Cylinder head porting refers to the process of modifying the intake and exhaust ports of an internal combustion engine to improve quantity of the air flow. Cylinder heads, as manufactured, are usually sub-optimal for racing applications due to design and are made for maximum durability hence the thickness of the walls.
For those not wishing to radically modify the ports, port irregularities and casting lumps (called flashing) are removed. The critical area of valve-seat-to-port is blended and the junction of the adapter to-manifold is also matched and blended, the complete port and combustion chamber reshaping, the area is shaped and contoured to maximum flow and while maintaining flow velocity for power increases throughout the RPM rang.
It is popularly held that enlarging the ports to the maximum possible size and applying a mirror finish is what porting entails. However, that is not so. Some ports may be enlarged to their maximum possible size (in keeping with the highest level of aerodynamic efficiency), but those engines are highly developed, very-high-speed units where the actual size of the ports has become a restriction. Larger ports flow more fuel/air at higher RPMs but sacrifice torque at lower RPMs due to lower fuel/air velocity. A mirror finish of the port does not provide the increase that intuition suggests. In fact, within intake systems, the surface is usually deliberately textured to a degree of uniform roughness to encourage fuel deposited on the port walls to evaporate quickly. A rough surface on selected areas of the port may also alter flow by energizing the boundary layer, which can alter the flow path noticeably, possibly increasing flow. This is similar to what the dimples on a golf ball do.
CNC porting is very precise because it is computer controlled. We can calculate the required cross sectional area and machine the port to that dimension every time (within less than .001”). We can also maintain consistent size throughout the port. Therefore, each port in the head will be identical and every ported head will be the same. This is a huge contrast to porting each head using grinding and sanding tools by hand.