You want as much compression as you can get away with to increase horsepower. In California, 10.5:1 is about the limit on pump gas because our gas changes blends from summer to winter. In a nutshell the summer/winter mix changes the gasoline's power output, thus decreasing engine power and possibly introducing ping or detonation. So how do we figure out compression and stay within a reasonable range for a street driver and great performance? It all starts with the bottom end; that includes the piston, deck height in relation to how far the piston is "in" or "out" of the hole, stroke, bore, head gasket thickness, head gasket bore, and what size combustion chamber is on the heads.
A piston that is -18.6cc and is out of the hole 0.004 is measured in a minus number (-0.004) with the online compression calculator provided, the same negative or minus CC number with pistons that have valve reliefs or dished. Example, bore = 4.030 with our .030 over Lunati 383 pistons, stroke = 3.750. Head gasket bore depends on the gasket manufacturer; let's say we are using the 1094 Fel-Pro gasket. The head gasket measures 0.041 thick compressed and 4.100 bore and if we are theoretically using a 64cc heads, this brings us to compression ratio of 9.6:1, but that only leaves us with .037 piston to head clearance.
Since our target compression ratio was higher than 9.6:1 we started to look at other options. So Gordy of Pfaff Engines and I started to look at flat-top pistons with -4.9cc valve reliefs to raise the compression. Calculating the same bore and stroke with a Fel-Pro 1044 head gasket (.051 with 4.200 bore) was getting us closer to 11:1 compression with the 64cc heads I already had. If we replaced the heads with 69cc heads we could bring that number down to 10.4:1, a perfect street compression. This would also give us an ideal .041 squish because the new flat-top pistons were out of the hole .010. Go online to the many online compression calculators to figure this one out. Here is one of my favorites: www.csgnetwork.com/compcalc.html.