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what I always believed

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Port and polish Often mis-used term. Back in the early days, aport was ground as large as you could get it and then polished to a mirror like finish with the thinking that smooth was better for air flow. It also looked pretty. When flow benches came on the scene, it was discovered that the polishing part was actually hurting the port. The greater the air speed, the more it hurt.

How can that be? There is this thing called a boundry layer which is actually caused by the friction of the air as it makes contact with the port wall. This friction causes the air to tumble at the point of contact. Compare this to a wave reaching the beach. The closer it gets to the sand, the higher the wave gets until it collapses. Now, with a polished surface, the air does the same thing but because of the drag caused by the polishing (yes, drag), the wave height becomes so high that it effectively reduces the size of the port. Eventually, the wave can no longer support it's self and collapses, only to start over again. Now you have turbulence.

Air needs a lubricant to eliminate this drag. The best lube for air is a cushion of "air". Fine scratches perpendicular to the direction of flow act as a tripping point for the air that contacts it. The air tumbles at every scratch forming what is called a "boundry layer" that about the same size high as the scratch is deep. The air flowing in the port uses this boundry layer as it's cushion or lube and therefor is more resistant to turbulence. You can demonstrate this to yourself by pushing your finger over a mildly abraded surface and then a piece of glass.

So after all that, the "term ported and polished" has stuck with us even though "polishing" hasn't been done for over 20 years.

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