70" 340 vs 72" 340 specs please

I cant seem to find the changes made from the early 430s to the 72,73 340 motors.I have read that the compression went from 10.4:1 to 8.5:1 but I couldn't find why.Anyone know what they changed on the motors or was it just gross and net HP and they were closer then they made them out to be.PRE 72 275hp 72,73 240 hp.