Chainsaw Porting Theory

[Keith Gandy]

Yup, that one too. I had thought you were at Mansfield. I've been in most papermills within 200 miles of Monroe.

Small world! Are u a millwright? We r back at it today down here!!! Lol!

 

[Barneyrb]

No, I used to work on drive systems for the pm's and have since moved into sales and got out of the service work.

 

[quietfly]

So far, on any Stihl 70cc-90cc that I have ever seen, you really cant go to big on the exhaust opening in the muffler.
They like them big, matter of fact, the bigger you go, the more power they have in the cut
The ported ones really come alive with a second large opening, like @Red97 makes
View attachment 12568 View attachment 12570 View attachment 12566 View attachment 12567

So on a big Stihl, like an 084, would they gain from running an open empty can?

 

[Moparmyway]

So on a big Stihl, like an 084, would they gain from running an open empty can?

I honestly don't know, but what I do know, is that there is one way to find out ........

 

[quietfly]

Actually i do want to try this, but i don't want to kill an otherwise cherry muffler on a guess.

 

[quietfly]

You know I like a higher compression ratio than some builders do. But experience has taught me that there are no one size fits all recipes.

For instance in a milling saw....long cuts, and extreme heat. This is one example where excessive compression would be counterproductive.

What can be done to help reduce heat in larger displacement milling saws? Can you over scavenge them and cause efficiency issues by running too little or no pressure?

 

[MustangMike]

It will if this is on the hood !!!
View attachment 12323

I see them in my rear view mirror all the time!!!

 

[merc_man]

Couple questions.

If you deck a cylinder do ya need to raise ports what ya take off the deck?

If you go wider on exhauast port do ya do the same on intake.

think i heard exhaust can be wide as 70 percent of piston skirt if ring ends let you. What is the rule of thumb for intake port width.

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[mdavlee]

Make a custom front plate with a big outlet for the 084 to try how much opening is ideal.

 

[Al Smith]

Now that deal on chopping half the front off a pressure can might bite one in the buttocks if you don't have enough carb backing you up .

A pressure can is a regulating device .Stop and think about how flow goes through the engine,pressure differentials .Some saws have barely enough carb to run with a slightly pressure relieved muffler .You over run one it will go lean out on you in a heart beat .

Now they won't all do that but anything over about a max of 125 percent it becomes less improvement .Usually 85 percent is considered about right .

A guy on another forum several years ago drilled a couple giant holes in a fairly new 088 Stihl .Proud as a peacock on how it ran but failed to retune the carb .Couple weeks later on flea bay damaged 088 Stihl 400 dollars,Cincinnati Ohio . .Myself,TW ,Ed H and several others cautioned him .Fell on deaf ears. Chit happens .

 

[jmssaws]

I usually don't widen the exhaust much if any but a 66 don't need any the exhaust port is large enough already, I change the shape of it but don't widen but just past the factory bevel.

Transfers are were I'm finding power, small changes in the transfers can make big changes overall.

 

[shorthunter]

I stand corrected, possibly, about transfer backflow.

Here is a great example of backflow in the transfers. This is from a 346xp, probably the strongest 45cc saw ever produced. You can see significant carbon buildup in the transfers.

 

[drf256]

Couple questions.

If you deck a cylinder do ya need to raise ports what ya take off the deck?

If you go wider on exhauast port do ya do the same on intake.

think i heard exhaust can be wide as 70 percent of piston skirt if ring ends let you. What is the rule of thumb for intake port width.

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The answers are:

1) Maybe
2) Possibly
3) Not sure there is one

It's all gotta work together, that's what's so confusing.

 

[drf256]

Here is a great example of backflow in the transfers. This is from a 346xp, probably the strongest 45cc saw ever produced. You can see significant carbon buildup in the transfers.

So you think carbon in your lowers are proof that burning charge goes all the way down to the case during initial transfer opening?

I can believe some backflow, but not that.

 

[paragonbuilder]

How else would you have burned residue (carbon) down there? Everything else is clean as a whistle.
Do you have a theory?


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[jmssaws]

I've never seen carbon in the transfers but perhaps on a shot p&r you could get some.

 

[Barneyrb]

Here is a great example of backflow in the transfers. This is from a 346xp, probably the strongest 45cc saw ever produced. You can see significant carbon buildup in the transfers.

I would be more inclined to believe that what you are seeing there comes more from rings not sealing or blowby. My thoughts are that when the piston is coming down the crankcase is pressurized and when the transfers are uncovered they are releasing the pressure. With blowby there is very little pressure on the case when at TDC and exhaust gasses which get by the rings would then escape down the transfers. Just my theory on that.

 

[jmssaws]

The last two posts are why there's carbon in transfers.

 

[drf256]

How else would you have burned residue (carbon) down there? Everything else is clean as a whistle.
Do you have a theory?


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Yes. Heat in the jug causes some of the fuel additives to burn/breakdown, and/or there's some combustion that makes it down there at some point.

Theory only, I have no proof.

Conceptually, I can accept some "bulging" of exhaust back into the uppers, but not that far down.

How much and how does a strato fill a transfer?

Again, zero proof. Interesting discussion.

 

[Mastermind]

Wanna get a curve ball in your theories? lol

http://www.dragonfly75.com/motorbike/portdurations.html