info to hold on to other than - "open it up a little".
The closest thing to calculation I did find was that the muffler port should be about 80% of the cylinder exhaust port, that is at least something to go on - however...
I dug up an article on making silenced performance enhancing mufflers (not tuned pipe) to high performance 2-stroke model engines, this article is about a famous and well proven design (in my country at least) that goes back several decades and is still in use with nothing to surpass it - except a tuned pipe of course.
The article contains a plan drawing for a 10ccm (.60ci) engine and a formula sheet for all dimensions based on engine displacement, these engines are operated at about the same rpm as a chainsaw. The muffler itself would be too big for a chainsaw and it should also ideally be adapted in length from the cylinder according to rpm which would also be difficult with a chainsaw, but the relations between engine displacement and muffler port is actually quite interesting.
(Metric)
The exhaust port formula is quite simple: 2 V(root) 2.03 x displacement = diameter
So, with a 72ccm displacement 2.03 x 72 = 146.16
Formula: 2 V(root) 146.16 = 24.179330016
Answer: a suitable total area for the muffler port(s) on a 72ccm 2-stroke engine is equal a circular area at about 24mm in diameter, that equals two circular areas at about 17,5mm.
Compared to my 381 kit saw the total muffler port area (2 ports muffler, 18mm circular + two vent slots) is about 400sq mm, it should be (according to the formula) at about 450sq mm.
(Edited)
The closest thing to calculation I did find was that the muffler port should be about 80% of the cylinder exhaust port, that is at least something to go on - however...
I dug up an article on making silenced performance enhancing mufflers (not tuned pipe) to high performance 2-stroke model engines, this article is about a famous and well proven design (in my country at least) that goes back several decades and is still in use with nothing to surpass it - except a tuned pipe of course.
The article contains a plan drawing for a 10ccm (.60ci) engine and a formula sheet for all dimensions based on engine displacement, these engines are operated at about the same rpm as a chainsaw. The muffler itself would be too big for a chainsaw and it should also ideally be adapted in length from the cylinder according to rpm which would also be difficult with a chainsaw, but the relations between engine displacement and muffler port is actually quite interesting.
(Metric)
The exhaust port formula is quite simple: 2 V(root) 2.03 x displacement = diameter
So, with a 72ccm displacement 2.03 x 72 = 146.16
Formula: 2 V(root) 146.16 = 24.179330016
Answer: a suitable total area for the muffler port(s) on a 72ccm 2-stroke engine is equal a circular area at about 24mm in diameter, that equals two circular areas at about 17,5mm.
Compared to my 381 kit saw the total muffler port area (2 ports muffler, 18mm circular + two vent slots) is about 400sq mm, it should be (according to the formula) at about 450sq mm.
(Edited)