When pistons are sold with a given compression ratio with a particular head chamber say 64 cc's resulting in 9.7:1 compression. Is that compression ratio assuming a zero decked block or original deck height. and how do they get their compression ratio without knowing what head gaskets are going to be used?

I believe the manufacturers will have like 9.025 deck height, .039 gasket and the bore size to get the compression of the piston for a 350. It really depends on the manufacturer and what they go by.

The reason I'm asking is I built my 383 with KB hyper's. They are the 18 cc dished pistons. These pistons are designed for 5.7 rods on a 3.75 stroke. KB say's it will give a 9.7:1 ratio with 64cc heads. I'm running 68 cc AFR 195 heads. In order to get better quench and a little more compression to go with my xe274 cam I used .015 steel shim head gaskets. My block has not been decked and is the original deck height. I'm guessing my compression to be around 9.3. I wish I had checked the deck height while i had the heads off but never thought about it at the time. The pistons are 0.30 over and the compression height is 1.433. I'm just wondering if I could use a little more compression to take full advantage of the cam.

Dave assuming the 9.025 average deck height, a 0.039" gasket like the FelPro 1010 or 1003, and the 18cc dish,,, your 383 with a 64cc head will be at 9.13:1

Have it zero decked next time it's apart and that jumps to 9.64:1 with a more optimum quench.

So.....Seeing that I have 68 cc chambers I'm lower yet. I do have .015 steel shim head gaskets though. That makes my compression a little low for the xe274 then doesn't it? 236@50 :( Damn the so called tech people!

So.....Seeing that I have 68 cc chambers I'm lower yet. I do have .015 steel shim head gaskets though. That makes my compression a little low for the xe274 then doesn't it? 236@50 :( Damn the so called tech people!

yes I would run around 10.1 for the that cam.

Well not too bad,, the 4extra cc in the chamber does a little, but the .015" head gasket more than makes up for it. Making those changes your at 9.29:1

I don't think the compression is hurtin you that much. Sure it's not optimum, but going to 10.5:1 wouldn't mke night and day difference IMO

Thanks for your help Marv!