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 twoand
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
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 midrange
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-
stroke engine does to produce exhaust,
and the noise that goes with it. The
engine draws a fuel/air mix from the
carburetor into the main crankcase
under the piston. It does this as the
piston moves up the bore to explode
the previously ingested fuel/air mix.
There is no timing system in a glow
engine, so the gases explode when the
right compression is reached to allow
the glowing glow-plug element to set
it off. (Gas engines do the same, except
there is a spark plug instead of a glow
plug. Spark plugs require external
current to activate them.)
The piston travels down again and
the exploded—and expanded—gases
begin to escape from the exhaust port.
The piston begins to compress the fuel/
air mix waiting below. This forces the
mix into and up the transfer porting.
These channels run up the outside
of the cylinder liner in the engine
casing. They guide the fuel/mix into
the combustion chamber to provide a
charge for the next explosion.
This is where you will hear the word
“timing” again. There are ports in the
cylinder liner that are positioned to let
the exhaust gases out before the fuel/
air mix comes in through its own inlet
port or ports. There are no cylinderhead
valves similar to an automobile in
this type of engine, so the control of gas
arrival and departure is achieved by the
positioning of the ports in the cylinder
stroke engine does to produce exhaust,
and the noise that goes with it. The
engine draws a fuel/air mix from the
carburetor into the main crankcase
under the piston. It does this as the
piston moves up the bore to explode
the previously ingested fuel/air mix.
There is no timing system in a glow
engine, so the gases explode when the
right compression is reached to allow
the glowing glow-plug element to set
it off. (Gas engines do the same, except
there is a spark plug instead of a glow
plug. Spark plugs require external
current to activate them.)
The piston travels down again and
the exploded—and expanded—gases
begin to escape from the exhaust port.
The piston begins to compress the fuel/
air mix waiting below. This forces the
mix into and up the transfer porting.
These channels run up the outside
of the cylinder liner in the engine
casing. They guide the fuel/mix into
the combustion chamber to provide a
charge for the next explosion.
This is where you will hear the word
“timing” again. There are ports in the
cylinder liner that are positioned to let
the exhaust gases out before the fuel/
air mix comes in through its own inlet
port or ports. There are no cylinderhead
valves similar to an automobile in
this type of engine, so the control of gas
arrival and departure is achieved by the
positioning of the ports in the cylinder
running leaner, but is actually running
faster—often with an increase of many
hundreds of rpm.
Depending on the exhaust design, the
tuning only happens at a certain rpm
and is not available throughout the rpm
range. Two primary factors influence
the width of the rpm range in which
such tuning can occur. The first one is
the preset timing of the cylinder ports.
You may hear terms such as “broad”
or “wide timing,” which mean that the
design is less sensitive to tuning and
will perform fine with a standard bolton
muffler.
The second factor is the length of the
tuning pipe. (In this case you should
regard a stock muffler as a short,
fat pipe). Generally, the longer the
muffler, the broader the rpm range.
Longer pipes typically give better
mid-range power and shorter pipes
give higher rpm at the top end. From
a sound reduction point of view, the
longer pipes give better results.
Because there is no way for us
to change the hardwired genetic
programming that dictates how we
react to sound, it falls to us to change
the sounds our engines make!
Correction
In my June 2012 “The Engine
Shop” column on servicing a twostroke
engine I wrote, “If you have
a 1/4 x 32 tap it will do no harm to
clean up the glow plug threads in the
head.”
At the end of my August 2012
column I added a late update on
glow-plug thread size that was
incorrect. Please be clear that it is still
1/4 x 32 for most glow engines.
Edition: Model Aviation - 2012/10
Page Numbers: 73,74,75,76
Edition: Model Aviation - 2012/10
Page Numbers: 73,74,75,76
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 twoand
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
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 midrange
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-
stroke engine does to produce exhaust,
and the noise that goes with it. The
engine draws a fuel/air mix from the
carburetor into the main crankcase
under the piston. It does this as the
piston moves up the bore to explode
the previously ingested fuel/air mix.
There is no timing system in a glow
engine, so the gases explode when the
right compression is reached to allow
the glowing glow-plug element to set
it off. (Gas engines do the same, except
there is a spark plug instead of a glow
plug. Spark plugs require external
current to activate them.)
The piston travels down again and
the exploded—and expanded—gases
begin to escape from the exhaust port.
The piston begins to compress the fuel/
air mix waiting below. This forces the
mix into and up the transfer porting.
These channels run up the outside
of the cylinder liner in the engine
casing. They guide the fuel/mix into
the combustion chamber to provide a
charge for the next explosion.
This is where you will hear the word
“timing” again. There are ports in the
cylinder liner that are positioned to let
the exhaust gases out before the fuel/
air mix comes in through its own inlet
port or ports. There are no cylinderhead
valves similar to an automobile in
this type of engine, so the control of gas
arrival and departure is achieved by the
positioning of the ports in the cylinder
stroke engine does to produce exhaust,
and the noise that goes with it. The
engine draws a fuel/air mix from the
carburetor into the main crankcase
under the piston. It does this as the
piston moves up the bore to explode
the previously ingested fuel/air mix.
