Miata Mailing List: November 1993, Message #42
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   From: gday@mprgate.mpr.ca (Gordon Day)
Subject: Performance Exhaust Systems
   Date: Wed, 3 Nov 1993 12:35:47 -0500


Al, I can't really give you a sophisticated explanation of it, just my
understanding from motorcycle exhaust systems.  If you have an
expansion chamber of a suitable volume and distance from the exhaust
port, then the vaccum created by the expansion of the exhaust gases
will (as you put it, the 'backwards traveling low-pressure wave') will
arrive at the port just as the exhaust valve opens.  From what you
wrote I'm sure that you already knew this and what you are really
after are the volume/distance numbers, but I don't have a clue.


>>>"al" == Al Jenab  writes:

al> Miata Ninjas, This may not be the proper forum, but I have a basic
al> automotive question for the Miata hot rodders out there in net
al> land.  What is "scavenging" in a performance exhaust system and
al> how does it work?  I know that the purpose is to induce a
al> low-pressure pulse at the exhaust port as the exhaust valve opens
al> in order to enhance exhaust gas flow out of the combustion
al> chamber.  What I don't know is how it's accomplished.  I've got a
al> vague idea of creating a backwards traveling low-pressure wave
al> which is timed at a certain rpm to achieve the effect.  How and
al> where is this wave created?