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

Jerry,
I don't know if it would help, but perhaps 2 smaller exhaust pipes, perhaps
square ones, could do the job and look pretty 'trick' too.
Just a thought!
George (down under)

I think two smaller down pipes of the same total area would be quieter.
Especially if they were square.

Paul Lamar
 
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