The compression ratio will be written above the engine number on the tab on the side of the top of engine block on the passenger side between the two centre exhaust pipes.
It should have a 8.3 or a 9.3 (please correct me if the numbers out not quite correct) above the engine number indicating the compression ratio.
The two engines are basically the same except for the pistons - so to upgrade from low compression to high compression you would need to change the pistons.
As a general rule HC will produce more power and more torque.
Garry
REMLR 243
2007 Range Rover Sport TDV6
1977 FC 101
1976 Jaguar XJ12C
1973 Haflinger AP700
1971 Jaguar V12 E-Type Series 3 Roadster
1957 Series 1 88"
1957 Series 1 88" Station Wagon
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