318 horsepower

Ah. Ok. there are several number syou need to get to calculate the compression ratio. If you dont know your piston specs for dish volume or valve relief volume these need to be measured for the calculators to work. For a set of heads, things like factory tolerances in castings and machining, valve jobs done in the past, maybe an engine rebuild, or valve replacement, or even sunk valves all combine to change the volume of the chambers. This is measured with a large burette or syringe, an alcohol based fluid, a small amount fo grease, and a flat plate of glass or lexan/ plexiglass about 1/4" thick with a countersunk hole in the center. You spread the grease around the circumference of the chamber, on the head gasket mating surface. You then place the plexi with the counterunk side up on the head, with the hole lined up near the top edge of the chamber. You can see if the grease is sealing all the way around by looking at the plexi. Air bubles mean leaks when fluid's in there. Then you fill the burrette or syringe with fluid (I use isopropyl alcohol w/windshield washer fluid to make it blue) and slowly release the fluid into the hole in the plexi, going slow to let the fluid displace the air completely. When it's full, and no air is int here, record how much fluid you unsed to fill it. That is that chamber's "CC figure". You basically do the same for the piston, but I like to use the 1" down method. you coat the bore lightly (piston down in the bore) with grease about 1.25" down from the deck surface. Then using a dial indicator (or ruler if you have to) bring the piston up until there is 1" of space between the deck surface and the top edge of the piston. (the flat just above the rings...not the center of a dish or dome). DO not move the piston in any direction but up, and if you go past 1" down, you have to go back and start from scratch with fresh grease. When the piston is 1" down, run a ring of grease around the top of the bore, again, hole in the plate near the highest side edge of bore (depends on what the shortblock is on/in). Then, fill and record how much fluid it took. That takes into account every nook and cranny, the shape of the valve head, the depth of the seats, the milling, the dome, valve releifs or dish shapes, the dead space over the rings..you name it. Then, when you're figuring the compression, you will need to modify the formula for the 1" downfill.