Porting a head is pretty self explanitory. The passages are opened up thus allowing more air/fuel to flow. Polishing is an old school term and is actually detrimental to performance! Yes... you'd think polishing the intake track very smooth would ENHANCE the flow and reduce friction, right? WRONG!!
Read this...
Last on the list of factors affecting airflow quality, is port finish. The fact is, a properly finished port is critical to the performance of the engine, as it is the final word on airflow quality. However, when "port finish" and "high performance" are mentioned in the same sentence, most enthusiast immediately believe that the finer the finish, the better the airflow. This is an easy trap to fall into, as utilizing the "finger test method," one would consider a port which feels smooth, must flow "smooth" as well. Unfortunately, this is not necessarily the case at all.
The airflow through a port of an internal combustion engine is what is known by fluid dynamists as "fully developed, turbulent flow." This simply means that the flow is not of a laminar state, and instead consists of molecules of air continually flowing at different velocities all the way from the center of the port, out to the edge of the port. For a perfectly straight pipe (as in our first example), the molecules in the very center are flowing the fastest, with those closer to the edges slowing incrementally as we near it. The molecules directly in contact with the edge of the port are in fact, "stopped" for all practical purposes and this is all due to the viscosity of the air itself. Potential problems due to these effects include flow shearing, as more molecules attempt to go faster near ones which are going slower. This tends to help promote fuel drop out and centrifuging, and poses further resistance to flow motion.
Since we cannot change the viscosity of the air, we must instead work to find a way to keep the flow from adhering to the port walls. One way to do this is to create a rough (textured) surface which in turn will promote what are called "Karman ring vortices." These vortices will "energize" the layer near the surface of the port wall, acting as some head modifiers have stated, "like needle roller bearings." The results of this phenomena act to reduce the adherence of the airflow to the port walls, allowing the bulk of the flow to proceed in a more homogenous, less disrupted fashion.
Just as cross sectional shape, size and consistency can be used to influence the airflow, port texturing and finish can likewise be used to our benefit. This means that, not only should a port be of a courser overall texture, but this texture should also be used to influence (or at least not degrade) the flow pattern in that specific area. If you haven't gotten all of this, don't worry. What it all boils down to is the fact that a finely polished port will not produce optimum results. We use anywhere from 36 to 80 grit on our manifolds and heads with the latter not being for the intake ports!
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I was hoping to get a free Tornado with my polish job....
