Husky 357 full crank vs. 359 half crank

[kodiakfisher]

There must be an engineering reason why the 357xp has a full circle crank.

I can only think that an object in motion stays in motion and with a full circle there is more energy via the greater mass of the full circle crank.

EHP what are your thoughts?

Kodiakfisher

 

[eyolf]

I probably have a very incomplete understanding of the principles involved, but the full-circle thing is to accomplish what outboard motor and motorcycle racers used to do when they "stuffed the crankcase". Reduce the amount of space available inside the crankcase and one can amplify the pumping effect at higher RPMs when volumetric efficiency is dropping off. Even air has mass, requires time and energy to overcome inertia; reduce the amount of air in the crankcase and reduce the stratification because spongy air doesn't force it's way around like a liquid or solid.

Of course, there is a trade-off. You can't move the air around the tighter spaces as well, so low-end and midrange performance is probably a little less. Its really a trick to squeeze just a little more effect out of raising the ports.

I've not seen the inside of the 359, but I have heard the full circles are made of sheetmetal and some kind of resin. Not so much weight, not as much flywheel effect as one might think.

 

[ehp]

eyolf you know more than you think or want us to know about you, yes it is a crankcase volume thing, the tighter the crankcase the more air you get to go threw the ports at a high rpm, but with the crankcase on a chainsaw being so tight to begin with this is not always a big help. Your crankcase volume has an area in volume below the dome of the piston at bottom dead centre and you divide that in to the total volume of the complete motor so the 357 will have a smaller volume than a 359 because of the full circle crank but if it is to tight you willnot make any power cause you cannot get enough air through the motor, no air no power.
the perfect number is 1= 1.2 so what does this mean
on a 3120 the ratio is 1 to 1.47 so for every cc of volume in the crankcase the cly. has 1.47 cc which means the crankcase is to tight to make the most horse power so then why not open up the crankcase well the more volume you make the case the more air will go threw it but also the more fuel the carb is going to have to pump now if you were running on gas this would not be a problem but on alcohol it is a problem cause if you make the case volume to give you 1.2 you are going to need almost double the fuel to go with the double horsepower you can make. On my computer programs it has all this plus I can see how much horsepower can be made before ever trying it on the computer dyno. Sure by the dyno numbers you can make huge power but the computer doesnot take in to fact how much power can the chainsaw parts stand before they fail, cranks pistons stuff like that. The big thing so far that I have found is will the crank stand the abuse, either the rod breaks or the crank bends on the flywheel side mainly because how much faster a alcohol saw picks its rpm's up. Yes we lighten the flywheel to almost nothing left and that helps

 

[ehp]

I have a 357 on alcohol and it runs good but no match to the 50, I am going to pull the bells off its crank and see how much more power I will get. That motor doesnot pass alot of air and needs more to make big power, this is one reason I like the 359 it does pass air and once ported does make good power

 

[Stihl 041 super]

?

Whats the difference between a full crank and a half crank anyway?

 

[Mange]

I could be wrong, but I think on a full crank there is plastic to complete the circle, it is a regular crank with disc's to take up space.

 

[kodiakfisher]

It sounds like the difference has to do with air movement at higher RPM's and possible comming closer to the 1=1.2 ratio EHP was talking about and in stock form the 357 does this better than the 359. Those that have the ability to modify a chainsaw appreciate the extra available air that the 359 has. In order to gain the advantage with the 357 you have to be high on the RPM's where as the 359 is more forgiving therefore feeling more torquey (is torquey a word?).

Does this sound reasonable to everyone. Mechanical Physics is really far outside of my education can you tell ).