Many '71s had closed chamber heads, but not all. Open-chamber heads have combustion chambers that are nearly round while closed chambers have combustion chambers that are smaller, oddly shaped and have a flat area as part of the chamber. The closed-chamber heads with flat top pistons and a stock gasket produce higher compression ratios than open chambers, and are more efficient besides. I wouldn't even guess what C.R you have-after 30 years, shops could have cut one or both of the heads, or the block, or sunk the valves during a valve job, all of which will change the ratio you get. In addition, there are three thicknesses of head gaskets available. Bottom line: to find what you have, someone will need to do some measuring of combustion volumes on your particular engine combination. This process is better described in whole articles by Isky, Crane Cams etc, but you do NOT want to run more than 9:1 compression ratio in a 351C with a stock cam and iron heads. Even then, you're probably going to need 92-octane gas. The more you modify the engine, the higher ratio you can use with the same octane, up to a point.
