Muffler Mod Theory

[Bowtie]

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 ...

so why dont you flush mount the tube???

 

[Oldsawnut]

I don't think it matters or the DP covers for the 066 and 044-046 would have tubes instead of holes in the cover... I think you just mainly need to let the exhaust out and quit overthinking it.. Unless you want to pipe it, then its not much of a work saw anyhow..

 

[motomedik]

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 ...[/QUOTE]

Good stuff! Been trying to figure what the deal was with the Dolmar. Just didn't seem 'right' and WAAAAY too loud.
Yes, I cut out the deflector. I knew I shouldn't have done it:bang::bang: COULDN'T HELP IT Seemed so RESTRICTIVE. What is it about the Dolies?? Why do they have it and Huskies don't!?? Something 'bout the porting? Same reason y'all say NOT to try to port them? At least I listened to THAT, so far...

 

[edisto]

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)

I'm going by what I've read in Jennings and Bell. As soon as the pulse reaches the end of a pipe or an expansion, the negative wave is reflected back. From what I understand, the cone doesn't "lengthen" the pipe, but prevents the loss of kinetic energy that a pulse loses hitting the open end of a pipe by being a more efficient inverter. This means the reflected negative wave retains much of its energy.

 

[tdi-rick]

I'm going by what I've read in Jennings and Bell. As soon as the pulse reaches the end of a pipe or an expansion, the negative wave is reflected back. From what I understand, the cone doesn't "lengthen" the pipe, but prevents the loss of kinetic energy that a pulse loses hitting the open end of a pipe by being a more efficient inverter. This means the reflected negative wave retains much of its energy.

I've only skimmed through Jennings a while back, haven't read any Bell in over twenty years, Bell was my kart engine builders bible.

Yes, absolutely correct.
From Smith and Morrisons work, they took pressure tappings and determined that the length and divergence angle changes the duration and amplitude of the scavenging pulse relative to a parallel pipe of the same length. Doubling the length of the parallel pipe yielded similar results to the optimum cone on the half length pipe on the test engine in terms of pressure curves. (a single cylinder four stroke) But a megaphone is superior as;

"A pressure wave entering the diffuser or megaphone is gradually reduced in amplitude with decrease in kinetic energy, This energy is however imparted to the waves resulting from the continuous partial reflection of the main pulse, and moves in opposition thereto, to the open large diameter end. Providing the megaphone is designed correctly, almost the whole of the energy content can be imparted to the returning train of reflected waves assisting in the scavenge, with only a small fraction of the original energy rejected to the atmosphere.
In addition, because of the continuous reflective action of the megaphone in comparison to the instantaneous action of a plain pip-end, the total period over which the scavenging waves arrive arrive at the exhaust valve is considerably prolonged."

I don't have the book (big $) but the latest and greatest treatise on performance two strokes is by Prof. Blair from Queens University Belfast, published by the SAE.
He was heavily involved in GP bike racing for the last few years of the Two Stroke era.
BTW, the MOTA software is based on Blair's work, and can supposedly be applied to any two stroke, including saws. Could save a lot of experimentation for someone that was serious about saw racing.

This could be interesting too, haven't read it yet, just downloading.
http://upetd.up.ac.za/thesis/submitted/etd-10312005-115545/unrestricted/01dissertation.pdf

 

[tdi-rick]

Of course that's all moot on a work saw, as there's no way anyone is going to walk into the bush with a proper pipe hanging off the saw.

I was just going to play and see if I could improve anything over the OE design while keeping it practical.

 

[husq2100]

i smell a WOJO...

although this has been a good thread

Serg

 

[motomedik]

Going back to the 7300 which now has no internal deflector/baffle- is it possible that acts as a sort of tuned outlet on the Dolmars? As well as silencer, of course. I feel as though upon removal, I lost more than the derestriction of the outlet gained me. I saved it, and might weld it back in, or try to build an internal Megaphone, if I'm feeling lucky. I've gathered, also, that I need to shorten and maybe change the angle slightly of my outlet tube. I think the opening is too much in front of the cylinder exit.

Thanks to all those that have shared their knowledge and made this such a great thread!

 

[tdi-rick]

Going back to the 7300 which now has no internal deflector/baffle- is it possible that acts as a sort of tuned outlet on the Dolmars? As well as silencer, of course. I feel as though upon removal, I lost more than the derestriction of the outlet gained me. I saved it, and might weld it back in, or try to build an internal Megaphone, if I'm feeling lucky. I've gathered, also, that I need to shorten and maybe change the angle slightly of my outlet tube. I think the opening is too much in front of the cylinder exit.

