Date:         Sat, 31 Aug 2002 16:04:16 EDT
Reply-To:     KimBrennan@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Kim Brennan <KimBrennan@AOL.COM>
Subject:      Exhausts...tuning.(was loud identity crisis)
Content-Type: text/plain; charset="US-ASCII"

In a message dated 8/31/02 6:28:18 AM, clive.harman-smith@NTLWORLD.COM
writes:


> I'd suggest that most holes in exhausts actually reduce power. A weak
> mixture is one result (hence coinnected to rough running and backfiring on
> the overrun), splashy irregular shockwaves in the syetm etc. etc. Any
> supposed power gains are usually psychological due to the loder noise.
>
It depends. My brother worked in a shop that built race engines. One thing he
learned after joining that shop was how an exhaust can hinder...or help
evacuating exhaust gases from the engine. This is the process of tuning the
exhaust. Most consumer vehicles have an exhaust system that isn't tuned all
that well (modern cars are better than older ones as emissions quality has
forced manufacturers to look at all aspects of engine management). The main
focus of the exhaust system in these consumer vehicles is to reduce emissions
(catalytic convertor) and reduce noise (muffler). Race engines, on the other
hand, don't have to worry about this (they also use different fuel, but
that's a different issue). In Race engines, the length of pipe used in the
exhaust system can have dramatic differences in the power output of the
engine. And it isn't a matter of pure length, but the ratio of volume of
gases passing through it to the length.

The wrong length pipe can cause a standing pressure wave in the exhaust
system, this means that is is harder for the engine to get the burnt fuel out
of the pistons, and causes a loss of power. Block your exhaust pipe and
you'll see what I mean. If the exhaust can't get out, the engine won't run
for very long. ON the other paw, if you have just the right length, the
exhaust system acts as a partial vacuum and actually aids in removing the
spent fuel from the pistons.

Why? Harmonics. You have pulses being caused by the pistons. These occur
rhythmicly and as with any other rhytmic system, you can get reinforcement of
the waves in a positive or negative fashion. In this case the waves are
pressure points in exhaust gases. High pressure is a positive reinforcement,
low pressure is a negative reinforcement.

All of this explains the large market in sports car after market exhaust
systems. It isn't just about noise, but various people trying to get a little
extra "free" power out of their engine, by tuning the exhaust.

Back to my original point. Depending on the size of the hole (and where it
is) you will probably end up with more power because of a less restrictive
path for the exhaust gases.