How do you Figure Piped Saws Compression?

[Ol' Brian]

.... IE you can have quit high static, but fairly low corrected compression.

Wouldn't the port timings have to be really screwed up for something like that to happen? Or way outside of the peak powerband?

 

[bwalker]

Wouldn't the port timings have to be really screwed up for something like that to happen? Or way outside of the peak powerband?

No, not really. The compression ratio for a stock chainsaw is very low. Lower than its static compression ratio would suggest.
Even a ported saw at 180-200 PSI will run fine on 87 octane pump gas so again its compression ratio is likewise low.

 

[Ol' Brian]

No, not really. The compression ratio for a stock chainsaw is very low. Lower than its static compression ratio would suggest.
Even a ported saw at 180-200 PSI will run fine on 87 octane pump gas so.agin its compression ratio is likewise low.

OK, so I'm probably confused... when you figure static compression ratio in a 2st... do you start at BDC, or when the last port is blocked off by the piston? I was thinking static figured from BDC, and corrected is when the last port closes (usually the exhaust port)?? It's been a long long time since I've wrapped my head around this 2stroke theory LOL

 

[bwalker]

OK, so I'm probably confused... when you figure static compression ratio in a 2st... do you start at BDC, or when the last port is blocked off by the piston? I was thinking static figured from BDC, and corrected is when the last port closes (usually the exhaust port)?? It's been a long long time since I've wrapped my head around this 2stroke theory LOL

Static is measured via a gauge screwed into the plug hole. Dynamic is measured by taking into account bore size, squish clearance, combustion chamber volume and exhaust port height.

 

[Ol' Brian]

Static is measured via a gauge screwed into the plug hole. Dynamic is measured by taking into account bore size, squish clearance, combustion chamber volume and exhaust port height.

OK... yeah that' starting to sound familiar. Makes sense. Small engine shop class in HS was a long time ago. My shop teacher was into dirt bikes and pipes and stuff and really liked to get into the weeds on this stuff, and we ate it up. Haven't thought much about it since.