Jerry Hey wrote:

First,  I want to thank Tracy and Laura for their hospitality in
hosting the Rotary Round Up.  It was great to actually meet so many
people who I only knew via the internet.   I learned a lot.

Richard Sohn, who impressed me greatly with not only his
fabricating
ability but also with his approach to problem solving, said that,
when
using a muffler (simple tangential) that has a lot of volume, the
tailpipe diameter can be reduced because as the exhaust gasses
expand
and cool inside the muffler,  they they are diminished.   If this
is
true, what size pipe would  be required for a two rotor at 7500
rpm?
Of course,  variables such as  volume of the muffler and the how
much
the exhaust gases are cooled, complicate a simple answer. Still  I
would appreciate anyone's thoughts. Do you agree in principle with
his
theory?   What are the auto racers doing?    I presently am using
a 3
inch tailpipe because I wanted to make sure there was zero back
pressure.  But a smaller pipe would  be a lot easier to live with
if it
would do the job.

  Jerry

Good question Jerry. (P1 x V1)/T1 = (P2 x V2)/T2
Therefore all other things being equal the area of the exhaust pipe
will be inversely proportional to the difference in exhaust temps.
The reference area is the total area of the two pipes coming out of
the
engine going into the muffler.

The temp  T is in Kelvin as I recall which is absolute temp.
Someone on here probably
can look up the conversion from centigrade or Fahrenheit to Kelvin
for us.

Lets say the temp out of the engine is 500 K and the temp out of
the exhaust
is 400 K than the area of the down pipe would be 4/5 of the total
area of
the exhaust pipes. Lets say the exhaust port pipes are 2" in
diameter so total
reference area would be 6.28 square inches. (pi x R^2) X 2. 4/5th
of that is about 5 square
inches. Dividing by pi gives 1.6. The square root of 1.6 is  1.26
which is the
radius of the down pipe or 2.52 inch diameter. It all depends on
the ratio of
the absolute temperatures of course. A long pipe will also have a
pressure loss
before it gets down to atmospheric. In other words the pressure at
the start
of the pipe will be higher than the pressure at the end of the pipe.

Check my assumptions, formulas  and numbers.

Paul Lamar

Using the Convert program: www.joshmadison.com/software/convert
I found the following values:
400K = 126.85C = 260.334F
500K = 226.85C = 440.334F

Yvon

Thanks Yvon

Paul Lamar

I found a free program for Linux called "units".

OK here are some actual numbers.

1800 F is 1255K
1600 F is 1144K

So the area ratio is 1144/1255 is .91. Not much.

Calc the new area. 6.28 times .91 is 5.72 square inches.

5.72 divided by pi is 1.82.

The square root of 1.82 is 1.34" radius or 2.69" diameter pipe.

Might as well use 2.75" inch pipe. Well it is a little bit smaller.

You would have to lose a lot of temp to make a big difference.
That lost heat would wind up under the cowl.

Paul Lamar

Thanks,   I am going to modify the present system but I will leave the
Tailpipe size as is.  Jerry

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