<snip>

Naa, if you measure the rectangular outlet of the basket it's slightly smaller than the cylinder port outlet, so some gasses are diverted through the perforations as the gasses expand.
You always get a reflected wave where a pipe ends, whether it does anything in this instance
IMO all it tends to do is break up the sound waves with minimal to slight back pressure, just diverting the bulk of the exhaust towards the bottom of the can.
Again, IMO you need some restriction to keep the inlet charge from screaming straight out the exhaust port. Those that have opened up the outlet on stock saws up to 150% of the cylinder opening area report very small gains in power over a 100% port opening but a big increase in fuel consumption, which makes sense.

 

[hh1341]

i smell a WOJO...

although this has been a good thread

Serg

What is a WOJO?

 

[edisto]

"In addition, because of the continuous reflective action of the megaphone in comparison to the instantaneous action of a plain pip-end, the total period over which the scavenging waves arrive arrive at the exhaust valve is considerably prolonged."

Jennings basically offer the summary without the data. Bottom line is a stronger pulse over a broader range (less peaky), and the same is true for the baffle cone on the other end of the chamber.

Thanks for the references...that sounds like a must have book, and I'm off now to check the thesis!

I'm also trying to figure out a way to get the scavenging of a megaphone without the loss of charge in an OEM-sized package. I have an idea I'd like to try, but like you said, it will be cut, weld, and hope, so I'll have to wait to give it a shot to make sure it doesn't melt any pistons.

Before I do that, I have to do the porting to get the right numbers...and before I do that, I have to get the cylinder...and before I do that, I have to get some money!

 

[weimedog]

i smell a WOJO...

although this has been a good thread

Serg

From doing a search, looks like uninformed has made this statement a few times on a few other technical threads...looks to have its roots in another member who must have done some less than stellar work and sold those services...maybe uninformed got stung?

 

[husq2100]

From doing a search, looks like uninformed has made this statement a few times on a few other technical threads...looks to have its roots in another member who must have done some less than stellar work and sold those services...maybe uninformed got stung?

nope, i have never had any business with S Wojo.

he was a member of this site once, before he was asked to leave....but he does like to come back under different alias's and stir the pot...MANY a time...

but like i said this is a good thread, som may not be him HAHAHA

Serg

 

[GASoline71]

Not to mention you can fork out 90 bucks for a muffler of his on ebay. That does no more than a stcok dual port will get you for about 25 bucks...

Gary

 

[scotclayshooter]

Timberwolf did tests on a 026/260 this is where 80-125% came from.

http://www.arboristsite.com/showthread.php?t=44816&highlight=spaghetti+method&page=5

Now im thinking if TW aint using pipes and just a hole in the front of the muffler, Pipes do nothing otherwise he would use them.

046s spaghetti method for measuring ports to judge your outlet size.
http://www.arboristsite.com/showthread.php?t=14863&page=2

 

[MR4WD]

Timberwolf did tests on a 026/260 this is where 80-125% came from.

http://www.arboristsite.com/showthread.php?t=44816&highlight=spaghetti+method&page=5

Now im thinking if TW aint using pipes and just a hole in the front of the muffler, Pipes do nothing otherwise he would use them.

046s spaghetti method for measuring ports to judge your outlet size.
http://www.arboristsite.com/showthread.php?t=14863&page=2

Don't know about that. Pretty sure if you wanted to get technical, you'd run an expansion chamber with a timed exhaust for the most power. I punched a 5/8 hole in my 385 husky muffler, then welded about a 2" long pipe to it. It was quieter, but and I'm fairly confident it made a bit more torque with the pipe than vs a straight hole.

I know your car probably isn't a 2 stroke, but pull the exhaust off at the cylinder head. See if it's got more, or less power. More than half the battle when building an exhaust on anything, is scavenging exhaust pulses to extract burnt fuel on the exhaust cyle.

 

[husq2100]

first of all Timberwolf really really knows his stuff...

second 2 stroke v 4 stroke is a completely different ball game...

Serg

 

[tdi-rick]

<snip>
Now im thinking if TW aint using pipes and just a hole in the front of the muffler, Pipes do nothing otherwise he would use them.

<snip>

Agreed, but I'm going to try one or two things, although I doubt there's much that would work in a stock sized muffler package, and a whole lot that can cock things up.
The cone exhaust TW put straight through the middle of the muffler on one of the 'build' saws looked good

 

[tdi-rick]

Don't know about that. Pretty sure if you wanted to get technical, you'd run an expansion chamber with a timed exhaust for the most power. <snip>

'course you would, and saw racers like TW do, often using programs like MOTA to get it in the ball park or better, but no one would run an expansion chamber/tuned pipe on a work saw, just not practical.

As SCS posted, TW did some tests on an 026 with various size openings and timed cuts to see what, if any changes happened. Well worth reading.

 

[husq2100]

Rick, do you any links to the TW cone muffler, pics or threads?

Serg