When comparing airflow numbers between cylinder heads, the folks at both Brodix and HVH point out that there are several things to consider (some of this is targeted squarely at the street-machine crowd, but it also applies to a large number of vintage Corvette racing applications). The following are general guidelines that anyone should consider when purchasing cylinder heads, no matter what the application.
Velocity is the Key
Large ports don't always equate to more power. This is especially true with street cylinder heads and, in many cases, dedicated race heads. Velocity is just as important as flow, especially for seat-of-the-pants performance that moves the vehicle when you step on the throttle. A smaller port volume generally equates to higher velocity for better street performance. The speed of the mixture determines how tightly the combustion chamber is packed. The more tightly packed the combustion chamber, the more pressure is developed when the mixture is ignited, pushing the piston with more force for more power. For example: A large port and a big flow number at 0.800-inch valve lift means low velocity (especially off-idle to 3,500 rpm) and results in less throttle response. For the street, velocity is the key to overall performance.
Why Small Ports Work
Typically, the smaller the valve diameter, the better the velocity and flow will be with the correctly sized port. This holds true for both the intake and the exhaust ports. When you can equal the flow of a larger port and larger valve with a smaller port and smaller valve, you have a much more efficient port, which will generally make more power and use less fuel.
The Range Of Flow
Compare peak flow and low lift flow. In many cases, enthusiasts tend to consider peak flow numbers only. They don't take low lift flow into consideration as an important factor in how a cylinder head performs. It's important to look at all the flow numbers, from 0.200-inch valve lift to peak valve lift, in order to determine the performance level of a head.
Flow versus Valve Lift
Big flow numbers usually equate to bragging rights, but you should always consider the camshaft as part of the package. If the camshaft has a maximum lift of 0.650 inch, worrying about cylinder head port flow numbers at 0.800-inch lift is a waste of time. The cylinder heads and the camshaft should be selected as a package.
According to the experts at HVH, a street driven engine spends most of its time in the mid-lift area of the camshaft (valve lift figures). Because of this, it is critical for the head porter to take the mid-lift figures into consideration. In fact, HVH spends much of its time working on flow in mid-lift areas, hence the strong flow numbers shown in the chart.
Street-driven Corvettes, drag race Corvettes, and road race Corvettes operate over wide rpm ranges. Circle track engines actually spend their time in a relatively narrow operating band. In a drag, street, or road race car, as the clutch is dropped or the engine comes off the converter and during each of the gear changes, the rpm drops. According to the experts, this engine speed drop can be in the range of 2,000 rpm in less than 0.05 of a second (that's a lot). This rapid drop in engine speed brings with it a rapid change in intake flow dynamics. That's why small high-velocity ports, small high-velocity intake manifolds, and relatively small carburetors are often quicker and faster than their "bigger is better" counterparts.