Chainsaw Pipe Exhaust theory

[Brewz]

Hi Folks.

I have been doing a bit of thinking about piped saws and am wondering if some folks with a bit of knowledge on them and the theory's behind how they help make power could add some food for thought?

Also, are they generally custom made or can they be bought from anywhere for popular saw models?

 

[mdavlee]

Custom made. It stuffs exhaust back into the chamber to fill more efficiently than a ton of unburnt mix going right out the exhaust.

 

[Brewz]

Ok, so the tuning bit is setting the pulse reflection to better achieve this?

 

[paragonbuilder]

Ok, so the tuning bit is setting the pulse reflection to better achieve this?

Size angle and distance determine when the wave reflects back. Too early and you stuff exhaust back in and prevent fuel charge from entering. Too late and it sucks the fresh charge out with the exhaust.
But it's only perfect at a single rpm, hence the tuning to your chosen point.

 

[paragonbuilder]

Tony, this was a good read on pipes
http://www.dragonfly75.com/motorbike/ECtheory.html

 

[mdavlee]

Ok, so the tuning bit is setting the pulse reflection to better achieve this?

Yep. Just about 1k rpm or so is where it will run best.

 

[Dieselshawn]

Pass the popcorn

 

[Al Smith]

You will find more information on tuned pipes on British motorcycle sites or RC sites than you will on chainsaw sites .

 

[spencerpaving]

I've done a lot of pipes....I've also wasted a lot of metal.....it's all about testing to find what works....some people think a pipe saw turns crazy rpm ....better off building a pipe that makes decent power and torque then you can gear up and have be more forgiving so it doesn't fall off the power band and die in the cut. ..jmo

 

[Gun32gtr]

Tony, this was a good read on pipes
http://www.dragonfly75.com/motorbike/ECtheory.html

I just read most of that and it went straight over my head haha.. Brewz I hope we can discuss this in Lamonds term

 

[Brewz]

YEah I read it and figured I would learn other things first

 

[smoker]

FYI something I found and saved

 

[RD35]

They say a picture is worth a thousand words.....but an animated picture (above) says it all!!!! Best illustration I've ever seen to explain how sonic impulse reflection increases compression and fuel charge to make more power.

 

[Lightning Performance]

 

[Gun32gtr]

That is a great little image

 

[MattG]

Ok, so the tuning bit is setting the pulse reflection to better achieve this?

This makes me think that naively drilling holes at the front of a muffler cover, may well be a silly thing to do, since this will minimize the strength of any pulse reflected back from here into the combustion chamber.

Thoughts?

 

[Brewz]

I have moved away from front openings and try and do an opening on either side of the muffler for equal pressure drop in the can.
I doubt it makes a lot of difference as the pulse reflection will be far to fast to stuff unburnt fuel and air thats exited the exhaust back in.

 

[MattG]

I have moved away from front openings and try and do an opening on either side of the muffler for equal pressure drop in the can.
I doubt it makes a lot of difference as the pulse reflection will be far to fast to stuff unburnt fuel and air thats exited the exhaust back in.

Being a science nerd, I did some calculations.......

Assume 7.5cm from the muffler opening to the muffler cover. Times this by 2, gives you 0.15 in metres. Divide by the speed of sound in m/s (that being 343). This tells me that the reflected pulse will take

0.15 / 343 = 0.00044 seconds to reach the muffler opening, after rebounding from the muffler cover.

Then consider a 2 stroke revving at 10000 rpm, (or wherever you are for peak torque under load). Divide by 60, gives you 167 revs/second, invert that value, and you have 0.006 seconds for a single rotation.

Since (0.0004/0.006 * 360) = 24 degrees of rotation.

(therefore at 10K, the crank has rotated ~24 degrees in the time taken for the pulse to reflect in the above example)

Feel free to flame me (I guessed one or 2 figures and rounded up or down in places), but this says to me that the timing of a reflected pulse at saw operating saws probably has some consequence. I can go through the maths in more depth if required.

 

[Brewz]

Cool!

I think it's a non issue in a saw muffler but I love the math!

I have toyed with the idea of a 90* angled steel piece in the saw running top to bottom to deflect the "too fast" return pulse to the sides of the muffler where the outlets are and stop it messing with the outflow from the exhaust port.

Prolly complete waste of time, but my mind thinks these things over without my permission

 

[MattG]

Cool!

I think it's a non issue in a saw muffler but I love the math!

I have toyed with the idea of a 90* angled steel piece in the saw running top to bottom to deflect the "too fast" return pulse to the sides of the muffler where the outlets are and stop it messing with the outflow from the exhaust port.

Prolly complete waste of time, but my mind thinks these things over without my permission

Hows about moving the muffler cover out a few centimeters, i.e. by making the can stick out more at the front? It wouldn't take much to make that 0.00044 of a second into 0.0006 second, which would equate to a more significant angle in the 10Krpm revving saw.

I'm not going to, cos I can't weld or fabricate, and I don't professionally tune saws. But tweaking pulse timings is exactly what bike and cart tuners do,

i.e. in #12 I believe the significant length is that parallel section mid-pipe. Whereas the tapered (out and in) sections widen the range of crank durations in which the pulse is influential (but in doing so, reduce the power of the pulse).

PS glad you liked my math!