BTW my speed of sound was based on a rough guess of 600 MPH.
The speed of sound varies with temperature so corrections must
be made for the actual speed of sound in the pipes.
I could be off by a lot. I'll have to look it up for 1500 degrees
or so.
Paul Lamar

For what it's worth I discussed some of this acoustic velocity stuff with
a guy who's spends his days designing sonar arrays for Lockheed Martin
and his evenings putting a Soob in Cozy so I suspect he might know
a bit about this.  He had the acoustic velocity for the Soob exhaust
at 1200 ft/sec from that I did the following to get a 1310 ft/sec
velociy for a 1400 F EGT. Here's a link to Phil's web site if you want to read his
theory on what's going on in an exhaust pipe.
<http://www.geocities.com/plmjohnson/exhaust.htm>

I assumed an average exhaust temp of 1100 F for the soob and 1400F for the 13B.
Unfortunatly I never did learn how he got the 1200 ft/sec acoustic velocity
which this whole house of card relies on.

1100F = 866K
1400F = 1033K
1200 ft/sec = 365.76 m/sec

// solve for rR/M at 1100F and 1200 ft/sec
c = sqrt(rRT/M)  // formula for acoustic velocity
c*c/(rR/M) = T
133780/(rR/M) = 866
rR/M = 154.48

// For 1400F from a rotary
c = sqrt(154.48 * 1033)
c = 399.47 m/sec = 1310.3 ft/sec

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