as mentioned dynamic compression is what controls what octane fuel you need, much more than static compression. And there are computer engine programs that will calculate dynamic compression, how prone an engine is to spark knock. I use performance trends engine analyzer, it gives me cranking compression, idle vacuum, dynamic compression, and calculates a spark curve for what ever octane fuel you want to use, including alcohol. For the cost of the program, you could not buy a set of pistons. It also has a calculator function for actual static compression using piston height and chamber CC's. When I take the time to measure, seldom doe an engine have as much compression as the paperwork states.