Miata Mailing List: November 1993, Message #53
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   From: meek@me.mech.utah.edu
Subject: Re: Performance Exhaust Systems
   Date: Wed, 3 Nov 1993 19:51:47 -0500

>Miata Ninjas,
>This may not be the proper forum, but I have a basic automotive question for 
>the Miata hot rodders out there in net land.  What is "scavenging" in a 
>performance exhaust system and how does it work?  I know that the purpose is 
>to induce a low-pressure pulse at the exhaust port as the exhaust valve opens 
>in order to enhance exhaust gas flow out of the combustion chamber.  What I 
>don't know is how it's accomplished.  I've got a vague idea of creating a 
>backwards traveling low-pressure wave which is timed at a certain rpm to 
>achieve the effect.  How and where is this wave created?  How is this related 
>to the length of the header tubes as opposed to the whole exhaust system?  
>Does the length/volume of the post-header exhaust system alone play a role, or 
>would you get the same performance with open pipes?  Is backpressure a factor? 
> Anybody know an SAE pub or a book on this?  Thanks in advance.
>-Al


I'll try a simple explanation.  This is sort thermodynamics 101 and
vibrations 101.
The power of the engine is basically a function of the temperature
difference and pressure difference of the cycle.  So you want a very high
temperature for the combusion part of the cycle and a very cool temperature
at the exhaust part of the cycle.  The same thing goes for the pressure. 
You want a large pressure change through the cycle -- a high pressure at
combusion and a low pressure at exhaust.  The exact thermodynamic laws we
won't go into.  So you can increase the pressure on the intake (turbo and
super charging) and/or lower the pressure on the exhaust.  Usually the
exhaust is not pumped out, though that will increase the power of the
engine and has been done but not as effectively as boosting the intake. 
You only have 14 psi to work with on the exhaust.  So what you try to do is
to design the exhaust to be "tuned".  The exhaust comes out of the engine
in a pulsing manner.  A scavenger or tuned exhaust pipe is of the length to
set up a standing wave (not a travelling wave) in the pipe -- essentially
like an organ pipe.  The node of the standing wave is at or near the
exhaust valve.  The node is the low pressure point of the standing wave. 
(lost yet?)  So the gasses coming out of the exhaust valve "see" low
pressure and so you get higher flow rates.  The only problem with the idea
is that it only works for a limited frequency range (or harmonics thereof).
 So the "tuned" exhaust is only tuned for a limited range of engine speeds.
 These are presumedly the speeds which you want to operate at.  Ideally,
you want a variable length exhaust pipe which automatically sets the ideal
length for the instantaneous engine speed.  You also want the intake pipes
at an ideal length.  So friends investigated the possiblility of a variable
length intake so the intake pressure could be increased (and therefore the
power increased) without a turbo charger.  It gets, as you can imagine,
rather complicated for a multiple cylinder engine and therefore is not very
practical.  For any intake and exhaust system you are basically trying to
have the minimal resistance to the flow of the gasses.

Hope that helps;

Sandy

Sanford Meek
meek@me.mech.utah.edu