Chainsaw Porting Theory

[mdavlee]

Now........don't be too hasty.

Point........counterpoint is the way to learn.

David nailed it, but no one was listening.



Wanna make it scream........straight out the front, but below the exhaust port works.

For a strong work saw.........out the side seems to make the most power at a lower RPM.

Shhh. We want to keep that to ourselves

Hahahahahahahahahahaha in my best DC voice

 

[Al Smith]

When TW did his research he had the results on a spread sheet .As I recall once you reach over 85 percent of the size of the exhaust port results drop off rapidly.It will increase slightly to about 125 percent then it stops .Noise by the way does not neccessarily equate to power .

 

[beaglebriar]

I've gone out the front on two of my saws and so for I'm happy with the results. They sound more badasser too.

 

[Danders]

When TW did his research he had the results on a spread sheet .As I recall once you reach over 85 percent of the size of the exhaust port results drop off rapidly.It will increase slightly to about 125 percent then it stops .Noise by the way does not neccessarily equate to power .

That fits with what I remembered as well. Power kept rising but at a much smaller rate, and the muffler wasn't muffling much towards the end either.

I've opened up most every saw I actually use and I wear hearing protection. I wonder if we could ever find ourselves on the wrong side of the law with muffler mods - alteration of emission controls or some such thing? No one has ever questioned me before when out cutting, but I suppose it's possible some busybody might make time for that.

 

[Al Smith]

I wouldn't worry too much about the chainsaw police issuing a citation for a noisey saw . For all intents a pressure can gutted muffler even with an 80-85 percent hole or holes is not as loud as some of my McCullochs .

 

[Danders]

The guy I cut with has a nice sp 81 that was his father in laws. The father in law bought it new but couldn't start it so it mainly sat in the garage for 40 years until my friend inherited it. Muffler on that reminds me of a sardine can.

 

[Terry Syd]

When TW did his research he had the results on a spread sheet .As I recall once you reach over 85 percent of the size of the exhaust port results drop off rapidly.It will increase slightly to about 125 percent then it stops .Noise by the way does not neccessarily equate to power .

His testing was also done to the FRONT of the muffler. He had a hole he could vary the size of. Since exhaust gases have mass, they tend to continue in the direction they are moving, a variable size hole on the SIDE of the muffler may have shown slightly different results, both in power and noise.

There was also the issue of his 026 being ported and having a fair amount of blowdown. A saw with less blowdown would likely have shown different results.

All in all it was a very good test he did that helped the chainsaw community come up with some guidelines that weren't previously available. I wish we could get more guys like him back into these technical discussions, especially those with access to dynos.

 

[Keith Gandy]

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

Thats what I use, solid front and 2 side outlets

 

[Four Paws]

On the topic of mufflers I have a wag.

A muffler is similar to a ported speaker box.

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.

Whoa, the speaker box...blast from the past. I built a lot of those in my day.

In regard to your second statement...something to ponder really. A guy could tune the box with port diameter and length. The larger tube has more cross-sectional area, and as such, can move more volume at a lower pressure. Consider this in relation to exhaust open timing. The larger tube also has more surface area (if length is held constant) and therefore can possibly have more frictional losses, especially if exhaust pressure, and subsequently velocity, is increased.

 

[Al Smith]

The guy I cut with has a nice sp 81 that was his father in laws. The father in law bought it new but couldn't start it so it mainly sat in the garage for 40 years until my friend inherited it. Muffler on that reminds me of a sardine can.

The SP 81 has a radiused muffler with a side discharge .That style is probably the best as far as a performance for the Mac 82 cc saws .The worst are the contraptions with a screen gizmo used on the 805 and later model 850's .

 

[shorthunter]

I have always figured that the hole was set below the exhaust port to eliminate as much resistance as possible. If you think about how the exhaust gases exit the cylinder it makes sense, they do not exit at 90°

 

[drf256]

I know we are on mufflers, but I've got a question that unrelated.

Has anyone gone with an assymetric exhaust port for the purpose of retaining the top ring gap in a wider piece of plating?

I'm specifically referring to Stihl's, where I'm looking to rotate the piston 180* so that I can grind fingers on the intake wall. I can widen the exhaust port, but more on the side of the lower ring gap and run only the top ring. That way, I have a lot of support for the one ring gap I'll be running.

If anyone has done this, did it throw off piston wash? Did you adjust the upper transfer on the side with the widened exhaust?

 

[Deets066]

I've tested it many times fellas, the bigger the better and on the side.

Close your side port and put the same size hole in the front cover and you will feel it loose.

2" long 1 1/2" pipe on the side were the factory outlet is on a big stihl makes the most power on my saws. I have tested this many many times.

Every time I added a pipe my 66 gained. I have one goin out the front but it isn't straight out. It hits the wall of the pipe and exits out both sides.

 

[Moparmyway]

Every time I added, my 66 gained.

Yup ......... I've seen that on every one that I have as well

 

[Stihlbro]

I know we are on mufflers, but I've got a question that unrelated.

Has anyone gone with an assymetric exhaust port for the purpose of retaining the top ring gap in a wider piece of plating?

I'm specifically referring to Stihl's, where I'm looking to rotate the piston 180* so that I can grind fingers on the intake wall. I can widen the exhaust port, but more on the side of the lower ring gap and run only the top ring. That way, I have a lot of support for the one ring gap I'll be running.

If anyone has done this, did it throw off piston wash? Did you adjust the upper transfer on the side with the widened exhaust?

Im not quite sure what your asking? Many cylinders from Stihl and husqvarna have assymetriccal ports from the factory,365 and 460's for sure. Get your area, bevel good for ring support, it should work?

 

[drf256]

Maybe I wasnt clear.

I need to leave a healthier amount of plating where a ring gap will ride.

Will leaving more on one side of the port than the other alter the flow in any negative way?

If so, is there a way to compensate for it, like raising the upper 1* higher on the side with less exhaust flow?

 

[Stihlbro]

I don't know how else to reply other than, I believe you will be fine. It's likely after some run time, being its a exhaust port, there will be some build up there from the heat.

If it easies your mind, we all want the picture perfect porting to show and share, but it doesn't have to be so precise. Some of the best performing cylinders I've been around are not picture perfect. One may think that the attention to small details add up, but I've yet to see it.

 

[drf256]

I don't know how else to reply other than, I believe you will be fine. It's likely after some run time, being its a exhaust port, there will be some build up there from the heat.

If it easies your mind, we all want the picture perfect porting to show and share, but it doesn't have to be so precise. Some of the best performing cylinders I've been around are not picture perfect. One may think that the attention to small details add up, but I've yet to see it.

I think it depends on which small detail one chooses to value over the others.

I agree, some good looking jugs dont perform as well as crude lookers. It all about where you wanna put the time in.

I like a smooth exhaust port, but polishing it to a mirror is now useless in my eyes.

 

[lwhaples]

Have done this to my2152,346,357

 

[czar800]

Every time I added a pipe my 66 gained. I have one goin out the front but it isn't straight out. It hits the wall of the pipe and exits out both sides.

View attachment 13936

How long have you been using the front cover set up?