Maybe a couple tenths at the most if that much. For a street car I would keep the comp in the pump gas limit. But if you build good quench and run the right cam you can run in the 10's with iron heads. I run 10.0 on the 440 in my 63 with 906 iron heads. I have quench as I use the KB quench pad pistons and I can run on 92 pump with 38 total and no ping at all. It has alot to do with the setup such as cam and quench and cyl pressure. If you dont have good quench I would keep it around 9.5 for iron heads and 10.5 with aluminum. With quench I figure about a 1/2 a point higher. Ron