My latest exhaust system uses a DNA muffler which has a 3" inlet and 3" outlet. When I first ran it, I was disappointed at how unacceptably loud it was. Then I remembered Tracy's statement about most guys using too large of an exit. So, I reduced the outlet to 2". Sure enough, it reduced the noise level significantly and there was no detectable loss of power. I'm still running it with the 2" exit.
Mark S.
On Mon, May 18, 2009 at 10:14 AM, Tracy Crook <tracy@rotaryaviation.com> wrote:
Your mathematics are sound George and using that approach will certainly keep back pressure to a minimum. But I think it is doomed to be noisy. That's just my understanding about how mufflers generally work and I could be wrong.
During a test of mufflers at a rotary fly-in some years back I was amazed at how effective the muffler became and how small the power loss was when we installed a 1" diameter muffler outlet pipe on the test stand engine (13B). 1" is admittedly too small but we learn by going to extremes sometimes.
Tracy
On Mon, May 18, 2009 at 2:40 AM, George Lendich <lendich@aanet.com.au> wrote:
Tracy,
The reason I ask is that I like to keep the muffler outlet the same area size or a little bigger than the exhaust outlet in the rotor housing which is usually about 2" dia.
I do understand the 2 and 3 rotors are staged and the exhaust and the exhaust (shock) waves are in sequence, but I like to keep the volume of the muffler and muffler exit to the total of the rotor exits i.e. 3x Pi.r squared (3x Pi 1 x 1) = 9.2 sq" or Radius of 1.71" = Dia of 3.42".
I thought 2.25 may have a little too much back pressure, as the rotary doesn't like back pressure.
Probably to do this exercise properly would be to calculate the volume of the heated and expanded exhaust gases and calculate the timing of the pulses to see how a particular sized muffler and exit size coped with the speed and volume of flow. It would be an interesting exercise.
George (down under)
Too soon to say really. I have not even had time to assemble the builders log to get ready for the DAR. Lack of engine power is definitely not what is keeping this thing grounded though : )
Tracy
On Sun, May 17, 2009 at 6:01 PM, George Lendich <lendich@aanet.com.au> wrote:
Tracy,
Any significant back pressure to speak of?
George (down under)
----- Original Message -----
Sent: Monday, May 18, 2009 12:11 AM
Subject: [FlyRotary] Re: 20b Exhaust
In general, the bigger the exit pipe, the louder the exhaust will be. I used 2.25" which I think is about the minimum for a 20B.
Tracy
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