When considering NA volumetric efficiency comes into play greatly. Also, in an all motor head you have to consider peak flow characteristics as well as flow velocity. Get one or the other wrong and you don't make the power. Get them both right and you have a great power curve and a much more "efficient" engine at using the parts you attach. Valve overlap and exhaust scavenging have to be considered when determining optimal primaries, secondaries, collector, as well as length of a header. Again, NA is much more of a science that I have not even begun to scratch the surface of. That's why when having a head ported for a serious NA build you have to find the right person to do it. Hell anyone can take and enlarge ports, deshroud chambers, do bowl work just to make them bigger. But knowing how to do it where you maintain optimum velocity of the intake charge and optimal scavening properties is where the real pros earn their money.