Polishing is not only unnecessary, it's actually harmful, in two ways. The first concerns the airflow itself. The experience of countless engine builders, plus research by Superflow, Boeing, and various members of the Society of Automotive Engineers, indicate that a smooth surface is not necessarily the slipperiest where air is concerned. Ever notice that an airplane's wings are not shiny smooth, but rough? The rough surface reduces air drag, and the wing glides through the air easier. Olympic bobsled teams stick sandpaper-like skateboard grip tape to the sides of the sled and the tops of their helmets for the same reason. Air tends to get lazy on a smooth surface. It's called the "boundary layer effect," and it refers to the fact that an ultra-smooth surface accumulates air pressure next to it. This pressure is really stagnant air, and it effectively obstructs airflow. A slightly roughened surface eliminates this boundary layer and increases airflow. The second way polish is bad concerns combustion. Intake tracts don't just flow air, remember. They flow an air/fuel mixture, whose behaviour is quite different from plain air. The same lazy air boundaries that constrict a polished port's airflow also make fuel "drop out" of suspension in that air. The mixture then enters the cylinder less thoroughly mixed, and poor combustion and reduced power result. For this reason, most builders glass-bead the port walls instead of polishing them. The slight roughness creates tiny eddy currects which keep fuel droplets suspended in the port airstream. Good combustion is thus ensured.
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