Quote Originally Posted by IC2 View Post
many Ford 302/351 engines used heads with EGR ports or holes in the exhaust port that allowed air to continue the combustion process, if necessary, in the exhaust system for emissions control. If for instance a head was changed at some point in the engines life, one ended up with those holes and the other didn't, you might have additional air flow right at that port. The heads could even have the same base part number and be a different configuration.
Beautiful assessment. I never would have considered that.