Mailing List Message #66050
From: Bobby J. Hughes <>
Subject: RE: [FlyRotary] Re: Mufflers
Date: Thu, 9 Jul 2020 12:00:12 -0500
To: Rotary motors in aircraft <>

Thanks Matt.  Hopefully you can return home before it gets cold. Looking forward to flight testing results. I suspect excessive back pressure is why Mark’s attempt failed and caused excessive EFT’s. I think he was seeing 1850 +F with his normal mixture settings. He rebuilt the engine and switched back to his previous muffler.  I think any 2.75” 321 tubing would have to be rolled and welded. It doesn’t appear to be available any longer.




From: Rotary motors in aircraft []
Sent: Wednesday, July 08, 2020 7:10 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Mufflers


Sadly, by the time the welder and I finished, I only got about 2 hours on the plane before I had to pack up my winter trip. I was planning to go back in March to FL (I live in Canada) to start flying off the 25 hours, but you know how that all went. So I'm just sitting here waiting for the borders to open up and fly my plane!

Early conclusion is it worked pretty darn good. I'd say I'm equivalent to an IO-360, maybe a tad quieter. Also on ground testing only, I quickly mocked up a long tube under the plane with holes drilled in it (with the end capped off), and it pretty much got rid of all the engine noise, only prop noise. Haven't flown with that yet. One thing I still want to do is test the backpressure. Making sure the CAN has enough holes inside.

The best price I found on 321 was from ProFabrication. I went with 2.75" 20ga (0.035") on the downpipe & outlet.

I did use SPD Exhaust for the double slip joints at the primaries and downpipe. 2" 16ga (0.06")  

For the CAN, McMasterCarr 24"x24" 18ga (0.05") rolled into I think a 5 or 5.5" can. You can go heavier ga, I just guessed.  

- Matt Boiteau



On Wed, Jul 8, 2020 at 2:35 PM Bobby J. Hughes <> wrote:

I’m at the exhaust header stage in my S21 FWF build. I’m considering a traditional header,  3-1 merge collector with 1.75” primaries to a 2.5” tube. I’m trying to be very weight conscious and looking for possible alternatives. The CA manifold can likely be built for nearly the same weight as the header design in the picture. I’m a little concerned with the tangential design. Mark S. built and tested one on his 20B. It ran hot and caused the outer water jacket seals to fail. Mark was not running a wideband so he couldn’t see a clear relationship between F/A and EGT’s.  He may have be running at or near peak EGT’s. EGT’s for the renesis can be kept to between 1500F -1600F with a little extra or a little less fuel. I will like install the coolant radiator on the belly so adding an external muffler would keep it off centerline.


I have located some reasonable priced Inconel 625 0.040 sheets. If rolled into a 5-6” tube, would this thickness be sufficient for a muffler shell or the CA / tangential manifold tube?




How is your CA header holding up?






From: Rotary motors in aircraft []
Sent: Saturday, January 25, 2020 7:53 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Mufflers


Okay so after 2 full days of having our welder out, finally brought the CAD design to life. Except both days were all nighters, so I was too tired and busy to take 'in progerss' pictures. Was tricky to build, which made the hourly welding cost sky high, more then the engine itself. Hopefully that means I can save money in the future on hearing aids.

Thanks Charlie for the tip on the spectrum analyzer. I didn't get any before numbers, but it's all low Hz now. That harsh high pitch is gone. Db's are still there, but I'd say with just the muffler design, it's comparable to typical lycoming. I can start to hear the prop noise over the engine noise. I measured back pressure around 3.5psi @ 2200rpm prop.

I ran out of time to flight test the long pipe under the airplane. I quickly held it up with my hands and the exhaust pretty much disappeared with the prop taking over the noise. Before flying, I want to add backpressure gauge to that part and drill holes to find a balance between backpressure and noise.


SO the "header" muffler should make quite like any other plane, and the "under belly" pipe will make you standout in silence.


Since this email type newsletter is from the dinosaur ages, I'll add pictures and video to


- Matt Boiteau

On 2020-01-18 5:29:51 PM, Matt Boiteau <> wrote:

Have started on the exhaust. Made all three primaries 2" double slip joints (that alone took almost a whole day with welder). The big can (CA - Centrifugal Accumulator) rolled into a 5" can and we'll secure it to the engine with straps.

