Howdy SLV49 and all:

Actual compression ratio is the ratio of the total volume of the cylinder with the piston at bottom dead center, the combustion chamber, compressed head gasket and piston dish, if any, to the same dimensions but with the piston at top dead center. As has been described, many factors effect this ratio, but once fixed, it is a mathematical ratio of these two volume dimension. If you change any of these factors you will change Compression ratio. This is a very mathematical equation.

In general, a point of compression is worth about a 4% power in/decrease, all else being equal.

Theoritical compression ratio is determined by the dynamic relationship of valve timing and power adders such as super chargers, turbo chargers and NOS. They are best measure by cylinder pressure.

Some real world realities;

Higher compression with a stock cam will generate more cylinder pressure than the same compression with a performance cam, because of valves opening earlier.

Compression also generates greater heat. It needs to be manages for best results. Heat is power- to a point.

It is best to include CR in the total over-all plan for your engine.

Adios, David