Check this one out for data on exhaust augmentors.
Lots of excellent data. Much money and time was spent so I would
copy the geometry exactly if I were doing it.

http://naca.larc.nasa.gov/

489.Manganiello, Eugene J Bogatsky, Donald An experimental investigation of 
rectangular exhaust-gas ejectors applicable for engine cooling naca-report-818 1945 

The fish mouth shape on the tip of this exhaust pipe is known to quiet a
internal combustion engine. You need to beef it up however as a rotary
is known to destroy these sorts of nozzles. My guess is 1/8th inch thick SS
sheet would be a minimum.

Paul Lamar
 
The AirCraft Rotary Engine NewsLetter.  Powered by Linux.
ACRE NL web site. http://home.earthlink.net/~rotaryeng/            
Copyright 1998-2002 All world wide rights reserved.