Good luck on the Hemi !!
On the Chevy thing, from 1971 on, all passenger car engines had lower compression.
The car block remained the same, so compression ratio was determined by the pistons and cylinder head combustion chamber.
As 1970 was the only year for 454 engines to have high compression, a common up grade was to install earlier 396-427-454 (1965-1970) heads on a 1971 or newer 454 to regain some compression.
Original high compression, say 10.25 to 1, was obtained both through domed pistons and smaller (closed) combustion chamber heads.
Special High performance engines, say 375 HP, 396, 425 HP, 427 OR 450 HP 454 used even higher domes on the piston to achieve approx. 11 to 1.
The Off Road L 88, 430 HP, 427, had even higher compression, 12.5 to 1, using very high domed pistons with open chamber (low compression) heads.
The basic open chamber design was carried over to the 1971 and newer engines with flat top, or even dished pistons lowering compression ratio considerably.
The one exception was the 1971 LS6, 425 H.P., 454, 9 to 1, with small dome pistons and open chamber heads.
Many 454 H.O. and 502 crate engines follow this small dome and open chamber design.
A basic flat top 454 short block with closed chamber heads can yield approx. 9 to 1 compression ratio.
The engine tag probably reflected this combination, head casting #'s would verify.
Spark plugs are another clue, as long reach, 44 XLS style plugs were only used in closed chamber heads.
1970 was the start of the short tapered seat R44TS style plugs so there are some closed chamber heads used Tapered Seat Spark plugs.
Note: All GM H.P. and compression ratios are for comparison/identification purposes rather than actual facts!!
Thanks
Randy
