Chainsaw Porting Theory

[CR888]

The term 'port and polish' probably came out of a 70's performance bike magazine and is a term still commonly used today by people modifying anything with a motor. A mirror finish can hamper performance in transfers and intakes however but if there was any merit to it, probably the exhaust port may have less carbon build up being smooth. God knows how much people have spent polishing ports to mirror finishes loosing hp along the way. Nice info from Mr David Young, good post!

 

[Mastermind]

I leave the transfers and intake with an 80 grit finish.......the exhaust at about 220. Nothing too shiny in my saws.

 

[Canadian farm boy]

I was alway told that you want the intake track on anything that is carbureted to have a slightly textured finish. The textured finish make a bit of turbulence and allows for the intake air to better carry the fuel. I believe this applies to throttle body injection as well. A textured surface can also help prevent fuel from puddling due to the air having a slight turbulence through to the CC.

If an engine is direct injected into the head or CC you can polish the entire intake track.

This is just my $.02. Some people may disagree

 

[Deets066]

When I finish porting I like to hit it with glass bead blaster, then I only smooth up exhaust after

 

[David Young]

I was alway told that you want the intake track on anything that is carbureted to have a slightly textured finish. The textured finish make a bit of turbulence and allows for the intake air to better carry the fuel. I believe this applies to throttle body injection as well. A textured surface can also help prevent fuel from puddling due to the air having a slight turbulence through to the CC.

If an engine is direct injected into the head or CC you can polish the entire intake track.

This is just my $.02. Some people may disagree

boundary layer like on a golf ball

same applies to the piston top.

ever see a polished factory piston top?


The all have machine marks. its to protect the piston crown.

 

[Canadian farm boy]

After the point at which the fuel and air are mixed is where you want the textured finish.

I've never seen a polished piston crown or combustion chamber. Turbulent air creates a boundary layer next to the surface and allows the intake air charge to hold the fuel better if that makes sense. Kinda hard to describe.

I've see lots of polished intakes but only on fuel injected engines with direct injection

 

[Stihlbro]

I can stare at this for hours, and find something different each time. Ponder looking for improvements.

 

[XP_Slinger]

boundary layer like on a golf ball

same applies to the piston top.

ever see a polished factory piston top?


The all have machine marks. its to protect the piston crown.

Yup, surface turbulence is very important to keeping the fuel atomized. If everything was "polished" as the popular nomenclature leads one to believe fuel would fall out of suspension and "stick" to polished surfaces where they cause changes in velocity. Similar to fog on the windshield of your vehicle. That's why, in my experimental learning curve on my 372, I didn't polish anything. I didn't leave things with big ugly grinding marks, but I did achieve a very nice, uniform, but slightly textured finish with a single cut carbide burr. Is it the best method? Maybe not. But hey, a fellas gotta learn some how...right?
I'm glad this subject came up. For a minute I thought this thread was dead. Keep it going!

 

[XP_Slinger]

View attachment 13669

I can stare at this for hours, and find something different each time. Ponder looking for improvements.

That is frigin awesome! And what kind of muffler is that?!?!?

 

[Mastermind]

Good threads never die.........they just fade away.

 

[Simondo]

That is frigin awesome! And what kind of muffler is that?!?!?

I didn't know they did a "CAT MUFFLER" on a 242 xp !! The green fuel cap model wasn't a Euro market saw i guess ?

 

[XP_Slinger]

The muffler on my green cap 346 NE doesn't look anything like that. It just has a very restrictive baffle in it with what looks to be some sort of ceramic coating on it.

Back to porting stuff. Ive read that many folks polish the exhaust to reduce carbon fouling. What's tumbling around in my mind is carbon forms because of high temps. There is usually more carbon in the exhaust passage then there is in the muffler itself which makes me think it's because the port is hotter than the muffler. With that in mind: would going bigger on a dual port muffler help reduce carbon fouling in the exhaust? Secondly: can you go too big on a muffler mod? I know how a tuned pipe works; but are these little cans doing anything to "re-stuff" the charge back into the cylinder? Always wondered if you can make a muffler flow too much on saw.

In the interest of reducing exhaust port temps, I wonder if fluting the outside of the exhaust would do anything to help dissepate the heat faster. Same concept as flutes on a rifle barrell. Provides more surface area for cooling. Could be the dumbest thing ever, but I've just been thinking.

 

[David Young]

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

 

[drf256]

Good threads never die.........they just fade away.

My my, hey hey...

 

[XP_Slinger]

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

Good post, I'm going to read that one a few more times. But the question still lingers. How much is too much with regard to muffler flow? Every saw is different, got it, but one would think that as performance increases the need for better flowing exhaust also increases. Gotta get the heat and extra exhaust out. And again; do factory mufflers have any sort of scavenging effect on the fresh charge trying to sneak out the exhaust port? If your crank case pressure isn't extreme I don't see how they would.

 

[tickbitintn]

Two schools of thought on exhaust port polishing;
-first is mirror like has less resistance to flow ... so it exits faster.....
-second is leave rough / textured to allow carbon coating ... blocks transfer of heat from exh gases to cylinder...

 

[David Young]

When at resonance the box(muffler) is in sync with the pulse generator

So we have inertia on our side because it takes energy to make a change from any given state.
When some goes out the box flexes when the exhaust closes the opposite happens to box it flexes the other way.

Think of a guitar string. Pluck at any speed you want may change amplitude but frequency is the length of the string. It vibrates back and forth at a decreasing magnitude.

This could cause a mild ramming. (Reversal).

Or it could set up for the next pulse which the box would be in sync and causing a slight pulling.


I'm confident this theory is right but I don't know how much it really impacts performance.

 

[shorthunter]

But the question still lingers. How much is too much with regard to muffler flow?

There was a wise man on Arboristsite, at one point, who answered this question. Maybe I can dig it up

 

[shorthunter]

Can't get my phone to open the link but if you google the addy, it comes right up

http://www.arboristsite.com/communit/threads/muffler-mod-how-big-is-too-big.44816/page-5

 

[Terry Syd]

When 'Timberwolf' was testing the velocity and direction of flow in the transfers, he had one graph that showed a reversal in flow in the forward transfers. This was done at high RPM and we figured it must have been a reflection off the front of the muffler that sent a positive wave back to the cylinder.

There was a possibility of getting that positive wave to reflect back just as the exhaust port closed (to pack the cylinder), but the wave was so short and of such limited duration that it couldn't reasonably be used in a work saw.