2. Bigger is not always better! A large port doesnt always mean more power. This is especially true with street heads and in some cases, race heads. Velocity is just as important as flow. 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 0.600" lift means low velocity (especially off-idle to 3500 rpm) and results in less throttle response. For the street, velocity is the key to overall performance.
3. Compare peak flow and low lift flow. Cylinder head buyers have a tendency to only consider peak flow numbers. Its important to look at
all the flow numbers, from .100" to peak, in order to determine the performance level of a head.
4. Type of valve influences flow. The type of valves used will influence how well a port flows. A valve with an undercut stem is less of a restriction to flow and will allow a port to flow more, filling the cylinder better and producing more power.
5. Consider this fact for street applications. Generally, in a street application, 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.