stuffing a crankcase

[muddstopper]

about a year ago, I bought a box full of junk husky 51's/55's. I have managed to build 4 complete saws using oem parts. I have a bunch of odd parts left so I have decided to build a cheap saw with all chinese parts. I plan on practiceing porting with the cheap parts. I split a 55 crankcase last night to install the china bearing and seals and got to thinking about maybe filling up the crankcase to lower crankcase volume. Since I dont know anything about high performance chainsaw building I am just guessing about how this stuffing business is supposed to work. My thoughts are that the piston and intake port control how much air gets in the crankcase. That shouldnt change by making the crankcase area smaller, as long as kept within reason. The only benefit I can see of filling the crankcase would be an increase in pressure and velocity on the exhaust stroke as the air is being pushed into the cyl as the piston if going toward BDC. My next question would have to be if one is planning on stuffing the crankcase, should the stuffers be added to the crank, or would epoxy inside the crank case work. Crank stuffers could probably be made to take up more area, but would be hard, (read expensive), to make. I kind of doubt any of the existing crank stuffers in models such as a 357 could be made to work. This going to be a cheap saw to play with and practice porting on. If it blows to pieces or runs poorly, I dont care.

 

[Franny K]

My thoughts are that the piston and intake port control how much air gets in the crankcase. That shouldnt change by making the crankcase area smaller, as long as kept within reason.

Stuffing the crankshaft will essentially change the compression ratio of the lower end leading to lower pressure on the up stroke leading to more air fuel mix, in theory anyway. This would be in intake/compression part of the stroke.

The only benefit I can see of filling the crankcase would be an increase in pressure and velocity on the exhaust stroke as the air is being pushed into the cyl as the piston if going toward BDC.

This would be in transfer part of the stroke

Is there really room to fill the circular portion of the crank cavity?

Might also think about balance. There is a lot of void in a crankshaft. I guess you can add heavier than steel material in place of steel on the fat end to make up for whatever addition on the void end but seems for a normal chainsaw crank might be problematic.

Have you looked at pictures of stuffed cranks or the exploded parts sheets for such models?

One thing I found rather interesting when more folks were posting pictures of the insides of chainsaws was for the top handle auto tune Husqvarna t540 which has a stuffed crank but the crank cavity was quite a bit bigger than the crank that was in it.

 

[muddstopper]

I'll admit I might be looking at this all wrong. Air is pulled into the crankcase on the compression stroke of the crank.The amount of air that can be drawn is determined by lenght of stroke and bore of piston. Changine case volume will not change these numbers. How much air a predetermined size stroke and piston can pull into the crankcase can be changed by removing any restrictions that prohibits airflow into the crankcase. Requardless the size of the air intake, the amount of air pulled into the crankcase is still limited by stroke and piston bore. Reducing crankcase volume should not increase or decrease the amount of air that can be drawn into the crankcase until there is some crankcase pressure change. This should only occur as the piston is traveling down in the bore. This should cause a increase in crankcase pressure until the piston passes the tranfer in the bore that allows the air to enter into the combustion chamber. The downward stroke of the piston should increase crankcase pressure and increase the velocity of the air entering the combuston chamber and help force the spent exhaust gases out of the exhause. I think this action would probably result in more of the air fuel mixture entering the combustion chamber as their would be less room in the crankcase to store any excess fuel mix.

I know I am just ratteling on and dont know exactly what I am talking about, So if someone wants to correct my guesses, feel free to do so.

 

[Franny K]

If it turned real slow things would be as you put above. The stuffed crank will make more suction at the peak suction the air/fuel will move faster, have more inertia. In response to the first 40% or so above.

 

[foeke]

Or it will run like a gem, and you will beat yourself over using cheap parts [emoji28].

Verstuurd vanaf mijn SM-G955F met Tapatalk

 

[happysaws]

Have you ever read Gordon Jenning's "Two Stroke Tuners Handbook"?

 

[muddstopper]

And half my post didnt show up.
Read????? Whats that?
truefully, I am a tinker, not a builder. I use my saws myself and dont work on them for other people. I think I learn more by watching someone that knows than reading a book. Books can go over my head and I cant ask a book a question when something isnt clicking in my noggen. I have been looking at the 55 I am building and have decided that stuffing this particular crank case just isnt possible. What little I might gain by decreaseing crankcase volume probably wont be worth the effort to get there. The crank just barely clears the crankcase as it is. The only way to decrease the volume would be to increase the material on the crank throws. This would require major machining so it aint going to happen. I'll put the case together tonite and measure squish and then decide if I will mill the head and then start working on porting.

http://www.amrca.com/tech/tuners.pdf

 

[Ozhoo]

357 stuffers can be purchased separately. Track a set down and do some figgerin from there.