dogdays is right on, flow has a lot less to do with it than the pulse pull effect from headers. Heck you could run no exhaust after the port at all for the ultimate flow but it still wouldn't make the power that good headers would. This is why when that magazine article years ago compared all the manifold from 273 junk to 340 gold there was barely a difference in power from the best to worst but headers added about 25hp on a basically stock 5.9 magnum.