Darin explains all of this in other videos. A wedge head motor is generally capable of rpms in the 9000-9500 range because the cfm/sq in is limited by the in line valve arrangement. A canted/rotated valve head can support more rpm because it can flow more cfm/sq in of valve area and because the chamber is usually more efficient. The hemi head is capable of the highest cfm/sq in flow, but the chamber design limits it's efficiency.