Chainsaw Pipe Exhaust theory

[Cyrille72FR]

A chainsaw "can box exhaust" cannot be tuned as well as a true 2 strokes exhaust.

But it still act a helmotz resonator : a volume and a tuned exhaust, if there is a tube in exhaust as in 261 (think of a bass-reflex loudspeaker).
Increasing the exhaust port surface will somewhen lead to no pressure build up in the can.
That's why, in my opinion, muffler mod with tubes does work.

I've seen a video where Hay Chaff & Sawdust shows that a specific diameter/length give the best power/torque on an 026.
I have two, so I figure I should measure the "tuning frequency" of this modded muffler.

 

[huskihl]

I went back and rewatched lutys "short expansion chamber", and another vid that he did on a similar idea, where he actually added a tiny expansion chamber to the box muffler.

Unless everything that I have ever read on expansion chambers is wrong, the reason that they are so long is because the timing of the negative and positive pressure waves is critical. So, I don't think that he is lying when he says power goes up, I just think that he is wrong about why power goes up.

Box mufflers rely on back pressure to keep charge from leaving the cylinder. If you are erring on the side of economy, that is going to leave exhaust gas in the cylinder, which will hinder combustion.

Opening up the exhaust (larger hole, more holes) increases the amount of time it takes to build up enough pressure to push back the exhaust flow, allowing more scavenging at the expense of fuel loss. That should (up to a point) increase power.

Adding the cone to the muffler increases the volume of the muffler, which should have the same effect as opening up the exhaust, i.e., taking more time to build sufficient back pressure.

In other words, I don't think that his modification could improve power beyond what you could do by enlarging the outlet or adding an additional outlet. And if the cone actually did reflect a positive pressure wave towards the port, that would be counter-productive because it would happen too early.

I believe most or all of what you said to be correct. I don’t know the exact length but the speed of sound at 12–13k RPM requires somewhere in the neighborhood of 24-28 inches of pipe before you get to the rear cone in order for the sound wave to stuff anything back in. Not saying that there aren’t performance gains by doing this and that inside, but like you said, I think there are similar or better performance gains by adding outlet area to the original muffler. These guys will spend countless hours fabbing up dog dicks of one sort or another sticking out of the side of a homemade muffler or measuring exactly how far they think they need to stick it inside the muffler, when they would be much better off taking that spent time on R&D grinding inside the cylinder

 

[edisto]

That's why, in my opinion, muffler mod with tubes does work.

A tube from the port in the could generate the negative wave for scavenging, which could be balanced by the development of backpressure in the can. But even getting that length in a reasonable space could be problematic.

 

[Cyrille72FR]

hello,

That's not what I meant, I mean when we implement a tube in a can exhaust, it behave as a bass-reflex : the tube is a band-pass.

I measured my MS261 exhaust which has a tube, it low-pass around 250Hz.
But I should redo the measurement with a "cleaner" protocol.

 

[edisto]

hello,

That's not what I meant, I mean when we implement a tube in a can exhaust, it behave as a bass-reflex : the tube is a band-pass.

I measured my MS261 exhaust which has a tube, it low-pass around 250Hz.
But I should redo the measurement with a "cleaner" protocol.

I understand how a Helmholtz resonator exhaust can produce cancelling sound waves to make the exhaust quieter, but can it play any role in scavenging?

 

[Cyrille72FR]

I understand how a Helmholtz resonator exhaust can produce cancelling sound waves to make the exhaust quieter,

It doesn't create wave, it just low-pass what the noise combustion and exhaust generate.

but can it play any role in scavenging?

I guess not around it's tuning frequency, where it's free flow, but upper band is a resistive load for the exhaust gas.
But it's seems to be really high in frequency to be effective... though I should consider that exhaust gas doesn't flow at speed of sound but higher.
I found 1100m/s for exhaust gas, and around 500m/s in exhaust port from 2 strokes for motorcycle competition.

If I look at the date for a 50cm3 cylinder exhaust tuned at 12500rpm, we "should" have a 20cm tube exhaust before the expansion chamber, which would be 40cm long and around 75mm wide at max.

