Muffler Mod Theory

[motomedik]

I have a theory that when modding a muffler, a tube should be welded or brazed in as a outlet, sized correctly, (length and I.D.) to promote good flow and cylinder scavenging. I see that some builders use a simple screened hole as an outlet, and it works fine. Is there a formula for the calculation of length and diameter of the outlet? Or is there no reason to bother? (Am I wasting my time welding and grinding, for no significant gain?)

 

[firebox40dash5]

I don't think length of the outlet has as much bearing as diameter. As stated in pretty much all the muffler mod threads, you want your outlet to be ~85% of the exhaust port area at the cylinder wall. Of course the smaller you go with your outlet, the more restriction increases with length, but most of the time the outlet pipe is pretty short anyway.

 

[Griner Bros.]

Also the tube should protude at least 1/4"-1/2" into the muffler ... NOT Flush ..

 

[motomedik]

My understanding of some exhaust designs is that a tube can, by using the exhaust's inertia, help to scavenge the cylinder and promote good filling of the next charge. The system must be designed with this principal in mind, and the tube sized, so that the air mass has enough velocity to accomplish this. This may mean that a smaller and/or longer tube has better flow. Or maybe this idea has no relevance here.

 

[Sawdustmaker]

I don't think length of the outlet has as much bearing as diameter. As stated in pretty much all the muffler mod threads, you want your outlet to be ~85% of the exhaust port area at the cylinder wall. Of course the smaller you go with your outlet, the more restriction increases with length, but most of the time the outlet pipe is pretty short anyway.

85%? i have seen that number a good bit but have also seen 125% Is 85 for stoc and 125 for ported?

I saw a 460 build where the builder started at 85% and went up to 125% and there were gains all the way through.

 

[motomedik]

We are primarily trying to tune for WOT, mostly, right?

 

[tdi-rick]

On a tuned expansion chamber type system, the stinger is critical in diameter and length for correct bleed off of system pressure, but I'm unsure if it has any significance on a small muffler type system, unless the outlet is oversized in diameter, then it might counteract that effect.

As firebox said, on a basic muffler most testing points to 85-100% of cylinder port area opening as being optimal, and after 85% it starts becoming a diminishing return, with an increase in fuel usage that apparently becomes significant over the 100% mark (for a work saw).

I'm actually installing a very small megaphone inside the muffler of one of my saws, and I've been tossing up whether to use a type of stinger or just a hole.
Unfortunately there is no data or testing on this sort of thing available to calculate something out, (that I'm aware of) so it's a suck it and see operation.

 

[rngrchad]

I can not attest to any gains in power on tube lengths but I will say: The length of the outlit "tube" will have a noticeable affect on how the muffler will sound. I've always felt that you should run as long an exhaust tube as possible, as it will make the saw sound much better. Just be sure to create the exhaust outlit in a way to avoid the operators face and hands.

 

[motomedik]

I was under the impression that the 85% was measured at the exit of the cylinder, so that would change my calculations somewhat, measuring at the cylinder wall.

 

[tdi-rick]

If you want to design an expansion chamber system, this program is good and it shows the calculations for everything, including the stinger.

http://www.users.bigpond.net.au/arrowkarts/private/Pipe_dsn.zip

 

[motomedik]

I can not attest to any gains in power on tube lengths but I will say: The length of the outlit "tube" will have a noticeable affect on how the muffler will sound. I've always felt that you should run as long an exhaust tube as possible, as it will make the saw sound much better. Just be sure to create the exhaust outlit in a way to avoid the operators face and hands.

Do you feel that the tube should or can be INSIDE the muffler, or does it have to be outside? I agree that the sound changes. My Huskie 61 sounds awesome and the gains were significant. With almost the same dimensions and design, my Dolmar 7300 is loud (obnoxious) and power gain was less than expected.

 

[tdi-rick]

I was under the impression that the 85% was measured at the exit of the cylinder, so that would change my calculations somewhat, measuring at the cylinder wall.

yep, measured at the piston face, not the port exit face.
Using the MOTA program it doesn't like the expansion in area from the piston to muffler face on stock saws, so I'm guessing that rate of taper is far from ideal too.

When I have built megaphones for 4 strokes merge collectors in the past, my rule of thumb was never to exceed a 15* divergence angle to eliminate flow separation and therefore turbulence. This seemed to be the accepted maximum angle of divergence.
I'm not sure if this holds true in all situations, but it served me well.

 

[Bowtie]

Also the tube should protude at least 1/4"-1/2" into the muffler ... NOT Flush ..

Oh really, explain that...opcorn:

 

[tdi-rick]

Do you feel that the tube should or can be INSIDE the muffler, or does it have to be outside? I agree that the sound changes. My Huskie 61 sounds awesome and the gains were significant. With almost the same dimensions and design, my Dolmar 7300 is loud (obnoxious) and power gain was less than expected.

