"Don Foreman" wrote in message
...
On Fri, 19 Oct 2007 16:55:04 -0500, "Robert Swinney"
wrote:

Did you ever hear the one "Nature abhors a vacuum." Pull the air out
of anything and gravity
pushing on the atmosphere causes it (the air) to rush in. Suction really
has nothing to do with it,
except that was the method used to eliminate the air. Gasses can be
eliminated in other ways, such
as the "getter" in the envelope of a vacuum tube at evacuation.

A correct answer would be "due to pressure differential". Manifold
pressure is seldom atmospheric. It's often lower (engine vacuum) in
an ordinary engine but it might be higher in a turbocharged engine.
"Suction" is created by a lower pressure region causing a pressure
differential, so "suction" is closer to right in this case.

The trouble with that is that "suction," like "centrifugal force," is not a
term that scientists or most engineers would accept, except in casual
conversation. Suction is just the result of a lower pressure acting
differentially to a higher pressure; centrifugal force is just the effect of
acceleration against the true force involved, which is the centripetal
force.

When I see "suction" used in a technical discussion I can accept it as a
casual term and assume that the person speaking, if he or she is technically
knowledgeable, knows there really is no such thing as "suction," as a real
force. But I'm not used to seeing it on a test of technical subjects.

There really is no "suction." And there really is no "centrifugal force."
They're useful concepts but they aren't technically correct.

--
Ed Huntress