To Increase CR (compression ratio) is the easiest way to boost hp. But Higher Cr means higher octane fuel. The higher the compression ratio, the higher the air temperature in the cylinder at the end of the compression stroke. Higher compression ratios, to a point, lead to higher thermal efficiencies and better fuel economies. Diesel engines need high compression ratios to generate the high temperatures required for fuel autoignition. In contrast, gasoline engines use lower compression ratios in order to avoid fuel autoignition, which manifests itself as engine knock, often heard as a pinging sound. Hence the need for Hi Octane.
-Mill the heads to increase the compression ratio. This is a simple, easy way to increase the compression of the engine without having to disassemble the top end.
For street engines with bolt-in cams, limit the milling to .050 inches. When the engine is assembled, use one of the .030 inch thickness head gaskets instead of the normal .040 or .060 inch gaskets. This will increase CR.
-Change to a domed piston. This method of increasing compression is more expensive, but more flexible than milling the heads. Most high compression pistons will come with valve relief's allowing the use of high lift cams. This JE domed piston raises the compression in a stock combustion chamber to 10.5:1.
-Welding up the combustion chambers and reshaping the chamber. This has to be considered the most expensive way to increase compression. It also requires the most expertise to properly perform the required work. Reshaping combustion chambers is most effective when performed by an experience engine builder.but in such cases you must check that the valves have enough clearance to miss the piston.
To decrease CR you can do the reverse of the 1st two above mentioned methods. and
-there is a much easier method of decreasing CR add a 1.4mm or thicker metal head gasket depending on what ratio you want.
But your cams lift have to be matched with the CR otherwise the effects might be disastarous