In Reply to: pistons posted by Guibo on September 08, 1999 at 00:34:15:
The actual compression ratio depends on whether or not the head has been cut, the head gasket thickness, the overbore size, valve protrusion, and the piston crown pop-up.
For a BMW with the E-12 (piano dome piston) the height of the dome above the piston deck will determine the compression.
1.5mm = 8.1:1
4.5mm = 9.0:1
6.8mm = 9.5:1
10.0:1 compression is only possible by using the 6.8mm dome and cutting the head.
Be careful, high compression without careful ignition timing and very high quality (high octane, fresh) fuel can be fatal.