Originally Posted by Riding Rich
No the PK80-J is allmost 69CC total. like 68.8 something. "stroke X bore dia."
But to find compression. "Corrected Compression"
We use the swept volume of the piston above the exhaust port.
Compression dosent start until the exhaust port is closed so we use the volume of the cylinder above the top of the exhaust port.
So thoose numbers at like 15 :1 or 16:1 compression are UNcorrected.
They take the complete volume of the cylinder and use for calculation but with the ports open there isn't much compression yet.
Other than the PSI in the CrankCase and PSI caused by Expansion Chamber slammin fuel back in the chamber.
Which will add some PSI to the actual calculated compression.
But lets see.
69cc / 5cc = 13.8 :1 Uncorrected Compression Ratio.
So be careful when you talk compression and make sure you know if were talking Corrected or UNcorrected compression.
It makes a big difference in the numbers.
Going much higher than 9:1 Corrected Compression probably won't help much.
at a certin point you actually have a detrimental effect.
Make sure you cange that caged needle bearing if you go high comp.
It puts much more strain on things.
GREAT INFO IM GOING TO BUY 1 OF THOSE PK 80 NEXT THANK YOU FOR POSTING THIS KIND OF MATERIAL !!!!!