well I can deff tell u that a lower CC head will up the compression not lower it.. here is my guess 11:1 if u were at 10:1

why I say that is cause my setup I am buying is a 8.5:1 at 68cc heads.. if I go to 72cc heads my compression drops to 8:1

so if u started at 72cc heads and were at 10:1 and u lower thre chamber size in effect upping the compression my guess is 11:1

if u went to a 68cc head u would prolly be at like 10.5-10.8:1

Dave