The inside pipe is 2.75" which has an area of 5.94sq". I'm not sure if there's a magic number, but all the holes I drilled equaled to just under double the area (11sq"). Most are 3/8, but I did add some 1/2 to get the total area up. Could we get away with drilling less holes? Not sure. I'll be able to drill out one side end of the CA and pull out the inner tube and change out. Will experiment later with less holes and monitor back pressure. Will weld a npt bung on the CA and use the wideband bung to measure the different.

The previous exhaust with just an Aero Turbine 2525XL, measured ~113dBA. With nothing, was closer to 120dBA. Funny around 2000 prop rpm was louder then at 2300 full static. 

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- Matt Boiteau

On 2020-01-07 12:19:23 AM, Matt Boiteau <> wrote:

Okay I'll make all 3 primary tubes (2" diameter) all double slip fit. Welding batwing tabs (picture below) on to either side on the slips, will hold the muffler to the header but allow expansion.


Having troubles finding 2.75" bends in 321, so I might have to just switch to 3" downpipe that is more common. Vband clamp before downpipe for easier removal and under the plane to allow adding additional exhaust setups (long pipe with holes drilled in it, fishmouth tailpipe, etc)

I found a company that makes flex bellows in 321. I'm just not sure if I should have it vertical on the downpipe or close to the muffler horizontal before the first 90degree bend downwards.

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Batwing tabs

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- Matt Boiteau

On 2020-01-03 4:22:18 PM, Neil Unger <> wrote:


                  As usual I have no idea.  I made all 3 slip joints and to date no grief in that area.  I am coming around to the stock manifold in some way.  Yes it is heavy, but appears to muffle the noise as well.  Know of 2 installations that use the stock manifold with a simple muffler after and both claim "acceptable" noise. It appears that the stock manifold has a big influence on noise??  Would prefer a DB reading, but if not available it is what it is.  All is compromise, weight, cost, time, but at the end of the day it has to work. 

Still working on the turbo even though the world is on holidays.  All to save my hearing.  So far the rotary is definitely "unique" as far as a turbo is concerned.  The heat generated exceeds all else.  Two things to date -- special exhaust wheel and water cooled bearing body =, all for heat.  Have modified the stock front plate on the renesis to take an electric water pump.  That is the simple bit.


On 1/4/2020 5:37 AM, Matt Boiteau wrote:

okay I found a good company called SPD Exhaust. They have everything you need in 321ss.

With the engine being an RX8, we have three exhaust ports. Should I weld the front and back solid to the muffler, and make the middle one a slip fit for expansion? Or vice-vesa?

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- Matt Boiteau

On 2019-12-28 11:03:42 AM, Finn Lassen <> wrote:

I did something similar with my RV-3 13B decades ago. It split open at the welds around one of the pipes from the manifold into the the big outer tube. True, it did use individual manifold base plates, not the connected factory manifold and the big tube was only 0.035.

Still, I would recommend slip joints on two of the three pipes, like I added on one of the pipes (cut through and a surrounding bigger pipe -- missing in picture).

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Can't remember why I went from the above to individual runners into a perforated pipe under the fuselage. Probably got lured by promise of increased power by tuned lengths and still uncomfortable by the muffler being inside the cowling.


On 12/28/2019 12:10 AM, Matt Boiteau wrote:

This is what I'm going to try in a few weeks. 

"Exhaust valve opens and a pulse of hot gas puffs out through a short pipe, then tangentially into a cylindrical canister. Being tangential, the pulse flattens out and travels helically (rather than bouncing and reverberating around) along the inside curved wall of the can, spiraling toward the exit, where it comes out more uniform in flow and so pretty quiet."


Outer pipe = 321 ss 0.050 thick. Flat plate bent to a 5" tube

Inner pipe = 321 ss 0.036 thick. 2.5" diameter

(I might use 302ss 0.065 since I already have it)


From my understanding, the area of the holes should be double the area of the inner pipe. 


- Matt Boiteau




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