JMO, General rule of thumb is 8.5cr @ 76cc - 64CC =12CC + 8.5 should get you pretty close to 9.7CR, without getting real technical with head gaskets, quench, etc. This should be fine with cast iron heads depending on Cam with out having to worry about pinging, with Aluminum, you can usually get away with 10.2cr Cam Pending. Too short of a cam duration bumps running CR up, long duration lowers running CR, too long a Duration cam with out other mods results in low performance.