from Bill Jepson...

Group,
 I am attaching several JPEGS of tangential mufflers. The smaller muffler is a variant on the cool tube theme, with the bounce panel added. The Jabaru uses a muffler of this style without the cool tube, and it's very quiet. I think the cool tube is a good idea on the rotary to help control temps....

An idea,

You could plumb the entry of his exhaust to the back of the evap core(s) via a plenum.  That would draw cooling air through the cores and would be especially effective at providing a good vacuum while taxiing on a hot day, when other means of cooling don't work as well.  As the RPM increased during climb or cruise it would work just as well (probably couldn't hurt).  This would essentially be an augmented exhaust as was described in CONTACT magazine a couple years ago. 

The advantage of Bill's set-up is the source of the vacuum would be close to the core on the left side, making a plenum simple to construct (add an extension tube out the front).

Chris Sargent