There is no timing system in a glow
engine, so the gases explode when the
right compression is reached to allow
the glowing glow-plug element to set
it off. (Gas engines do the same, except
there is a spark plug instead of a glow
plug. Spark plugs require external
current to activate them.)
The piston travels down again and
the exploded—and expanded—gases
begin to escape from the exhaust port.
The piston begins to compress the fuel/
air mix waiting below. This forces the
mix into and up the transfer porting.
These channels run up the outside
of the cylinder liner in the engine
casing. They guide the fuel/mix into
the combustion chamber to provide a
charge for the next explosion.
This is where you will hear the word
“timing” again. There are ports in the
cylinder liner that are positioned to let
the exhaust gases out before the fuel/
air mix comes in through its own inlet
port or ports. There are no cylinderhead
valves similar to an automobile in
this type of engine, so the control of gas
arrival and departure is achieved by the
positioning of the ports in the cylinder
running leaner, but is actually running
faster—often with an increase of many
hundreds of rpm.
Depending on the exhaust design, the
tuning only happens at a certain rpm
and is not available throughout the rpm
range. Two primary factors influence
the width of the rpm range in which
such tuning can occur. The first one is
the preset timing of the cylinder ports.
You may hear terms such as “broad”
or “wide timing,” which mean that the
design is less sensitive to tuning and
will perform fine with a standard bolton
muffler.
The second factor is the length of the
tuning pipe. (In this case you should
regard a stock muffler as a short,
fat pipe). Generally, the longer the
muffler, the broader the rpm range.
Longer pipes typically give better
mid-range power and shorter pipes
give higher rpm at the top end. From
a sound reduction point of view, the
longer pipes give better results.
Because there is no way for us
to change the hardwired genetic
programming that dictates how we
react to sound, it falls to us to change
the sounds our engines make!
Correction
In my June 2012 “The Engine
Shop” column on servicing a twostroke
engine I wrote, “If you have
a 1/4 x 32 tap it will do no harm to
clean up the glow plug threads in the
head.”
At the end of my August 2012
column I added a late update on
glow-plug thread size that was
incorrect. Please be clear that it is still
1/4 x 32 for most glow engines.
Edition: Model Aviation - 2012/10
Page Numbers: 73,74,75,76
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 twoand
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
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 midrange
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-
stroke engine does to produce exhaust,
and the noise that goes with it. The
engine draws a fuel/air mix from the
carburetor into the main crankcase
under the piston. It does this as the
piston moves up the bore to explode
the previously ingested fuel/air mix.
There is no timing system in a glow
engine, so the gases explode when the
right compression is reached to allow
the glowing glow-plug element to set
it off. (Gas engines do the same, except
there is a spark plug instead of a glow
plug. Spark plugs require external
current to activate them.)
The piston travels down again and
the exploded—and expanded—gases
begin to escape from the exhaust port.
The piston begins to compress the fuel/
air mix waiting below. This forces the
mix into and up the transfer porting.
These channels run up the outside
of the cylinder liner in the engine
casing. They guide the fuel/mix into
the combustion chamber to provide a
charge for the next explosion.
This is where you will hear the word
“timing” again. There are ports in the
cylinder liner that are positioned to let
the exhaust gases out before the fuel/
air mix comes in through its own inlet
port or ports. There are no cylinderhead
valves similar to an automobile in
this type of engine, so the control of gas
arrival and departure is achieved by the
positioning of the ports in the cylinder
stroke engine does to produce exhaust,
and the noise that goes with it. The
engine draws a fuel/air mix from the
carburetor into the main crankcase
under the piston. It does this as the
piston moves up the bore to explode
the previously ingested fuel/air mix.
There is no timing system in a glow
engine, so the gases explode when the
right compression is reached to allow
the glowing glow-plug element to set
it off. (Gas engines do the same, except
there is a spark plug instead of a glow
plug. Spark plugs require external
current to activate them.)
The piston travels down again and
the exploded—and expanded—gases
begin to escape from the exhaust port.
The piston begins to compress the fuel/
air mix waiting below. This forces the
mix into and up the transfer porting.
These channels run up the outside
of the cylinder liner in the engine
casing. They guide the fuel/mix into
the combustion chamber to provide a
charge for the next explosion.
This is where you will hear the word
“timing” again. There are ports in the
cylinder liner that are positioned to let
the exhaust gases out before the fuel/
air mix comes in through its own inlet
port or ports. There are no cylinderhead
valves similar to an automobile in
this type of engine, so the control of gas
arrival and departure is achieved by the
positioning of the ports in the cylinder
running leaner, but is actually running
faster—often with an increase of many
hundreds of rpm.
