Bore vs stroke

[Tony Snyder]

Here is one for you 2 cycle engine experts. On a 4 cycle I thought that long stroke vs bore gave a torque advantage, and that a large bore vs stroke gave a high rpm power advantage.

In a 2 stroke where everything has to happen in fewer crankcase degrees, is an over square bore an advantage at all? It seems that the 2159 Jonsered runs pretty good with a larger bore than stroke.

 

[bwalker]

Since saws dont really need low rpm torque a short stroke motor is not a diadvantage. On the contrary a short stroke high rpm motor will have a higher chain speed due to an increased reliable rpm limit. It will also make more power at this increased rpm than a long stroke would if you wrang it out to the same rpm.

 

[Tony Snyder]

The new 2159 Jonsered runs a 47mm bore by a 34mm stroke. Isn't that radically over square, even for chainsaws?

 

[bwalker]

No, not really. Its actually pretty conservative compared to a f1 motor which have really short strokes. Most 125cc dirt bikes are oversquare as well.

 

[Dagger]

Tony, most saws today are oversquare, as we call a bore larger than stroke., which is, as said above, the more reliable way to get high rpms..it has to do with piston speed. no matter what engine we are building today, we are limited to a piston speed , measured in Feet /sec. or min. to the available alloys and cylinder coatings. When the piston speed goes up..so does the wear factor.
Since your customers have demanded higher chain speeds for a faster cutting saw in todays market, the realiable way top get it is shorten stroke to retain max piston speed while getting higher rpms.