I guess it depends on who's stating the "old theory"...

Darin Morgan: "Make no mistake, velocity is the primary variable in the design of the entire induction system. I often say that my job title should be Velocity Manager instead of Cylinder Head Designer. Air speed is 10 times more important than raw flow numbers. If you kill the velocity by 10 percent, you will kill almost 40 percent of the wave and ram energy that dynamically fills the cylinder! Raw airflow cfm is an important variable as well; it's just not as important. If you buy a cylinder head that is properly sized for a flow of 400 cfm and your engine is only asking for 350 cfm, you will not only fail to achieve the power potential that the 400 cfm would have given you, you will also fail to reach the power that the 350 cfm would have given you. That's because you killed all the air speed in the induction system. If your engine is asking for 350 cfm and you give it a properly sized cylinder head flowing 350 cfm, your airflow demand is matched and your air speed is matched. You then have a chance of achieving the power potential that 350 cfm can give you."

"How much power potential can 350 cfm give? Well, that depends on a host of variables such as engine speed, overall induction system design, and piston speed. To put it in basic terms, the less restriction you have in the induction system and the more freedom you have to attain increased engine speeds, the easier it is to extract the full potential of the 350 cfm available. Most people don't know how much airflow their engine is actually asking for! This leads to builders wanting to purchase cylinder heads with way more airflow than their engines can possibly use. The end result is a low air-speed induction system that can't properly fill the cylinder by means of dynamic inertia and harmonic supercharging, which means the engine will never reach its full power potential."

"That said, a good cylinder head port design will flow a lot of air for its valve size. The bad news is that a bad port design will flow just as much if not more air! Airflow alone won't tell you if a port design will reach its power potential with 100 percent certainty. Everyone knows that it's easy to compare two 23-degree small-block Chevy heads with 220cc ports. Just pick the one with the most flow, right? That's about all the average builder can do, and in a lot of cases it's hit-and-miss. There are multitudes of ways to achieve that 220ccs. You can have a big pushrod pinch section and a very small bowl area, or a huge bowl area and a super small pushrod pinch area. One 220cc port can actually be choked off at the pushrod, short-turn radius, or throat area, hurting top end power. Another 220cc port design can have too small of a bowl area and too large of a choke and hurt power and torque equally across the entire power range. Having extra airflow isn't always bad, but it can't come at the expense of air speed. The ports must be sized properly. The amount of air Pro Comp Eliminator engines are asking for are exactly how much the heads flow, and that's not a coincidence. People want to make cylinder head design simple, but it's not. It's very complex and interdependent on a massive amount of variables."