Exhaust Systems- Using Tuned Pipes
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The greatest advantage gained from using a pipe is the increase in combustion chamber pressures that are
generated during the combustion process (by virtue of the denser air / fuel charge). As a general rule, a piped
engine will require a higher volume combustion chamber (lower compression ratio) to reduce the initial
compression. As stated earlier, heat is the greatest undesirable result of most engine modifications, the higher the
compression and the higher the rpm, the more power and the MORE heat produced. Hence when engines are
modified to produce more power, then modifications are also necessary to dump the accompanying increase in
heat.
The most common signs of over heating are blown plugs and a ‘sand-blasted’ looking head. A sand blasted /
pitted head is actually caused by detonation ... which can be caused by under compression, and running lean to
compensate, or by over compression at any needle setting.
Tuning
For optimum performance, the following guide may prove useful (for with or without a pipe).
A) OVERHEATING / HOT WEATHER
Increase combustion chamber volume or head (squish band) clearance
Increase stinger diameter or decrease stinger length (to reduce pipe pressure)
Decrease prop load.
Improve cooling
Decrease nitromethane content of the fuel (if using nitro)
B) COOL RUNNING / COLD WEATHER
Decrease combustion chamber volume or head (squish band) clearance
Decrease stinger diameter or increase stinger length
Increase prop load or
Decrease cooling
Increase nitro content of the fuel (if the competition rules allow nitro)
C) HIGH HUMIDITY
Decrease prop load
PIPE LENGTH
PIPE TOO SHORT
a) frequent blowing of glow plugs
b) Sand blasted head
c) over heating engine
d) difficult to get 'on the pipe', sensitive needle
e) excessive carbonising of the head
f) engine sags under load, ie, long climbs
g) 'Harsh' running
PIPE TOO LONG
h) very easy on needle
i) smooth, quiet running
j) prone to burbling, rich running
k) no power
l) none of (a) to (g) above, try shorter and see what happens.
Like propellers, only trial and error will prove which pipe is best suited to your engine requirements.
ADJUSTING Quiet PIPE LENGTHs
It is rare that a pipe will not need adjusting in length, in fact the only way a pipe can be optimised is to vary the
length to observe the changes to determine the best length. (This can also be done by varying the prop load and
observing the pipe characteristics).
To shorten a pipe, remove material from the alloy header pipe, as this will lessen weight and help prevent fatigue
of the alloy. Save the off-cut and use for extending the pipe.
To extend a pipe, use the header off-cut, pushed inside a 75mm length of silicon coupler. Many pilots will have
three silicon extensions in their flight boxes, at say 8, 16 and 24mm lengths.
In some circumstances there will be a diameter difference between header and pipe, where possible, use the
larger diameter.
Source : n/a
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