I was possibily a bit to brief in my previous post. What I wish to point out is that when there is any doubt about the compression ratio of an engine it is best to actually measure the trapped compression ratio. This is the compression ratio calculated using the trapped swept volume of a cylinder. i.e the volume of the cylinder from the point of exhaust port closure to the tdc position.
As an example, if the exhaust port on a standard stroke(54mm) banshee opened at 27mm from tdc, the calculated trapped compression ratio would be exactly half of the static (also known as geometric compression ratio).
In a two-stroke the cranking compression is directly related to the trapped compression ratio! If you raise the TCR, the cranking compression ratio will also rise. The static compression ratio has no relevance to the cranking compression.
Factors that can cause the cranking pressure to appear different to what was expected include.
1/ Gauge accuracy
2/ altitude
3/ warm or cold engine.
4/ squish clearance
5/ dome shape of piston
6/ cylinder bore
7/ exhaust port timing
8/ worn rings
9/ worn bore
Trapped compression ratio is sometimes known as dynamic compression ratio. However the name recommmended by the SAE is trapped compression ratio. Does not matter to much what it is called as long as you realize that they are the same thing.
Below is a small sample table for the calculated trapped compression ratio. CCV is the combustion chamber volume in CC.
Exhaust Port Opens from tdc
CCV 26mm 27mm 28mm
13 7.43 7.68 7.93
14 6.97 7.2 7.43
15 6.58 6.79 7.01