I have a 305 from an 85 Z28 4bbl, i'm researched on google and found that they had 9.5:1 compression, but what kind of piston's did the motor come with? Where they flat, dished or domed?

My set of now are flat top pistons with 2 valve reliefes (they have 0.030 stamped on them), but I cant remember if I had the shop put in a new set or just use the ones that were in there.

 

If the pistons now have "0.030" stamped on them, they are likely not the originals.

The original pistons should have been flat tops with four valve reliefs.

 

Ok, so with only a 2 valve, would that effect compression at all? I'd imagine if so, not by a noticable amount.

 

No, probably not by much. Stock 4 valve relief flattops usually end up with about -6cc worth of total "dish" volume. 2 valve relief (aftermarket) pistons would be not quite half that. About -4cc being typical. A tenth or two of a point difference in compression maybe.