Norman,
You are of course correct, but the advertised or static compression ratio (CR) calculates the ratio based on the intake valve being closed at BDC. As we know, this is not the case but it is the industry standard. We know what the stroke is 3.543" and that the advertised CR is based on the intake valve being closed for the entire stroke.
What the dynamic calculation does is it relates the stroke to the compression. If the intake closes at 51 degrees ABC the stroke in the formula is changed from the stock 3.543" to 3.02" because that is when the valve closes and the compression starts to build in the cylinder. They call this "Effective Stroke."
I often use this site for the calculators:
www.wallaceracing.com
The dynamic CR calculator is under "D."
Here is an example with the Megacycle Mk2:
When you push "Calculate" the info disappears and a new screen pops up:
It's best to capture a screengrab of the first screen because it will disappear with the new calculation.
The calculator is using the same formula that we are familiar with, but it is changing the stroke to match the valve opening.
The value is using this information to match the fuel to your CR. You don't want to use fuel with a higher octane than necessary because its volatility is suppressed the more the octane is boosted. It is also important as general information as someone who is running a 9:1 piston with a Mk2 is getting somewhere around 7.5:1 DCR and not the high performance that they may be expecting.
David