On kart systems where the primary expansion chamber was also the muffler, the stinger was separate to the final converged cone and definitely entered the muffler area to operate properly.

 

[edisto]

My understanding of some exhaust designs is that a tube can, by using the exhaust's inertia, help to scavenge the cylinder and promote good filling of the next charge.

That doesn't work unless the tube leads directly out of the exhaust port. The sonic pulse from the exhaust will reflect back a negative pressure wave when it reaches an open end in the tube. That negative pressure is what helps scavenging.

On a tuned expansion chamber type system, the stinger is critical in diameter and length for correct bleed off of system pressure, but I'm unsure if it has any significance on a small muffler type system, unless the outlet is oversized in diameter, then it might counteract that effect.

The restriction of the stinger produces backpressure that helps the positive pressure wave reflected back from the constriction in the expansion chamber to force charge that followed the exhaust back into the cylinder.

I'm actually installing a very small megaphone inside the muffler of one of my saws, and I've been tossing up whether to use a type of stinger or just a hole.
Unfortunately there is no data or testing on this sort of thing available to calculate something out, (that I'm aware of) so it's a suck it and see operation.

The megaphone will do more harm than good unless it is preceded by an inlet pipe 6 or more exhaust port diameters in length, and slightly larger than the diameter of the port. If it is too close, it reflects the wave back too soon (while pressure still is high in the port).

 

[Dan_IN_MN]

If you want to design an expansion chamber system, this program is good and it shows the calculations for everything, including the stinger.

http://www.users.bigpond.net.au/arrowkarts/private/Pipe_dsn.zip

tdi-rick

Thanks for posting the link to the expansion chamber design. I've been looking for something like that for a while. I just downloaded it.

Dan

 

[eyolf]

My understanding of some exhaust designs is that a tube can, by using the exhaust's inertia, help to scavenge the cylinder and promote good filling of the next charge. The system must be designed with this principal in mind, and the tube sized, so that the air mass has enough velocity to accomplish this. This may mean that a smaller and/or longer tube has better flow. Or maybe this idea has no relevance here.

There have been a few discussions around 2 stroke theory right here at arbo-site, and there are some pretty good websites that even have video illustrations and expansion-chamber design calculators. But compare the volume of an expansion chamber with the box on your chainsaw. IMO, there are not much performance improvements to be had once most of the backpressure is relieved.

Pushing the tube into the muffler can would have a slight effect of raising backpressure because the outlet would be somewhat insulated from resonant waves within the can...sound waves bouncing around inside would slightly hinder the migration of gasses out. But only very slightly, and probably not at a useful (or wide enough) RPM range.

But they sure look cool. Where I live nobody is checking, but relatives on the west coast say that forest service people and even wildlife officials can boot you out of the woods for having a modified muffler on a saw. Probably fire danger, but I can see where a butcher job could cause trouble.

 

[edisto]

Also the tube should protude at least 1/4"-1/2" into the muffler ... NOT Flush ..

In an expansion chamber, it does not matter whether the outlet intrudes into the expansion chamber or not (unless it is moved past a critical point in the baffle) in terms of power, but it does make the chamber quieter.

 

[Griner Bros.]

Remember .. the 'can' muffler is at best a BAD design for a 2cycle trying to produce high power on a given power ban ...
It's a total compromise, made to fit the space given on a chainsaw ... reduce sound ... and produce some back pressure ...
Basics to mod a 'can'
1. %'s of 80%-125% of port size ok on stock saws ...
2. 100%-150%+ of port size on modded saw with port work ...
3. Don't flush mount the tubes to the can ... insert the tube 1/2" or more ..
4. Try to mount the tubes as far from the port as possable ...
5. Try not to open new ports directly in front of the cyl.exhaust
port ... without some type of a deflector between them ........... (Dolmar's have deflectors)

All saws respond differently to muffler mods .... most all produce more power .. cooler head temps. ... & A LOT MORE NOISE ...

 

[tdi-rick]

<snip>

The restriction of the stinger produces backpressure that helps the positive pressure wave reflected back from the constriction in the expansion chamber to force charge that followed the exhaust back into the cylinder.

It's basically a controlled bleed.

The megaphone will do more harm than good unless it is preceded by an inlet pipe 6 or more exhaust port diameters in length, and slightly larger than the diameter of the port. If it is too close, it reflects the wave back too soon (while pressure still is high in the port).

Any references ?
Don't recall the six times port diameters, but it makes sense.
I realise that if it's too close the reflected wave will be too soon, what a meg does though is artificially 'increase' the length of pipe, at least according to Smith and Morrison, who's work is the basis of all exhaust tuners. (Scientific Design of Exhaust and Intake Systems)

Timber Wolf had built a saw where he had a cone go right through the middle of the muffler can, so it was pretty short and it worked well from what I saw on the web.
I'll dig up a pic.