From: Theo_Green@firstar.com
To: lancair.list@olsusa.com
Message-ID: <8625693B.004BE8EB.00@mail.firstar.com>
Date: Mon, 14 Aug 2000 08:49:30 -0500
Subject: RE: exhaust backpressure
Reply-To: lancair.list@olsusa.com
Mime-Version: 1.0

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Sorry for the late post, but backing up many of Brent's points. I posed the
question to a friend who is a managing engineer at Harley-Davidson's test
division. Here is the reply:

There are some differing theories on all of this and I believe most of it is
application and engine specific, not generalities.  For example, just
crushing the end of the pipe to an unknown oval will probably net you a
goose egg in performance (i.e. 0 net performance improvement.)  However,
there are a number of exhaust manufacturers who have gone to great lengths
to prove an ellipse is the best shape, of course size and cord length etc.
is very important.  Many calculations/theories/testing/$$$$ behind all that.
We spent quite a bit of time and $$/research developing cross over pipes to
equalize pressure cylinder to cylinder on our V-Twins for the purpose of
improving horsepower.  In fact it is a common misconception that our exhaust
systems are extremely restrictive compared to after market pipes/systems.
What we lose in noise we gain in improved scavenging capability and reducing
back flow due to restrictions etc. blah, blah, blah.  Bottom line is there
is a tremendous amount of science that goes into an exhaust system, size,
shape, length, intake design, valve placement, port flow, port size, port
shape, etc. that goes into these systems.  There is no one correct answer
either.  Just adding our cross over added almost 15 horse to our bikes!
Generally speaking though, if you are after horsepower at top speed, big
pipes, with crossovers rule.  If you are after low end torque, then smaller
(to a point) pipes with crossovers rule.  Where the cross over is added by
the way is also important.  Old "big three" muscle car's always had a cross
over, usually behind the tranny.  These seemed to have been more important
with stock exhausts than with after market though?  However, I do not hear
of a cross over ever hurting.  Nor have I ever heard of an elliptical or
oval shaped output ever hurt?  Bottom line is it would probably not hurt to
put in a cross over and change the outlet shape.  I just remembered there is
a company that is producing elliptical tubes for their system as well.  Good
luck with the choice!


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