"Del Johnson, North Point Solutions, Inc" wrote:

I want to throw out some ideas I have about designing an exhaust
augmentation system and get your feedback:

Idea #1:  It would seem like there would be an advantage to using a short
exhaust stack leading up to the augmentor
since there is a lot of heat radiated from the two exhaust pipes.  Once the
exhaust gases enter the augmentor they mix with cooling air from the
radiators.  The augmentor piping can be made out of cheaper, lower
temperature tolerant material.  One of the old Contact magazine articles
(I'll have to look it up if there is interest with this group) indicated
that the augmentor tube was warm to the touch (presumably not hot enough to
burn flesh).  The tests that were performed did not use a Mazda engine but
never-the-less it is obvious that you would significantly reduce the exhaust
temperatures by combining it with the cooling air.

The advantages are:

1.  The sooner you get control over the hot exhaust gases the safer you
system will be.  By only exposing the hot exhaust stacks for a short
distance you minimize your exposure and improve upon the safety of your
overall system.
2.  The exhaust system (two exhaust stacks) are not likely to fail and do
not require elaborate couplings (none required) that are typical problems
that you have to worry about with a long (tuned) exhaust system with
muffler.

The disadvantages are:

1.  There would probably be some loss of engine performance due to the short
exhaust stack length ( Does anyone have an idea of what the performance drop
would be?)
2.  The system lacks a muffler - augmentors compound the exhaust noise.
This leads me to my second idea:

Idea #2:  This is as much a question as an idea.  Is it possible to
incorporate some kind of muffler technology in the augmentor tube.  The
'augmentor tube muffler' would have to offer a low resistance to the air
flow.  Any ideas of what should be used?  Implementing the first idea
depends on being able to manage the exhaust noise.  You can't implement the
first idea without moving the muffler system into the augmentor.

The advantages are:

1.  Whatever materials that you put into the exhaust augmentor whould have a
much longer life because it is not exposed to the super hot gases.
2.  There are a greater variety of materials and options that can be use
because the materials don't have to hold up to high temperatures.

The disadvantages are:

1.  No off the shelf 'augmentor tube mufflers' that you can purchase.
2.  Any restriction in augmentor gas flow will decrease the effectiveness of
the augmentor to pull cooling air through the cowling.
3.  Unless someone has successfully done this, it may take several design
attempts to come up with a workable solution.

Since this is one of my typical brain storming ideas I would welcome any all
responses from interested builders.

Thanks!

Delbert Johnson

One thing you must be sure to do is introduce cooling air around
the exhaust pipes as close to the engine as possible. I suspect
that could be a factor in the recent exhaust pipe failure and subsequent 
off airport landing of the exhaust augmented Power Sport powered Velocity.
 
Paul Lamar
 
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