There's much more to cylinder-head selection than just choosing the head with the best flow numbers. In fact, as we've explained, large intake-port volumes with high peak-flow numbers may not be the right choice at all. There are actually several criteria for selecting the best overall cylinder heads for your application. The best way to look at cylinder-head flow is to closely examine and compare the entire flow curve from low lift to max lift.
If you must look at only one point, most airflow experts prefer to look at mid-lift flow. For street engines, airflow at 0.400 inch is a good place to evaluate all cylinder heads. The main reason for this is that maximum lift flow at 0.550- or 0.600-inch valve lift is at the ragged edge of most streetable valvetrains. More importantly, the valve is only at max lift once in the entire valve-lift curve, while the valve achieves mid- and low-lift numbers on both the opening and closing sides of the valve-lift curve.
Cylinder-head selection also means evaluating exhaust-flow numbers as well. A cylinder head with killer intake numbers but poor exhaust flow is not going to perform as well as a head with a slightly weaker intake but an excellent exhaust port. It is possible to prop up a weak exhaust port with longer exhaust duration and more lift, but generally that engine will not perform as well as an engine with a strong exhaust port and a more ideal camshaft. One way to evaluate the relative strength of an exhaust port is by comparing the exhaust flow to the intake flow as a percentage at the same valve lift. This is generally referred to as the exhaust-to-intake (E/I) relationship. For example, let's say the intake port flows 250 cfm at 0.400-inch valve lift and the exhaust port flows 187.5 cfm. Exhaust-port flow divided by intake-port flow will express this relationship as a percentage. In this case, the exhaust flows 75 percent of the intake at the same valve lift.
Conventional wisdom holds that anything more than a 75 percent E/I relationship is considered good. Eighty percent is even better. But anything more than this might be suspect. In other words, a great E/I may point to a weak intake port rather than to a strong exhaust port.