Chainsaw Pipe Exhaust theory

[paragonbuilder]

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.

It may have some consequences, but there is no way to tune a muffler can for a saw. The pulse would be way to fast as your math shows, if it is correct. The pulse needs to be right before exhaust closes and at the desired rpm or it is either hurting the saw or doing nothing.


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[MattG]

It may have some consequences, but there is no way to tune a muffler can for a saw. The pulse would be way to fast as your math shows, if it is correct. The pulse needs to be right before exhaust closes and at the desired rpm or it is either hurting the saw or doing nothing.


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I'm kinda just playing with the maths out of my own curiosity to be frank. But before I did so I didn't realise that the delay of the pulse (0.00044) was actually a reasonable fraction of the time period of a rotation at 10k.

 

[MattG]

The deal with the pulses around the opening is not hard and fast......essentially at varying revs we are comprising purging efficiency (which will prefer a negative pressure) over charge compression (which will prefer a positive pressure).

 

[bikemike]

Some aftermarket moped performance pipes are a good start to a pipe for a saw.
I have 1 on my 49cc echo and it makes great power and a broud range of power and chit tons of torque. I have no regrets from a pretty strong saw to a lot more saw than it was. I like it

 

[bikemike]

I didn't do much math vs fitment on 2 saws I piped. But I got luck and think the style of header tube and length of it plays a big role in how and where it makes its power and tourqe

 

[bikemike]

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?

moped pipe on the echo. Parts together a pipe for the Poulan

 

[Czed]

@GD350TURBO
here's some pipe info

 

[Lightning Performance]

View attachment 69412 moped pipe on the echo. Parts together a pipe for the Poulan

Do the math and see how those pipes stack up.

 

[Chainsaw Jim]

I'm curious how this pipe would compare to a pipe calculated and built specifically for this saw.

The smoke coming up is from a pinhole in the flange weld.

 

[Tomos770]

I didn't do much math vs fitment on 2 saws I piped. But I got luck and think the style of header tube and length of it plays a big role in how and where it makes its power and tourqe

 

[Tomos770]

Some aftermarket moped performance pipes are a good start to a pipe for a saw.
I have 1 on my 49cc echo and it makes great power and a broud range of power and chit tons of torque. I have no regrets from a pretty strong saw to a lot more saw than it was. I like it

Most Scooter pipes are built for 70ccm cylinder kits.....some are for 50ccm

 

[Tomos770]

Interesting idea...

 

[J.Noss]

I've been interested in 2-stroke expansion chambers and looking at this animated representation of how they function brought up a question:

That is- wouldn't the modern strato-carb saws have much less benefit from the effect of the expansion chamber pipe? Since those engines at a certain point in the rotation cut off fuel flow introducing only air, wouldn't that then eliminate or seriously reduce the power-gain from the pipe?

Its worth a thought anyways, perhaps the point the expansion chamber is sorta sucking the fuel through is different from what point the strato bit cuts fuel-mix but I'd think its quite possible that the strato would kinda interfere with optimal function..

 

[Al Smith]

As a suggestion you might be interested in essentually a self tuning pipe like KTM motorcycles use .It uses a spring loaded relief valve which in essence controls the amount of back pressure of the returning echo of sound waves .Google probably can find the info ..I worked on only one model of KTM a few years back and it was a crotch rocket .Something around 20-22 HP from 200 cc .As it was the relief valve was rusted shut and the pipe was only reponded at nearly wide open .Afterwards it responed at just about every RPM's and was like rocket ship

 

[Al Smith]

In addition to the above you'll find more information on RC sites than you'll ever find on chainsaw sites .RC people are good about giving out info,chainsaw tuners are not,just the way it is .

 

[92TRX300FW]

I've been interested in 2-stroke expansion chambers and looking at this animated representation of how they function brought up a question:

That is- wouldn't the modern strato-carb saws have much less benefit from the effect of the expansion chamber pipe? Since those engines at a certain point in the rotation cut off fuel flow introducing only air, wouldn't that then eliminate or seriously reduce the power-gain from the pipe?

Its worth a thought anyways, perhaps the point the expansion chamber is sorta sucking the fuel through is different from what point the strato bit cuts fuel-mix but I'd think its quite possible that the strato would kinda interfere with optimal function..
View attachment 460521

builders do not build piped race saw with the strato ports and there is almost no result by butting a pipe on a stock chainsaw

 

[edisto]

That is- wouldn't the modern strato-carb saws have much less benefit from the effect of the expansion chamber pipe? Since those engines at a certain point in the rotation cut off fuel flow introducing only air, wouldn't that then eliminate or seriously reduce the power-gain from the pipe?

My understanding is that the strato loads the "front" of the transfer with air so that you can time the engine to push out more exhaust without throwing fuel into the exhaust port. Better scavenging without wasting fuel.

The first wave with a pipe (created by the diameter of the pipe increasing) creates negative pressure that draws more charge through the cylinder into the exhaust, which would improve scavenging, but toss away fuel.

I think that the real advantage of the expansion chamber is the positive wave created by the restriction at the end of the chamber. This stuffs that fuel in the exhaust back into the cylinder. That compression of the charge allows more air and fuel in the cylinder; it is basically supercharging.

I don't see how a strato design would interfere with either process, but I could be missing something. I've never had a strato saw in my hands.

 

[edisto]

Interesting idea...

I have never built an expansion chamber (so take this with several grains of salt), but I have done a lot of research into how to do so.

The design in the video relies entirely on back pressure to minimize fuel loss. I don't see it being any different to "cut and try" in terms of common muffler mods for chainsaws. The reason that expansion chambers are so large is because it is the timing of the reflected waves that matter, and that depends on exhaust speed.

An opening of the exhaust into the chamber causes a reflected wave of negative pressure. That has to occur at the right time to pull more charge from the crankcase. The cone (as opposed to an abrupt opening) widens that timing window a little.

At the opposite end of the chamber, the restriction reflects a positive pressure wave. That's what stuffs the charge that was pulled into the exhaust back into the cylinder. If his cone was reflecting positive waves, they would occur way too early, which would be counterproductive.

The only way to make a shorter expansion chamber exhaust is to find a way to speed up the waves. Higher temps would do it, but not enough to make that much of a difference in length.