Cylinder Heads 101

Andy, What they are talking about is a thing called a "boundry layer". any material flowing donw a tube has friction from the sides of the tube. In a port's case, the material is air, carrying fuel in peices as small as molecules, and as large as visible drops. The fuel is heavier, and tends to want to "fall out" of the air stream. By leaving the rough surfaces, you create all kinds of tiny turbulence along the walls. These tiny eddies and stuff makes it harder for fuel drops to attach to the walls. Also, you reduce the surface area for heat transfer from the port wall to the mixture by a bit, which can also lead to fuel puddling at low engine speeds and larger overlap cams. Really, there is always a boundry layer of some size, and a rough surface vs smooth makes a very small difference in the thickness of the layer. On the exh side, smooth surfaces are harder for the carbon and stuff to attach to, so usually you want a nice smooth exh side. In intkae ports, I dont even touch the floor. The majority of the flow ina stock port is along the top and the shorter side wall (the "inside" port wall) until you get to about an inch upstream of the sort side radius into the bowl. If you've ever looked into a clean intake port, you can see the fuel stains where the air is moving slowly. The goal of a head porter is to keep the port as small as possible for the given engine, to keep the port stable and fast. Not to straighten every bend, and enlarge everything to the max.