http://www.georger.com/toad.htm On this page I devised a spreadsheet that takes the CR into consideration for a Volvo B20. I started with some measured values before I started the rebuild in hopes of arriving at the desired compression ratio. The first set of numbers are for a stock engine and to arrive with the same value as published stock, I had to ignore the "ring land" or the area between the top of the piston and the cylinder wall down to the first ring. The result gave me the 8.7:1 published number. The critical value is the head volume in CC's. This value is arrived at by measuring the amount of liquid that will fit into the combustion chamber with the valves closed. Turn the head upside down and place a flat piece if plexiglass with a a small hole in the top to "fill" it from. I had a glass cylinder from a science class with a stop-cock at the bottom. I measured that 57 cc's of liquid fit into the combustion chamber. Knowing the bore and stroke, I can tell what volume ( 496.38 cc's) is being compressed into the combustion chamber and thereby have the variables identified. Since I bored it out .030 I had more volume (504.93)and I knew that to still use "pump" gas I couldn't go over 10:1 so I determined that when I shave the head I will take a precise amount off to reduce the height and therefore reduce the volume of the chamber the piston is forcing the air fuel mixture into. By reducing the combustion chamber to 47.5 cc's I am achieving a CR of 9.7:1 and I did this be shaving the head. The performance is just what I wanted and I have driven this configuration 30,000+ in the past 2 years (93-94 OCTANE)without any issues. I have never checked the pressure build-up in the cylinder ("compression check") because it checks the condition of the engine as a machine ....the fit of interworking parts. The ability to hold pressure tells you about the condition of the "fit" of piston ring to piston and thus to the cylinder wall and also the valve seating to the cylinder head. You can have a good stock engine give a higher "pressure" test than a worn high compression engine. Compression rations are built into (designed) the component selection process in conjunction with the engine assembler. Engine condition is determined in the form of a "leak down" test resulting in a percent of lost compression. Check out the page listed above for my results.
PS I have the 9.7 engine in my 1800 but I also did the same to my 544 (virgil) but kept the stock cc in the head because I drive the 544 all winter and cold cranking has a lot to do with winter driving and starting.
Christopher Virgilthevolvo