This month, I will explore engine exhaust tuning. One of the most interesting aspects of being involved most of my life with internal-combustion engines is sound.
There is something hardwired in us that causes us to react
differently to different sounds, and differently to the same
sound. A jet passing overhead is so commonplace today that
most never look up unless it is low or unusually loud.
There is one sound that seems to have a built-in degree
of sensitivity to our brains. Namely, the high-pitched sound
of an insect that we instinctively know could hurt us. Flying
insects that produce sounds such as the unmistakable whine
of an “attack” mosquito, all the way to the lumbering tone of
an incoming bumble bee “bomber,” will get our attention.
It may be that these primitive reflexes saved early humans.
One thing for sure is that those reflexes are still there!
My theory is that the autonomic response to that kind of
noise is why high-revving model engines have the ability to
both draw attention and invoke reactions in and from people
who live near our flying sites.
Before I go any further, it is worth stating that this is not
an anti-engine-noise piece, but a deeper look at why our
engines can annoy and why mufflers, designed correctly,
improve the situation for all of us.
Enjoying man-made noise is subjective. When I compare
the noise of a Cox .049 at maximum
rpm to the noise of six 12-cylinder
Merlin engines at the Battle of
Britain memorial flight (Lancasters,
Hurricanes, and Spitfires) making low
passes, I enjoy the Merlins.
The massive roar and throbbing of
a World War II Rolls-Royce Merlin
engine is music to my ears, but the
Cox .049 is like fingernails on a
blackboard! And yet, as a small boy,
nothing was sweeter than getting that
little CL engine to scream and fly in
circles until I was too dizzy to stand.
As a Pattern pilot in the 1990s, I
was required to get my 1. 60 two-and four-stroke engines down to
90-decibel sound readings at 9 feet
from the measuring meter. At the
same time, I fought to get more
power to haul my competition
airplanes straight up without losing
airspeed. If the airplane slowed down
too much it would lose heading and
points in the round.
Tuning an engine with an exhaust
A soda can is
used as a muffler
1974, in an attempt
to reduce exhaust
Below: The growl of big gas-ignition engines can
be tamed using tuned “cans” designed to also
increase the power.
Extreme silencing can be achieved using an
exhaust header and a carbon-fiber tuned
pipe to gain power while decreasing noise.
system was not a new concept. Most of the work was
focused on getting more top-end power. What happened
in the Pattern world was that the focus shifted to the mid-range and more torque at the lower top-end rpm. This was
because a slower-turning propeller produces less noise.
Larger propellers were used with higher pitch to get the
same speeds as before with significantly less noise. Engines
were heavily cowled, and even the carburetor intake noise
was reduced with air filters similar to those in automobiles.
Let’s take a simplified look at what a glow-powered two-
73 Model Aviation OC TOBER 2012 www.ModelAviation.com
THE ENGINE SHOP
Tune your engine exhaust like a pro