I'll be honest I thought buying one of the full rebuild kits would be enough. I'll be doing some more reaserch it seems haha!
Not quite that easy. The compression is largely based on the cc's of your heads; the chamber. You can guess at it based on factory numbers which can be easily gotten off the web -but may not be accurate as heads vary and you may not know if your heads have been milled down at some point. I am using the iron 7K3 heads which are said to be 96cc's. I did some clean-up grinding/polishing in the chambers just below the intake valve. The height at which your valves seat after your valve job & the type of valve contour can effect cc's. Milling of the head can reduce cc's. So that all said, with heads assembled, my heads measured out at 98cc's.
The other measurements needed are the head gasket; thickness you are using and its bore size, not your cylinder bore size as the gasket is usually larger. Head gaskets come in different thickness. Common is .038"-.040", but can be less or more depending on what you want. My 455CI has Cometic gaskets (the best & most expensive) at .027".
Then you need to know how much area above the top of the piston at its top most travel, or top dead center (TDC) -this is often called "down in the hole". Pontiacs typically are .015"-.020". My 455CI was .020" as measured with the crank/rods/pistons fitted. This is where you will see the term "zero decked" used where the top of the block is milled to bring it level with the piston. I don't care to do this, but the real purpose is to develop a good "squish" area between the top of the piston & the head at the flat area (that part that isn't the combustion chamber) of the head that sits over the piston. The number to get is typically said to be .035", but this depends on your piston type as forged expand more than cast & rod type -aluminum rods need more clearance as they can stretch. When the piston squeezes up to the head, it pushes the gas mixture in that area into the combustion chamber & eliminates any chances for pre-ignition/knock -which is the true reason you are doing this. So you may want to go .040" - .045". With my 455CI, .020" down in the hole, forged pistons, and .027" head gaskets, I have a quench area of .047". So when everything heats up and expands, that total number will be smaller and right where I want to be.
Ok, now you have your head cc's, your head gasket cc's, and the amount of area above the piston ( bore size x .020") in cc's. The last part of the formula to determine compression will be the amount of area found in the valve relief's at the top of the piston. This is where you match your pistons to the head cc's you are using. Valve reliefs can be 6-30 cc's as a means of adjusting the compression ratio you are seeking. Of course the heads are also the way to adjust cc's by selecting a chamber with smaller or larger cc's, BUT, if you already have the heads or you have the smaller chambers, then you use the piston to dial in the desired cc's needed to get a pump friendly compression. On my 455CI I used the K-B ICON FHR forged pistons which had 10.8 cc's at the valve relief area. Taking all the numbers I had and adding them up, my compression sits at 9.02 to 1. 9.25 would have been ideal with 9.5 the max with iron heads unless you wanted to run race gas/additives. This is just where I want it for iron heads. If I had ran aluminum heads, I'd have shot for 10 minimum or 10.5 which would have probably been ideal. Compressions below 9.0 are not to be feared as they will still make your engine haul ass and be able to use low octane gas. The trick is to select a cam that raises what is called dynamic compression, which is typically the Comp Cams 110 LSA ground cams. I used the XE274 in my previous '72 400 with 7K3 heads and the replacement pistons in a rebuild kit which probably gave me 8.2 to 1 compression. Engine pulled like a train, but power dropped off like a stone at 5,500 RPM's which is a good thing for stock rods.
That compression is now used to select a camshaft designed for your engine's compression, and the max RPM you plan on spinning it under wide open fun -which is also based on the internal parts selected so you don't grenade the thing; everything has to be matched. Cam choice has other variables as well, but compression is the largest consideration in making a cam selection.