Miata Mailing List: November 1993, Message #29
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   From: Al Jenab 
Subject: Performance Exhaust Systems
   Date: Tue, 2 Nov 1993 17:34:49 -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