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There are some phrases, words or terms used in the
porting world that really deserve an explanation. By clearing up the
"how and why" of these things, I hope to improve the understanding of proper flow testing.
The following are the most common questions we get.
CFM @ 28" This
term is used to define the test pressure at which the CFM was reached.
In order to duplicate a test, you always use the same test pressure. In
some cases 10", 25", 36" and a few other pressures are used. The
greater the pressure, the higher the CFM. 28" is generaly accepted
as the standard.
Test pressure Pressure
can be expressed as positive or negative number and these are often
referred to as depression for negative (vacuum) or plain pressure for
the positive. Flow testing intakes is always done with vacuum and
exhaust is done with pressure. So when you ask, "what was the test
pressure?", you get your answer knowing that it's negative or positive
based on the type of port, intake or exhaust.
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.
Depression A condition caused by too long a period of time away from the track!
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