Compression Ratio equals Total cylinder volume divided by compressed volume.
X= The rest of the combustion chamber volume including the inverted dome in the piston and the clearance volume.
From the back of the Bentley, page 27 of technical data, the compression ration of the 2.1L is 9.0:1. I used a large plastic syringe I purchased from Advance Auto. Using this I came up with a Head Volume of 16cc for the 2.1L, 12cc for the 1.9L and 50cc for the inverted dome in the piston. The metal head gasket thickness came in at .09cm (6.2cc volume). For a cylinder displacement I arrived at 527cc (2109cc/4)
Plugging in the above values and using 16cc for the Head Volume I came up with x=49.875cc(not too far off the amount I measured).
If we figure the Compression ratio using the 12cc value of the 1.9L heads and using the value of X above, I come up with a compression ratio of 9.5:1.