So not a day after wondering why the size of the secondaries
was backwards from what seemed seat-of-the-pants right to
at least two of us here, I got the answer. The answer is that
the equation governing the relationship between pressure in
a vessel and the interior area of that vessel is:
P = F/A
That is, pressure is equal to force *divided by* the area. Or, the
amount of pressure in a container is inversely proportional
to the area. Since a longer tube has more area, there's less overall
pressure if the collector is farther from the valve seat.
Further introspection indicates that lower pressure is going to
have a beneficial effect at low RPM also because the velocity
of the exhaust gases -- which depend on the piston velocity,
which in turn depends on engine speed -- will be fairly low.
Low velocity translates to low pressure, which in turn translates
to a situation in which low backpressure would be a Good Thing.
At high RPM, however, the pistons are going to be moving
much faster, hence gas velocity in the exhaust will be much
higher. This higher-velocity gas will be better able to overcome
pressure in the downstream part of the system, and also there
will be less time between valve events so a shorter distance
between valve and collector will be more effective.
Also, of course, high RPM doesn't mean more time between valve
events, it means less time between valve events. So you want a
"higher-pitched" tube for a 5000 RPM scavenging effect than you
want for a 3000 RPM scavenging effect, and higher-pitched in this
case means shorter. Which also has an added beneficial effect due
to the equation.
Hope this clears it up.
--Scott Fisher
--
Scott Fisher
fisher@avistar.com SEFisher@AOL.com
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