 

[Fabulous]

I believe most or all of what you said to be correct. I don’t know the exact length but the speed of sound at 12–13k RPM requires somewhere in the neighborhood of 24-28 inches of pipe before you get to the rear cone in order for the sound wave to stuff anything back in. Not saying that there aren’t performance gains by doing this and that inside, but like you said, I think there are similar or better performance gains by adding outlet area to the original muffler. These guys will spend countless hours fabbing up dog dicks of one sort or another sticking out of the side of a homemade muffler or measuring exactly how far they think they need to stick it inside the muffler, when they would be much better off taking that spent time on R&D grinding inside the cylinder

The speed of sound is constant regardless of rpm … temperature is “the” factor here - these “dog dicks” you refer to can and will outperform any stock can muffler. It’s physics … You take a 562xp and just open the stock port up and viola! instant power increase - take the same saw and bolt on a “dog dick” that’s been thought thru , designed and tested and forget about it - the can will be down by a noticeable amount. Note : this isn’t considering the NOISE factor

 

[edisto]

The speed of sound is constant regardless of rpm … temperature is “the” factor here - these “dog dicks” you refer to can and will outperform any stock can muffler.

RPM and temperature are directly related.

No-one said that there isn't an improvement. Just that the improvement is a matter of increased volume slowing the build up of back-pressure, which helps with scavenging. Nothing to do with sonic waves.

Opening up the muffler would produce the same effect.

My thought is that increasing volume, instead of increasing the opening of the outlet, might make the pressure increase more peaky, which may be advantageous, but would be difficult to time properly.

 

[huskihl]

The speed of sound is constant regardless of rpm … temperature is “the” factor here - these “dog dicks” you refer to can and will outperform any stock can muffler. It’s physics … You take a 562xp and just open the stock port up and viola! instant power increase - take the same saw and bolt on a “dog dick” that’s been thought thru , designed and tested and forget about it - the can will be down by a noticeable amount. Note : this isn’t considering the NOISE factor

….. says the guy who’s been trying to make a living selling dog dicks for 5 years…

It’s all been tested, it’s all been tried. The tiny bit of gain in horsepower is negated by the volume and torque lost compared to the original can

 

[huskihl]

The speed of sound is constant regardless of rpm

Never said it wasn’t. But you need a longer pipe for maximum performance at targeted lower RPMs than you do for maximum performance at higher RPMs due to the speed of the engine

 

[Ketchup]

Not saying that there aren’t performance gains by doing this and that inside, but … I think there are similar or better performance gains by adding outlet area to the original muffler.

… they would be much better off taking that spent time on R&D grinding inside the cylinder

Amen.

 

[Outback]

So this is the cutting edge of performance? Your saying the front tube length vs the expansion chamber diameter dicktaintes the rpm you can lay the pipe? Does it or does it not require the other saw to have a bark box? I used my super secret AI to generate this diagram. Also I might have to make this for one of my saws.

 

[Fabulous]

RPM and temperature are directly related.

No-one said that there isn't an improvement. Just that the improvement is a matter of increased volume slowing the build up of back-pressure, which helps with scavenging. Nothing to do with sonic waves.

Opening up the muffler would produce the same effect.

My thought is that increasing volume, instead of increasing the opening of the outlet, might make the pressure increase more peaky, which may be advantageous, but would be difficult to time properly.

If it can’t get out it can’t get in simple as that - remember : it’s called a muffler for a reason - not a performance field tested pipe , the hotter the pipe gets the faster the molecules move - physics 101. A piped chainsaw top end can be made to run hotter at a lower rpm by pivoting on the dogs and pulling up on the pistol grip vs a higher rpm build cutting at 2000rpm more but running a can muffler limiting its torque.

 

[Fabulous]

….. says the guy who’s been trying to make a living selling dog dicks for 5 years…

It’s all been tested, it’s all been tried. The tiny bit of gain in horsepower is negated by the volume and torque lost compared to the original can

Yah , there’s a redneck out there with a quickly growing utube channel , hat chaff and sawdust is the name , that just might disagree with you Huskihl … in fact he’s got a Dyno and his results speak for themselves. Saying it’s all been tested, all been tried is akin to saying you’ve solved the game of chess

 

[huskihl]

Yah , there’s a redneck out there with a quickly growing utube channel , hat chaff and sawdust is the name , that just might disagree with you Huskihl … in fact he’s got a Dyno and his results speak for themselves. Saying it’s all been tested, all been tried is akin to saying you’ve solved the game of chess

He will probably threaten to sue me as well

 

[edisto]

If it can’t get out it can’t get in simple as that - remember : it’s called a muffler for a reason - not a performance field tested pipe , the hotter the pipe gets the faster the molecules move - physics 101. A piped chainsaw top end can be made to run hotter at a lower rpm by pivoting on the dogs and pulling up on the pistol grip vs a higher rpm build cutting at 2000rpm more but running a can muffler limiting its torque.

