Got to agree with twisted on Compression, Static and dynamic compression are 2 very differnet things, you can have a high static cr but if the dynamic cr is to high it will require race fuel, the camshaft will dictate what your DCR is. I ran 12.5's in 283 daily driver and even drove the truck fully loaded pulling a fulley loaded trailer from Texas to NH on pump gas and never had an issue. Even an engine with only 10.1cr and aluminum heads can require race gas if you have to small of a cam installed to blead of compression. As far the 12cr in that 6 chances are that the previous owners have the head shaved to the max along with pistons, a stock 250/292 head with out shaving it would probally drop you a full point minumum maybe even as much 1.5 points. Most of these guys will shave these heads .120 possibly putting you down to 58cc chambers vs stock 72-76cc chambers.