Depending on the exhaust design, the
tuning only happens at a certain rpm
and is not available throughout the rpm
range. Two primary factors influence
the width of the rpm range in which
such tuning can occur. The first one is
the preset timing of the cylinder ports.
You may hear terms such as “broad”
or “wide timing,” which mean that the
design is less sensitive to tuning and
will perform fine with a standard bolton
muffler.
The second factor is the length of the
tuning pipe. (In this case you should
regard a stock muffler as a short,
fat pipe). Generally, the longer the
muffler, the broader the rpm range.
Longer pipes typically give better
mid-range power and shorter pipes
give higher rpm at the top end. From
a sound reduction point of view, the
longer pipes give better results.
Because there is no way for us
to change the hardwired genetic
programming that dictates how we
react to sound, it falls to us to change
the sounds our engines make!
Correction
In my June 2012 “The Engine
Shop” column on servicing a twostroke
engine I wrote, “If you have
a 1/4 x 32 tap it will do no harm to
clean up the glow plug threads in the
head.”
At the end of my August 2012
column I added a late update on
glow-plug thread size that was
incorrect. Please be clear that it is still
1/4 x 32 for most glow engines.
Edition: Model Aviation - 2012/10
Page Numbers: 73,74,75,76
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 twoand
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
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 midrange
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-
stroke engine does to produce exhaust,
and the noise that goes with it. The
engine draws a fuel/air mix from the
carburetor into the main crankcase
under the piston. It does this as the
piston moves up the bore to explode
the previously ingested fuel/air mix.
There is no timing system in a glow
engine, so the gases explode when the
right compression is reached to allow
the glowing glow-plug element to set
it off. (Gas engines do the same, except
there is a spark plug instead of a glow
plug. Spark plugs require external
current to activate them.)
The piston travels down again and
the exploded—and expanded—gases
begin to escape from the exhaust port.
The piston begins to compress the fuel/
air mix waiting below. This forces the
mix into and up the transfer porting.
These channels run up the outside
of the cylinder liner in the engine
casing. They guide the fuel/mix into
the combustion chamber to provide a
charge for the next explosion.
This is where you will hear the word
“timing” again. There are ports in the
cylinder liner that are positioned to let
the exhaust gases out before the fuel/
air mix comes in through its own inlet
port or ports. There are no cylinderhead
valves similar to an automobile in
this type of engine, so the control of gas
arrival and departure is achieved by the
positioning of the ports in the cylinder
stroke engine does to produce exhaust,
and the noise that goes with it. The
engine draws a fuel/air mix from the
carburetor into the main crankcase
under the piston. It does this as the
piston moves up the bore to explode
the previously ingested fuel/air mix.
There is no timing system in a glow
engine, so the gases explode when the
right compression is reached to allow
the glowing glow-plug element to set
it off. (Gas engines do the same, except
there is a spark plug instead of a glow
plug. Spark plugs require external
current to activate them.)
The piston travels down again and
the exploded—and expanded—gases
begin to escape from the exhaust port.
The piston begins to compress the fuel/
air mix waiting below. This forces the
mix into and up the transfer porting.
These channels run up the outside
of the cylinder liner in the engine
casing. They guide the fuel/mix into
the combustion chamber to provide a
charge for the next explosion.
This is where you will hear the word
“timing” again. There are ports in the
cylinder liner that are positioned to let
the exhaust gases out before the fuel/
air mix comes in through its own inlet
port or ports. There are no cylinderhead
valves similar to an automobile in
this type of engine, so the control of gas
arrival and departure is achieved by the
positioning of the ports in the cylinder
running leaner, but is actually running
faster—often with an increase of many
hundreds of rpm.
Depending on the exhaust design, the
tuning only happens at a certain rpm
and is not available throughout the rpm
range. Two primary factors influence
the width of the rpm range in which
such tuning can occur. The first one is
the preset timing of the cylinder ports.
You may hear terms such as “broad”
or “wide timing,” which mean that the
design is less sensitive to tuning and
will perform fine with a standard bolton
muffler.
The second factor is the length of the
tuning pipe. (In this case you should
regard a stock muffler as a short,
fat pipe). Generally, the longer the
muffler, the broader the rpm range.
Longer pipes typically give better
mid-range power and shorter pipes
give higher rpm at the top end. From
a sound reduction point of view, the
longer pipes give better results.
Because there is no way for us
to change the hardwired genetic
programming that dictates how we
react to sound, it falls to us to change
the sounds our engines make!
Correction
In my June 2012 “The Engine
Shop” column on servicing a twostroke
engine I wrote, “If you have
a 1/4 x 32 tap it will do no harm to
clean up the glow plug threads in the
head.”
At the end of my August 2012
column I added a late update on
glow-plug thread size that was
incorrect. Please be clear that it is still
1/4 x 32 for most glow engines.
