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.