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The rotary is loud because there is no exhaust valve to meter flow from none to much and back to none again. The rotary opens exhaust flow instantly to full flow with no restriction at all. The early 13Bs had a wedge shaped device in the Periphery port to break up the speed of part of the flow and remix it with the high velocity flow. Very effective. The Mazda race muffler was short and fat, for mounting below the rear bumper of steel bodied race cars. It too used the perf tube idea, but had a small space in the start where the tube was not connected to the inlet tube. Then a short space with a second bulkhead to support the perf tube. The bulkhead had many small holes to allow flow to the outside of the perf tube. So, when you looked through the inlet you could see right out the back with no restriction at all. The room at the front was to allow the high pressure pulse to drop most of its power. The outer volume was filled with stainless scrubbing pads or Lava rocks for use in gas fire fire places. The lava rocks didn't last long. Perhaps one weekend of racing. I have no data on the stainless pads. It would seem to me that stainless pads would last longer. Ports were provided to install the stuffing. The exhaust thinking of that time was to have two long pipes to the back of the car then joined right before the muffler. Moving the outlet as far as is possible from the ears is better. Aiming the outlet away from the vehicle is good. You must assume that the muffler will be beaten to pieces with time. Round shapes only. Flat pieces will fail early. Racing cars shift between 9,500 and 10,000 RPM. Airplanes less than 7,000 RPM. Not nearly as much power or mass flow. We were below 103 Db with a copy of the Mazda race muffler. That is measured 50 feet from track edge at a location where full throttle would be in use.
Lynn E. Hanover
In a message dated 1/19/2020 4:50:37 PM Eastern Standard Time, flyrotary@lancaironline.net writes:
Just for point-of-reference, have any of you guys used your dB meters around Lyc & Cont powered a/c? Checking an RVx or EZ with similar HP at roughly the same positions relative to the exhaust should give you some perspective on relative sound energy vs the rotary. If there's one available, also check a Cessna 180 on takeoff, also at the same distance from the plane as from the homebuilts on takeoff. Now if you want to know why the rotary *seems* so loud, download a realtime spectrum analyzer on your phone. You'll need to take those measurements at some distance away from the plane(s) to get realistic readings (the phone can't handle 100+ dB levels very well at all), but I'll bet you see some significant differences between the spectrum of a rotary and the RV/EZ group, and some at least passing similarity between the rotary and a C180, if the 180 driver is using full throttle & rpm during the takeoff run. Similar situation for a T6 (Harvard) doing a high speed pass at full rpm. Charlie On 1/19/2020 2:51 PM, Neil Unger 12348ung@gmail.com wrote:
Mat,
Picked up the muffler that Chris uses and I am amazed that it works. Chris seems to have a gift of falling on his feet. It is very simple and he claims it is very quiet, but with a slight power loss. Will photograph it, but is based on an ATC OZ muffler. Checked my DB readings last week, and only got as far as 3000 RPM which is the quiet range! 2000 RPM 85 DB 2500 RPM 95 DB, 3000 rpm 100 DB and outside the cab 110 DB. NO wonder at full noise it gets annoying. Obviously my system is useless for noise reduction, with certainly no back pressure at all. I am guessing that to have noise reduction I will have to wear some restriction which is power loss. Same story -- just what compromise can I live with?
Will attempt to describe the muffler once I have measured it on the bench. MY thanks to Chris for loaning this to me.
Neil.
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.
- Matt Boiteau
On 2020-01-07 12:19:23 AM, Matt Boiteau mattboiteau@gmail.com <flyrotary@lancaironline.net> 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)
Batwing tabs
- Matt Boiteau
On 2020-01-03 4:22:18 PM, Neil Unger 12348ung@gmail.com <flyrotary@lancaironline.net> wrote:
Mat,
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.
Neil.
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?
- Matt Boiteau
On 2019-12-28 11:03:42 AM, Finn Lassen finn.lassen@verizon.net <flyrotary@lancaironline.net> 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). 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. Finn On 12/28/2019 12:10 AM, Matt Boiteau mattboiteau@gmail.com 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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