Tuned Pipe Theory and
PMB Model Boats
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Tuned Pipe Theory Part1
How Two-Stroke Expansion Chambers Work.
You know that changing the exhaust
pipe and pipe length on your boat can have a marked effect on the
engine's power characteristics, but do you by how much and why ?
How Much. A two stroke 125cc engine with standard
exhaust system can combust no more than 125cc of fuel air mix. A
two stroke 125cc engine with good tuned exhaust system can combust
approx 180cc of fuel air mix.
Why. Simply put, it's because the two-stroke exhaust
system, commonly referred to as an 'expansion chamber' uses
pressure waves emanating from the combustion chamber to
effectively supercharge your engine.
In reality, expansion chambers are built to harness sound waves
(created in the combustion process) to first suck the cylinder
clean of spent gases--and in the process, drawing fresh air/gas
mixture (known as 'charge') into the chamber itself--and then
stuff all the charge back into the cylinder, filling it to greater
pressures than could be achieved by simply venting the exhaust
port into the open atmosphere. This phenomenon was first
discovered in the 1950s by Walter Kaaden, who was working at the
East German company MZ. Kaaden understood that there was power in
the sound waves coming from the exhaust system, and opened up a
whole new field in two-stroke theory and tuning.
An engine's exhaust port can be thought of as a sound generator.
Each time the piston uncovers the exhaust port , the pulse of
exhaust gases rushing out the port creates a positive pressure
wave which radiates from the exhaust port. The sound will be be
the same frequency as the engine is turning, that is, an engine
turning at 24,000 rpms generates an exhaust sound at 24,000 rpms
or 399 cycles a second--hence, an expansion chamber's total length
is decided by the rpm the engine will reach, not displacement.
Of course those waves don't radiate in all directions since
there's a pipe attached to the port. Early two strokes had
straight pipes, a simple length of tube attached to the exhaust
port. This created a single "negative" wave that helped
suck spent exhaust gases out of the cylinder. And since sound
waves that start at the end of the pipe travel to the other end at
the speed of sound, there was only a small rpm range where the
negative wave's return would reach the exhaust port at a useful
time: At too low of an rpm, the wave would return too soon,
bouncing back out the port. And at too high of an rpm, the piston
would have traveled up the cylinder far enough to close the
exhaust port, again doing no good.
Indeed, the only advantage to this crude pipe system was that it
was easy to tune: You simply started with a long pipe and started
cutting it off until the motor ran best at the engine speed you
So after analyzing this cut-off straight-pipe exhaust system,
tuners realized that pressure waves could be created to help pull
spent gases out of the cylinder. Following this, The tuners
realised that these pressure waves could be utilised still
further by using a divergent cone to increase the strength of the
negative wave and then that a convergent cone added to this would
increase power still further as explained next....
The exhaust opens on the down
stroke and a pressure wave emanates from the exhaust port into the
header pipe. This pressure wave travels through the exhaust gases
that are in the pipe at the speed of sound.. It’s the pressure
wave that travels at this speed, not the exhaust gases themselves.
(Imagine a stream and you throw in a rock. The waves from that
rock will travel down the stream faster than the speed of the
water.) Anyway, the wave reaches the front divergent cone and a
weak negative wave (negative pressure or ‘suck‘) (laws of
physics) is sent back to the exhaust port which reaches the
exhaust port while the transfers are open helping to remove
exhaust gases from the cylinder which in turn helps fresh mixture
from the crankcase up through the transfers into the cylinder. (
some of which will enter the front part of the header)
The length of the front cone and its
distance from the cylinder (header length) determines the amount
of time that the pressure reducing wave from the exhaust does it
work in emptying the cylinder of exhaust gas and then assisting
the fresh mixture up from the crankcase into the cylinder. If
header is too short then the wave energy from the front cone is
wasted because the negative wave ( the ‘suck’) arrives at the
exhaust port while the cylinder pressure is still high after
combustion. It should arrive there when the pressure in the
cylinder is low but there are still exhaust gases that need to be
extracted. If the header length is too long then the wave is
arriving later than optimum and the exhaust gases are not fully
removed from the cylinder. The front cone needs to be long enough
to generate a wave to help the fresh mixture into the cylinder but
it also needs to continue working long enough to allow some fresh
mixture into the first part of the header. This is the mixture
which will be forced back into the cylinder. If it is too short,
then it does not allow mixture into the header. If its too long,
then it reduces the length of the rear cone and that needs to be
long enough to force all of the unburnt mixture in the header to
be forced back into the cylinder. The pressure wave continues into
the rear cone and immediately sends a positive pressure wave (laws
of physics!) back down the tuned pipe towards the exhaust port
forcing the unburnt fresh mixture back into the cylinder. The
strength of the wave increases as the rear cone gets smaller and
the length is made so that the returning pressure wave from its
very end at the junction with the stinger coincides with the point
of exhaust port closure. When this most critical length
of stinger to exhaust port) is correct, then maximum power is
achieved. If this critical length is too short then the returning
wave forces hot gases back into the cylinder, dramatically
increasing cylinder combustion temperatures. If this length is too
long the maximum power will not be achieved because maximum
supercharging or cylinder filling will not occur, although power
in the corners will be better because the tuned length will
coincide more with the reduced rpm in the corners.
In conclusion we can see that the
front cone length and distance from exhaust port is very important
to achieve maximum cylinder filling and to pull some mixture into
the header and the distance from piston to start of stinger is
extremely important to get maximum filling ( supercharging) of the
cylinder. . When we adjust the tuned pipe length on our engines we
are moving several things at once, the start of front cone, the
end of front cone, the start of rear-cone and the end of rear
cone/start of stinger.
