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Date:         Fri, 17 Sep 2004 02:41:50 -0400
Reply-To:     Dennis <guskersthecat@YAHOO.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Dennis <guskersthecat@YAHOO.COM>
Subject:      Re: Exhaust Pipe Question

On Thu, 16 Sep 2004 23:10:46 -0500, Robert Cardo <rrecardo@WEBTV.NET> wrote:

>You are 100% correct on the exhaust scavenging theory from the exhaust >valve/ port area Dennis. >However, in an exhaust system that relies on a converter with pin hole >sized passages for the exhaust to flow through the reverb effect it >totally lost and the efficient extraction of the exhaust gasses from the >cylinder's is lost. >Put the brakes on the exhaust flow with a plugged, half plugged, or >even in good shape, with the tiny holes the exhaust gasses have to flow >through to get to the other with the converter in the way the reverb >effect gets lost in a non performance style system. >This is just the way it is. >Removing the stone from the converter alone will decrease the >restriction in the exhaust and allow the engine to breath, and perform >with ease. >( A Lot better than with in place and in tact anyway ) > >RC

There are two principles at play here, one is gas particle movement, the other pressure waves. Pressure waves approximate the speed of sound (varies with temperature) and travel differently than gas. Where the gas flow may be restricted by the cat, pressure wave tuning relies on the fact that movement is omnidirectional in the exhaust system.

The pressure wave resulting from combustion passing the exhaust port moves down the primary header pipe, and back up any tubes available to it at the collector, traveling at the speed of sound. The gases however, do not behave this way....so it's not the gas behaviour we look at about when talking pressure wave tuning. Because the cat is located after the primaries, it doesn't prevent the pressure wave from doing it's job. Removing it would have no effect on pressure wave tuning the header.

Of course if you have a plugged cat, then exhaust gas will pile up pretty quick and negate the wave tuning effect...but then again, if your cat is plugged, your likely running way too rich, and the cat is just a symptom of some other problem.

Also, given that the thread started on mufflers, then reducing pressure at the cat outlet by using a non-restrictive muffler would only serve to improve its flow.

The high flow cat on my conversion is rated to 4.5L, far in excess of the 2.0 litre I4 ... and likely not much of a restriction. If you cut one apart you'll likely be surprised by just how open the monolith is. We don't have emissions testing here but I was happy to buy and install it (with maybe an HP or two lost) just to have the van running as clean as possible.

In any case...makes for good discussion.


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