Chainsaw Porting Theory

[drf256]

Good Morning boys and girls, i'm your
substitute teacher, Mr. Buckwort....

Got any pics of yourself?

I always wanted to bang the sub. You probably ain't my type though.

 

[Stihlbro]

Mr Buckwort( my hand is raised) , I have a question? Are you trapping more when the intake is closing later from cutting the base?

 

[wigglesworth]

View attachment 13183


Mr Buckwort( my hand is raised) , I have a question? Are you trapping more when the intake is closing later from cutting the base?

Well, your intake duration is "slightly" increased due to the later closing, so in "theory" there is a slight amount more in the case, "IF" that slight amount more isn't reverberated back into the intake tract due to the pressure wave caused by the downward motion of the piston.

Kind of a crude example here, but Imma go with it. Pretend your crankcase is a tin can. It holds "X" amount of water. If you pour it in slowly, it still holds "X" amount of water. If you spray it in with a high pressure hose, guess what, still holds "X" amount of water. If you put a lid on it fast, or slow, it still only holds "X" amount. No real way to change it with port timing without opening up the crankcase, or adding stuffers, in my eyes anyways.

Make sense?

Morning by the way. Recess will be at 9:00, with snack and story time at 11:00, followed by nap time at 12:00....

 

[Stihlbro]

So how do you know your crankcase is full, spit back thru carb?

 

[drf256]

Yes teach.

So smaller case volume doesn't increase pressure on its own.

So are crank stuffers, full circles, etc only beneficial in models where the case volume is too large for the jug displacement?

Allow you to open the case or jug in other places and still maintain pressure?

 

[drf256]

But, even though your case is smaller, you do have a larger relative size piston compressing the case.

You're changing the case volume more in terms of expanded vs. compressed case.

So I'd venture to say you actually are increasing case compression by reducing case volume.

 

[wigglesworth]

Yes teach.

So smaller case volume doesn't increase pressure on its own.

There is displacement of the piston. The smaller the case, with a bigger piston is gonna give you greater displacement, I.e. Pressure.

So are crank stuffers, full circles, etc only beneficial in models where the case volume is too large for the jug displacement?

I've seen stuffers in lots of old skool saws, with huge displacement, in the 100cc area and in my thought process, the stuffers where there to increase the working rpm of the motor.

Everyone seems to agree to that as well, that that is the result of decreasing crankcase volume, is higher working rpm, or shifting the powerband up, but usually at a loss of torque, no?

Allow you to open the case or jug in other places and still maintain pressure?

I read somewhere that the actual pressure in the crankcase, between the intake closing, and the transfers opening is minuscule. Like, below a couple psi.

 

[wigglesworth]

So how do you know your crankcase is full, spit back thru carb?

Good question. I got a couple that blow crap all over you when there running. I guess there full. Lol.

 

[Stihlbro]

Blow/spit back at idle or at wot?

 

[wigglesworth]

Blow/spit back at idle or at wot?

WOT.

The Durand 372 is the worst. I know why though, and honestly would take a bath in gasoline just to run the saw, so it don't bother me much. lol.

 

[drf256]

Everyone seems to agree to that as well, that that is the result of decreasing crankcase volume, is higher working rpm, or shifting the powerband up, but usually at a loss of torque, no?

Makes sense. Like an 046 vs an 044/046 hybrid.

The only way it makes sense is higher rpm.

 

[Stihlbro]

But not at idle? Interesting! I bet it saturates the air filter , but wait...........is from a full crankcase or reverberation from when the piston stops the flow?

 

[Mastermind]

I must interject.....

How can cutting the base decrease case volume?

The piston will still be traveling the same height, so where is the decease?

Teach me.

 

[Mastermind]

Great discussion BTW.......

I'm playing the part of counterpoint today.

 

[Stihlbro]

Im in time out, I can't speak to anyone at the moment. The teacher is mean today.

 

[czar800]

I must interject.....

How can cutting the base decrease case volume?

The piston will still be traveling the same height, so where is the decease?

Teach me.

If I got it right.... It's in the transfers???

Can I go to the front of the class? Lol!

 

[dall]

lol spent many days with nose to chalk board and in principals office

 

[czar800]

I skipped way too much school.....

 

[Four Paws]

Im in time out, I can't speak to anyone at the moment. The teacher is mean today.

Then PM me

 

[paragonbuilder]

If I got it right.... It's in the transfers???

Can I go to the front of the class? Lol!

But that would only be true if you didn't raise them back up.
Damn, I was seeing it how Al does til Randy had to throw in the whole piston moves the same amount fact.
Since the entire base is sitting on the case, nothing more protrudes into the case to reduce volume except the top of the transfers are lower until raised back up.
Unless the cylinder has case extensions that don't need to be trimmed...then it would reduce volume a bit.
Do I get a sticker now?


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