some general saw questions

[mcb]

1) been searching the site for a few weeks and i find no mention of gas ports (vertical holes in piston crown into top ring land to get combustion pressure and vaccuum to expand and contract ring, respectively) do they not work on a two stroke? if not, reason why.. ?


2) are saws internally, externally or non-balanced? will taking weight off the reciprocating assembly without modifying the rotating assembly cause trouble? or lightening/radiusing the crankshaft without altering top end weight?

 

[timberwolf]

I played a bit with vent holes in the piston crown, was alright, but nothing magic, also with the carbon in a two stroke they will plug up, maybe not the best to keep blowing carbon and exhaust gas in behind the ring on the long haul. Maybe for racing they could help control ring flutter, but they don't have any place in a work saw.

Crank is ~50% balanced, that is it offsets about 50% of the piston and rod mass. this dampens the up and down vibs but adds some front to back. Balance in a single cylinder two stroke is just a compromise as it's not possible to truely balance such an engine unless with countershaft(s) and weights. I would not think it an advantage to remove material from crank, this will increase base volume and decrease base compression. Some tight based saws with an overbore piston and pipe might take well to the extra base volume, but most cases it won't help. Other places to reduce weight if you wan't, but that's only going to help if you are trying to win a race.

 

[mcb]

"base volume" term was stumping me but you just opened my eyes to a whole other concept.. that the crankcase is its own form of manifolding. wow.


ive always treated 4 stroke intake runners like a waiting room to a doctors office. the more patients i can fit into the waiting room without reducing velocity, the more can rush in the door every time it opens.

but to consider that a runner or manifold (the crankcase) actually has a compression ratio.. thats something that would have never occured to me. big help, thank you!

(now im tempted to start filling the bottom ends with devcon!)