The cycle of a typical internal combustion engine is called the four-stroke cycle. The piston rises and compresses the FAM before the spark ignites it. This increases the pressure and heat of the mixture near to its auto-ignition point. This will enable the mixture to burn at its fastest and consequently the fuels energy will be most efficiently harnessed. However if the compression ratio is increased too much then detonation may occur.
To raise the compression ratio material could be either skimmed off the top of the cylinder head. However the limiting factors with this modification are that the valves become increasingly close to the pistons, as well as increasing shrouding factor. Also the head blots may need modifying. Another disadvantage is that if ever the head needed skimming this may not be possible as the clearance between the valves and the pistons may already be at a minimum, meaning a new head is required.
A better way of reducing the compression ratio is to install high compression pistons. The top profile of these pistons increases the compression ratio without the associated shrouding factors of the previous method. The block will not have to be skimmed and there will still be generous clearance between the valves and pistons. This means that in future the head can be skimmed if needed. This method of increasing the compression ratio could be more affordable in the long term.