"The better measure of head efficiency is computing the coefficient of discharge. This is simply the airflow at each lift point divided by the valve curtain area, which is valve circumference multiplied by lift. Let's say we have two heads that both flow 300 cfm at 0.400-inch lift. One has a 2.200-inch valve and the other has a 2.100 valve. If you multiply each valve by Pi to get the circumference, you can then multiply that figure by the lift to obtain 2.76- and 2.63-inches, respectfully. Now divide the 300 cfm by each valve circumference to obtain 108.7 and 114.0 cfm/inch, respectfully. I know that is an odd unit of measure, but it is the correct terminology. In this example, you can see the smaller valve clearly has more velocity, will be easier to cam, and will generally run faster. This is the best way for the average consumer to compare two heads to one another."
Tony McAfee: "The area of a port dictates how much flow is capable through a given orifice size. We probe different parts of port in order to measure velocity in feet per second. People often use too big of a cylinder head on a motor. Our 227cc small-block Chevy heads flow 50 cfm more than our 180cc as-cast heads, but won't make as much power on a typical 350 because the engine can't use the head's flow potential. Now if you put a bigger cam in it, raise compression, and install a better intake manifold, then the 227cc heads will work much better. Generally, as the rpm range of an engine goes up, the more air it can use. The more cycles per second a motor turns, the more velocity and airflow it needs. High-velocity ports aren't as important, but are still necessary on a high-rpm motor. After about 350 feet per second, fuel becomes detached from the air, so there is a tradeoff. Sharp areas near the valve job helps put the fuel back into suspension. This is important, because even with good atomization, some fuel still runs down the walls of the port. That said, without testing it's hard to figure out what head you really need. Your best bet is asking people with experience building similar combinations to yours for advice."
Darin Morgan: "The short-side radius is a big factor and is tied to the main overall advantage of raised intake runners. By raising the port entrance, you're taking air speed velocity load off the short-side radius. The lower the port gets, the more the air speed at the short-side radius goes up, and the more critical its shape becomes. Lower ports force you to lower the air speed to get the air to negotiate the short turn, which wastes energy. Ideally, the short-side radius needs to be shaped to have the highest air speed throughout the rpm range without disrupting the boundary layer. In essence, the short-turn radius controls torque, mid-range, and top end power. If you stand it up and make it really fast, you will increase torque and kill top end. If you lay it back and reduce air speed, it will kill low-end torque and increase top end power. Not only do raised intake ports offer more latitude in shaping the short-side radius, they also yield smaller fast-burn combustion chambers and offer a better overall induction system path and design. The higher ports move more air through the engine since the airflow path is straighter, and enables a properly tuned intake manifold runner to be aimed directly at the carburetor booster. The straighter flow path keeps the air/fuel mixture in suspension far better than any low-port flow path ever could, since there's less frictional loss and fuel fallout. An airflow mixture entering the combustion chamber from a straight high-port induction path is more homogeneous and burns faster, producing more power with better BSFC numbers.