Model Forum / Radio Controlled / Air Models / September 2006

Adding large silicone tubing to your muffler extension will quiet it
significantly, but will also steal some power.

You can probably get a muffler extension.

I know they are available for YS engines.  I have
one for a YS 91 AC.  They cut the noise considerably
and are or were used by pattern pilots to meet
noise requirements.

            Marty

One thing to be aware of; once you get the exhaust noise _way_ down,
other sources of noise become realatively more important and may need
to be addressed too.
These include prop noise (which can be a biggee), vibration transmitted
to the airframe thus making the airframe vibrate, and engine air intake
noise (can also be surprisingly loud)

Martin

I use one on an  OS 52 that has worked very well for me.  It's a
resonator type that doesn't produce any negative effects on the
operation of the engine - in fact I often slope soar a Kadet LT50 with
the engine at a tickover idle for upwards of half an hour, and it has
no tendency to load up, or lean out due to the drop in fuel head over
that extended idle period.
Don't have a drawing, so I'll  try to paint it in text.  I wanted at
least 10:1 expansion ratio for quieting performance, so cross section
area of the chamber is 10X the cross section of of the exhaust header
pipe.  Length is about 6", an somewhat arbitrary "TLAR' dimension that
intuition said would provide sufficient volume, and be near some small
ordered harmonic of the exhaust frequency.  I think can presume from
what I wrote so far that there is nothing critical in the dimensions,
and you would be right!  Inlet and outlet pipes extend the length of
the chamber and terminate embedded in the end plugs.  They are spaced
evenly across the diameter of the chamber, i.e., each centered half
the radius out from the chamber CL.  Both inlet and exhaust have holes
drilled over their length and circumference, I used about 20 in each
sized so the total of their area is at least 2X that of the exhaust
header.  That was a 2nd iteration BTW - not enough holes used
initially, causing noticable increase in backpressure that affected
mix setting adversely.  I hadn't made them big enough to account for
edge effects on exhaust gas flow.   The connection is made to the
stock muffler with a short length of automotive fuel/vapor tubing.
This tubing has required replacement at about 1 year intervals as it
tends to harden and crack in time.  The connection to to the short,
smooth stock muffler outlet is made more secure by cutting a groove
around it to fit a C-clip.  This gets a bite on the connector tubing
and prevents slippage.  When installed it is rotated with the outlet
pipe oriented down (bottom of chamber, so oil condensate gets blown
out wit hthe exhaust stream.  FWIW, all material is scrap aluminum
tubing from a local salvage yard at 2 bucks/lb.  The ends plugs were
machined by a buddy for prees fit.  Never had come apart except used a
bench press to dismantle it for for the modification mentioned above.
That's about it - I had intended for it to replace the stock muffler,
but it worked so well in my test config as an aftermuffler that I left
it that way.  Been working fine for more than 10 yrs now.
As for performance, it drops measured dBA level at 3 M by 5-6 dB at
WOT, depending on prop used.  More than I expected to get, as the OS
52 is pretty darned quiet out of the box.  At idle it's undetectably
silent during my slope soaring jaunts.  Sometimes I blip the throttle
just to be assured that it is still running.

Abel