The following article is broken down into four sections, click on each to link to that section:
Bowl - area of the port directly above the valve.
Throat area - area of the port looking in from the manifold surface.
Short side radius - the tight curve on the port floor as it turns from the throat and enters the bowl.
Undercut stems - valves that have a smaller shaft diameter above the head of the valve in the bowl area to improve flow.
Bowl blending - smoothing the bowl (area directly above the valve) to the throat (area of the port looking in from the manifold surface) to remove any high spots left in the head by either the casting process or any machining process.
Port match - machining the throat (area of the port looking in from the manifold surface) to the same size as the gasket opening. This is typically done so at the gasket surface of the port is the same size as the gasket opening and then blended into the existing port by ½" to 1" within the port. Also refereed to as gasket matching.
Three angle valve job - the process of cutting the valve seats at three angles (typically 30°, 45°, and 60°) to smooth the transition between the port and the combustion chamber.
Volumetric efficiency - a ratio of the mass of air actually flowing through the port verses the mass of air theoretically capable of flowing through the port for the given volume. The high the number the better.
Home porter - the amount of work that can be done by the average person with a die grinder and no flow bench to clean up a set of head to improve flow with out damaging the heads or killing the flow velocity (also see stage 1).
Pro porter - the work done by a professional engine shop with a flow bench to maximize the volume of air that can go through a port while still maintaining acceptable port velocity (also see stage 3).
Stock - heads that have no clean up of the cast condition. All casting imperfections and machining edges are left untouched.
Stage 1 - heads that have minimum clean up work to improve the volume of flow over the as cast condition. Typically heads in this category will have the bowls blended, gasket matching done, and a good three angle valve job.
Stage 2 - heads that have cleanup work such as full exhaust porting in both the bowl and throat area. The intake will be gasket matched and the bowl will be fully ported. Both ports will usually have some work to enlarge the ports to improve airflow. Typically heads in this category will have some combustion chamber work also to improve flow.
Stage 3 - these heads will have full race porting on both the intake and exhaust side. The ports will be enlarged to the maximum possible with the optimum cross section and contour to get the most flow possible. The volumetric efficiency will be very high on these heads.
CNC machining - a process where a computer controlled equipment is used to machine each port out with a minimal of handwork. This operation has the advantage of making all ports exactly alike.
Extrude hone - a process where abrasive putty is force through the ports to remove material. This process has the advantage of leaving a port with a very high volumetric efficiency.