TO BE FINALLY CONCLUDED
Tuned Pipe Theory
COMPARING PIPES AND PIPE LENGTHS
|The tuned length L as shown in
the diagram is the length that most people use as a comparison. This is OK
as a comparison but the length that is most critical is TL. Many different pipes can
be used on an engine but that tuned length TL will always remain
the same within a few millimetres for a specific rpm (if all other factors remain
constant, nitro content, oil content, air density, temperature etc).
This applies to all two stroke model engines , petrol (gas) or Glow powered
(nitro). We know this from many many bench and on the water
tests conducted on many different engines. To elaborate: If you were
running a tuned pipe at its optimised length (the length that is giving
most power or speed) and that pipe had no flat in the centre section and
you wanted to change to a pipe with a flat in the centre or belly section.
You should measure TL on the old pipe and then set TL on the
new pipe to the same length to give you a starting point for
TUNING THE EXHAUST SYSTEM
is decided by rpm, exhaust timing
and speed of sound within the exhaust system. The last part should remain almost the same whatever you do to the exhaust timing or rpm.
1. Shorten the pipe length and rpm will be higher, lengthen pipe length
and rpm will be lower.
2. If you increase the exhaust timing and rpm stays the same then pipe length
will need to be longer.
3. If you increase the rpm but exhaust timing stays the same then
the pipe length has to be shorter.
If you can measure the rpm of your
motor and exhaust timing, then you can use a simple calculation to show how
much you need to change the pipe length when altering ex timing and
Here are some some simple calculations for
engines where the exhaust gas temperature is not affected by nitro content.
( For these calculations you can measure the pipe length
between whatever points you want to, but the norm is from plug to widest
part of cone.)
If you take a boat that is
running well and pipe length has been optimised then to alter rpm or
exhaust timing the following calculations may help to find the new
required pipe length.
length from manifold face to widest part of front cone.
Just as a theoretical example ....If rpm is 15,000, exhaust timing is 175degrees(duration) and pipe length is
Firstly you work out the constant for your set up. So..
K = R x L
K = 15000 x 330 = 28286
In this example that would give a K number of 28286 and as I wrote before, K will remain
the same whatever you do..
If you want the engine to rev at 16,000, the equation changes to ..
L= E x K
therefore L = 175 x
28285 = 309mm which is the new pipe
length from manifold face to widest part of cone.
This would make the new pipe length 309mm.
If you wanted to increase exhaust timing to 180 degrees and run at
L = 180 x 28286
------------------- = 339mm
length should be separated from stinger diameter because although they are
linked, in practice you would need to make a big change in stinger length
to affect the backpressure. Stinger diameter is crucial to the pipes
operating temperature and hence the power production. If the stinger is
bigger than optimum them making it even bigger will have little effect but
by sleeving it down then you will be able to find the size that gives best
power. Normally a smaller stinger will improve top end power because the
exhaust gas temperature will increase which will have the effect of a
shorter pipe length. If you go too small on the stinger then power will
suddenly start to drop in the corners and the motor will begin to
overheat. To get the best power its usual to lengthen the header and make
the stinger smaller to get the best overall performance. A bigger stinger
will have the effect of spreading the power band but the engine will
not make the same peak hp.
Stinger length is important because its part of the pipe resonance. The
wrong stinger length will reduce performance at the upper end of the rpm
band. i.e. between peak torque and peak bhp.. There will be maybe one or
two stinger lengths that will cut the rpm off at a certain level reducing
the 'overrev' which gives the best top speed. There will be one stinger
length which gives the best overall power and over-rev. I find no way to
calculate that stinger length, trial and error is the only way. It's not
dependent upon engine size, just on the pipe design. For example, my best
.21 pipe runs over 100mm stinger but my best .90 runs around 60mm. One
thing though, very short stingers up to 20mm long don't normally work and
extremely long stingers of 150mm to 200 mm can work very well. Once the
best stinger length is found, it does not seem to vary if the pipe length
PS On stinger length, it is only a few percent performance difference but
every little helps!!
The speed of sound within the exhaust system is dependent upon
the EGT (exhaust gas temperature). The higher the temperature the longer
the pipe length must be for a given rpm.. EGT will vary with the following
factors..Stinger diameter (smaller stinger = higher EGT) , fuel
needle setting. (a leaner mixture will raise EGT.) fuel mix. High
oil content reduces EGT, high nitro content also reduces EGT.
|A few helpful facts. The
volume of a pipe is only really related to the displacement of the engine
because the various diameters of the pipe ( header, belly and stinger) are
a function of exhaust port area, and if an engine has a bigger displacement,
it usually has a bigger exhaust port area. It's often said that a bigger
volume pipe is less peaky or it has a broader spread of power. This is not
actually so. The volume takes care of itself when the pipe is calculated.
The important things are firstly (and most importantly) the length from
piston face to start of stinger and secondly header length, cone
lengths, belly length, and then header diameter, belly diameter and
stinger diameter. Normally a good pipe will have a belly cross sectional
area of about 10 times the exhaust port area with a stinger diameter of
about 0.5 to 0.6 of the exhaust port area and the header around 1.2 times exhaust
port area. By exhaust port I mean the actual port in the liner not the
port where the exhaust manifold bolts on. If we take 2 pipes with the same
cone lengths and total tuned length then the pipe with the largest
volume will will require a smaller stinger diameter to maintain the
same EGT (exhaust gas temperature) within the pipe.
MEASUREMENT WITH IPHONE
|Dave Marles. November 2004