Why quote my post if you aren't going to address anything that I said?

Or did you simply not understand anything that I said?

A tuned pipe offers amazing increases in performance because it helps scavenging with a negative pulse, and then supercharges the cylinder with a positive pulse.

The downside is that in order to do so, they have to be long enough that they are not practical. Physics 101. Look at a picture of one.

The "dog dicks" that you seem to be defending are not tuned pipes, because they don't work with timed impulses. A box muffler keeps unburned fuel from being exhausted by building up back-pressure. That restricts the efficacy of scavenging. Opening up a muffler, or increasing the volume of a muffler will delay the buildup of back-pressure, or reduce the amount of back-pressure, which improves scavenging. Physics 101.

That will increase power, but nowhere near what an actual tuned pipe could do, because it is not long enough to have the reflected positive impulse arrive at the right time to push charge back into the cylinder.

 

[huskihl]

Why quote my post if you aren't going to address anything that I said?

Or did you simply not understand anything that I said?

A tuned pipe offers amazing increases in performance because it helps scavenging with a negative pulse, and then supercharges the cylinder with a positive pulse.

The downside is that in order to do so, they have to be long enough that they are not practical. Physics 101. Look at a picture of one.

The "dog dicks" that you seem to be defending are not tuned pipes, because they don't work with timed impulses. A box muffler keeps unburned fuel from being exhausted by building up back-pressure. That restricts the efficacy of scavenging. Opening up a muffler, or increasing the volume of a muffler will delay the buildup of back-pressure, or reduce the amount of back-pressure, which improves scavenging. Physics 101.

That will increase power, but nowhere near what an actual tuned pipe could do, because it is not long enough to have the reflected positive impulse arrive at the right time to push charge back into the cylinder.

This guy gets it^

 

[Fabulous]

Why quote my post if you aren't going to address anything that I said?

Or did you simply not understand anything that I said?

A tuned pipe offers amazing increases in performance because it helps scavenging with a negative pulse, and then supercharges the cylinder with a positive pulse.

The downside is that in order to do so, they have to be long enough that they are not practical. Physics 101. Look at a picture of one.

The "dog dicks" that you seem to be defending are not tuned pipes, because they don't work with timed impulses. A box muffler keeps unburned fuel from being exhausted by building up back-pressure. That restricts the efficacy of scavenging. Opening up a muffler, or increasing the volume of a muffler will delay the buildup of back-pressure, or reduce the amount of back-pressure, which improves scavenging. Physics 101.

That will increase power, but nowhere near what an actual tuned pipe could do, because it is not long enough to have the reflected positive impulse arrive at the right time to push charge back into the cylinder.

You are correct that a dirt-bike type expansion chamber will produce the largest gains but just so you know …. Back in my day guys would “tune” their 2-strokes by attaching a long straight-pipe to the cylinder and adjust the length to where the engine ran best! In fact, Dyno Joe did a video showcasing this concept.

 

[Ketchup]

General theory question that might apply to “dog dick” style pipes:

If the ideal distance of an expansion pipe is 40cm, can a 20cm pipe utilize the sound wave on its secondary reflected pulse? What about 10cm?

Related question:

What negative effects does the sound wave have if it hits the exhaust port too early in the cycle?

 

[edisto]

but just so you know …. Back in my day guys would “tune” their 2-strokes by attaching a long straight-pipe to the cylinder and adjust the length to where the engine ran best! In fact, Dyno Joe did a video showcasing this concept.

Just so you know...the long pipe is the first step in an expansion chamber. The pulse leaving the opening reflects a negative pressure wave that helps with scavenging. A straight pipe works at only a narrow range of rpms. A cone reduces the strength of the pulse, but spreads it over a wider range of rpms.

Having seen the length of the pipe required, it should be obvious to you that the "dog dicks" are not tuned pipes.