Sound waves are pressure waves, but pressure is
To accurately measure the pressure of any
oscillating wave one needs sophisticated high speed transducers.
These are very expensive and are generally computer controlled.
It should be noted that waves in a medium (pipe
for example) are oscillating. With an oscillating waveform, the
amplitude of the wave will be constantly varying. There is usually a
negative and a positive component (value). The shape of the wave can
be uniform or very non-uniform (like I suspect an exhaust pressure
wave would mimic)
When measuring any wave with a mechanical
device (such as a pressure gauge) this gauge is very over-tasked for
this sort of a measurement. The result would be a RMS (root mean
squared) value, AT BEST, and would not accurately represent the
Let’s assume you have a steady frequency
sinusoidal wave in the pipe (which we do not but we will assume for
the moment) Taking a measurement with a gauge will give you the RMS
value of this wave. The RMS value will be an “average” at best.
Now assume a pressure wave that is constantly
varying it frequency, and amplitude (like a true pipe pulse) and
now, you get a pressure that can not be measured correctly with a
simple gauge-type device.
As we rise in elevation, atmospheric pressure
decreases. This is a given. So, if you do NOTHING but rise in
elevation, you drop pressure. So, ANY pressure measurement in a non
sealed medium will be LOWER just from the rise in elevation.
Now the pipe pulses are treated as sound waves
and the speed of sound is used in these calculations. The speed of
sound (in a non-sealed medium) is temperature dependant. As we rise
in elevation, temperature usually decreases and therefore; the speed
in which the pipe waves travel are slower and this why we need to
have increased pipe heat as elevation increases. You can increase
the pipe heat via several methods. 1) Minimize radiation losses via
wrapping the pipe with a TRUE heat barrier. 2) Increasing the
internal gas heat via added power (engine enhancements etc.) these
are a few. OR, you can alter the tuned length of the pipe to
compensate for the slower travel.
So, one would surely benefit from a pipe that
is designed around the lower atmospheric pressures that are present
at the higher elevations.
BUT, let’s say you increase your engine’s
internal heat and pressure via some true power increasing
modifications. Now the internal pipe pressure may still be lower
than if you were at sea–level BUT, it would be higher than if your
engine did not have the power enhancements and you would have more
rapid travel in the pipe.
So, the higher in
elevation we go, the less power the engine makes unless something
changes to process more air through the engine (Engine enhancements
etc.). The naturally aspirated engine loses power because it can't
process any more air. The engine can't magically grow displacement
or increase rpm
So, what is the point to all this?? ONE point
would be that the increase in elevation will produce less internal
engine pressure and, as a result, less pipe pressure. Increasing the
pipe’s internal pressure via stinger choking or restrictive
silencers CAN allow for more pressure to be placed back in the
engine via the “plugging pulse” of the pipe. BUT, this can come at a
cost. The pulse’s strength and length are determinant mainly by the
cone sections of the exhaust, NOT the stinger. The diameter of the
stinger is a critical component. WHY? Because it has some control
over the pipe’s operating temperature (internal heat). Too large of
stinger and you can lower the pipe heat to a point of power-loss.
Too small and you get an exhaust that can not bleed off pressure
effectively and the engine will be the recipient of this
“non-exhausted” heat and can cause engine failure, especially on
long WOT pulls).
Get the stinger diameter just right and the
engine acts like a tad shorter pipe and you can gain some power.
NOTE: I said “STINGER” not “SILENCER” The silencer is located AFTER
the stinger. Yes, the silencer does effect the rate of exhaust
bleeding but having too large of a silencer does not effect the
stinger’s operation. It does effect overall pipe temp.. having too small of silencer diameter can
effect overall pipe operation due to raising the internal heat and can cause
engine failure with prolonged running.
OK, so what do we know?
1) Increasing elevation will decrease pressure
2) Pipe pressures are largely determinant on the power
of the engine.
3) Internal pipe heat affects the speed of the
waves in the pipe.
4) The proper way to increase pipe heat and
pressure is to design a pipe that accounts for the higher elevation.
5) The Stinger section of the pipe has a strong
effect on the RETURNED internal pipe heat to the engine and how fast
the pipe cools or heats.
6) Increasing the returned pipe pressure via
stinger or silencer choking can cause severe engine failure but can
also aid in some cases.
7) Internal pipe pressure can not be accurately
measured via ANY mechanical gauge.
8) More power creates more internal
pressure/heat and can allow a pipe designed for low elevation to
work at high elevation.