There is no way that you can screw anything into your spark plug hole and it tell you the compression ratio of the motor. There are too many variables like cylinder head chambers,camshaft design, cam timing ,valve overlap, and how fast the starter is spinning the motor. In the past I had a motor that was exactly 13 to 1 . The first camshaft that I had in it ,had a cranking compression of 240psi. The only thing I changed was the camshaft to a larger nitrous cam and the cranking compression dropped to 185. The only correct way to accurately calculate compression ratio is to dissassemble the motor.