Chainsaw Porting Theory

[Al Smith]

Now this business of either machine marks on the piston face or smooth might need some clarification .Now of days on automotive stuff it has the machine marks but they weren't always that way .If you look at older designs it was smooth .Certain ones for example hemi domed where smooth .Some like Jahns pistons from years ago shined like a mirror .
With chainsaw engines it went the same way .

So evidently it must be by design because with modern methods on high speed CNC machinery they could come out with a mirror finish if they wanted them to .

In case anybody wonders I've installed and maintained machinery for one piston line at Honda and 4 or 5 at Ford .

 

[Deets066]

On the topic of mufflers I have a wag.

A muffler is similar to a ported speaker box.

It should be tuned to the max power of the motor. The rpm can be viewed as frequency.

We would want a muffler/ box with a resonant frequency of our target rpm or frequency. Let's say 10krpm or 10 kHz.

To find this we need the compliance VAS of the the speaker/ pulse generator/ piston port opening I don't know how to do that but. We know the smaller the box the higher the frequency.

Operating at less than resonant frequency limits or essentially closes the outlet. Good news is it will not be as loud.

The port. Let's say we use a 1/2" piece of emt. If the pipe is 2" long and only extends 1/4" out of the muffler we subtract the tube area from the muffler area. It tunes our box for a higher rpm

If we stick the tube 1 3/4 " out of the muffler the area is added to the box. Our resonance frequency will be lower. Because we made the box bigger.

Now when it comes to diameter of the tube. Larger is more efficient. Really no surprise. A 3/4" EMT will be more efficient. The reason it friction along the walls of air hitting itself at high speed.

Round is the best shape because it will have the most open area for a given perimeter.

So this is my guess hope I explained it well. I wish someone knew a bunch about box design.

David

My brother went to school for that kind of stuff. He used to be a fanatic about it. He would first find the resonate freq. of the car and then tune the box to the exact freq. it made for one bad ass stereo let me tell ya. I'm just not sure how he would apply that to chainsaws

 

[drf256]

The speed of sound is fast enough for this to occur, for sure.

I need to find the time to try some deflector cones inside the muffler. Hoping to have one for 026 and one for a 262 done by the CT GTG. We should have a chainsaw Dyno there thanks to @wcorey and @malk315. Would be very interesting to swap mufflers only and see what happens.

 

[smokey7]

This muffler talk is closer to home for me. I feel you can go too big, also think that it's a tuning device. I more or less sneak up on the type and size of the changes I make. This is great stuff guys, I have learned so much from all of the discussions we have had here. Thank you all

 

[XP_Slinger]

The speed of sound is fast enough for this to occur, for sure.

I need to find the time to try some deflector cones inside the muffler. Hoping to have one for 026 and one for a 262 done by the CT GTG. We should have a chainsaw Dyno there thanks to @wcorey and @malk315. Would be very interesting to swap mufflers only and see what happens.

Looking forward to seeing your results.

 

[XP_Slinger]

My brother went to school for that kind of stuff. He used to be a fanatic about it. He would first find the resonate freq. of the car and then tune the box to the exact freq. it made for one bad ass stereo let me tell ya. I'm just not sure how he would apply that to chainsaws

Sounds like you picked up a lot of info from your brothers profession. It very much applies to this subject, sound waves are sound waves. However the difficulty lies in finding the ideal resonance of a chainsaw muffler. I tend to think that these mufflers provide a pressure barrier that somewhat keeps the majority of the fresh charge from escaping. I'm sure there is a certain amount that goes out the muffler, but finding the optimum amount of back pressure while providing maximum engine performance is a dificult science. Especially for guys like me that don't have a dyno. "Feeling" a slight gain or loss in performance is a highly subjective method and can be influenced by state of mind.

 

[XP_Slinger]

So how about fluting the outside of the exhaust to increase surface area for cooling? If everyone has stopped laughing from the first time I brought it up I'd like to hear some thoughts. I keep thinking about it and I think it might be beneficial. Heat is the enemy of all engines and any means of reducing heat helps them out.

 

[czar800]

I agree there is a ton of great information in this thread! As for me and mufflers I've alway go larger than the outside edge where the muffler bolts to the exhaust port...

 

[smokey7]

Wow that would be a huge hole. I tend to think that a hole area around 60% 70% of the actual port size by the piston has worked very well in the mods I've done . I usually have a good balance of power noise and fuel economy. I've gone bigger and smaller at a trade off to one or both of my other criteria I look at.

 

[huskihl]

Each saw may be different. Stihl 026 and 066 at least run better the bigger you go. Maybe most stihls like it because of exhaust port width. A husky has a much wider port already.

 

[riverrat2]

Each saw may be different. Stihl 026 and 066 at least run better the bigger you go. Maybe most stihls like it because of exhaust port width. A husky has a much wider port already.

Bull P_ _ssy!!!!! be careful many times more ends up actually being less!!!!

 

[darkimpulse]

Bull P_ _ssy!!!!!

Um, ok?

 

[huskihl]

Bull P_ _ssy!!!!!

To which part? Timber Wolf had dyno results on an 026 showing the bigger the better. And deets, mmw, jms, and I have noticed it in an 066. Maybe porting has something to do with it. Not sure

 

[huskihl]

Bull P_ _ssy!!!!! be careful many times more ends up actually being less!!!!

Many times, yes. Not all times. Each saw is different

 

[Moparmyway]

Bull P_ _ssy!!!!! be careful many times more ends up actually being less!!!!

The tach dont lie
I see more RPM in the cut with 044's, 066's, and others when mufflers have huge dual openings like these

 

[czar800]

Wow that would be a huge hole. I tend to think that a hole area around 60% 70% of the actual port size by the piston has worked very well in the mods I've done . I usually have a good balance of power noise and fuel economy. I've gone bigger and smaller at a trade off to one or both of my other criteria I look at.

I've never been concerned about fuel economy or noise on a saw that I'm porting. I have a few stock saws if needed.
I know my methods are not very scientific You can see that in my 026 build thread. I have good gains and I'm happy at the end of the day.

 

[Mastermind]

The tach dont lie
I see more RPM in the cut with 044's, 066's, and others when mufflers have huge dual openings like these
View attachment 13830 View attachment 13832

Notice that these big outlets are on the sides of the can......

I think (and have no way to prove.....yet) that really matters. I'd rather have the front cover flat, and smooth than to have an opening straight out of it.

 

[smokey7]

Yes I think that helps alot with the flow hitting front of the can to break up sound and keep some of the fuel from escaping so easily. Sone of these muffler mods seem way too big to me, only because I keep picturing the size of the exit on a piped motor of much larger displacement. I know it's two different animals but it's always on my mind when I mod one.

 

[czar800]

Notice that these big outlets are on the sides of the can......

I think (and have no way to prove.....yet) that really matters. I'd rather have the front cover flat, and smooth than to have an opening straight out of it.

Ive had the same thoughts about opening up straight out the cover.

 

[Moparmyway]

Notice that these big outlets are on the sides of the can......

I think (and have no way to prove.....yet) that really matters. I'd rather have the front cover flat, and smooth than to have an opening straight out of it.

I dont like straight out of the cover ............. IE: the OEM dual port
I agree, side outlets run better