Articles of Interest
Spark Plug selection
written by Keith Pickersgill 20 March 2008
Firstly, let us differentiate between "Heat" and "Temperature".
Think of Temperature as the concentration of heat in a component (just as
Pressure is the concentration of gas in a chamber).
Temperature is measured in Celsius or Fahrenheit, whereas heat is a
measurement of energy, i.e. Joules.
The heat-range of the spark-plug does not influence the temperature of the
engine itself, it is instead an indication of how much heat is retained by the
plug itself and how much is transferred through advection to the cylinder-
head via the metal thread.
The plug heat-rating determines the heat-flow path from the electrode to
the metal thread. The shorter the heat-path, the more heat flows to the
cooling cylinder-head, the cooler the electrode remains (cooler plug), and
the opposite for longer heat-paths. This fact implies that a "cooler" plug may actually deliver a hotter CHT reading (Cylinder Head Temperature)!
Why a range of different plugs?
The spark-plug must become hot enough to burn itself free of contaminates, but
not so hot that the electrode burns away, which would increase the gap
until it misfires.
The actual temperature range between adjacent plug models is quite wide
(I seem to remember more than 120C) and there are very large overlaps
between two adjacent plugs in the heat-range models, so often you could use
either/or one heat-range model up or down.
To determine the correct plug for your local engine, elevation, fuel and
oil, start with a brand new plug of your best-guess, then:
1) set the gap as accurately as you can.
2) Put about ten hours normal flying on the plug.
3) Strip the plug and examine the electrode as well as the gap.
If the gap has widened, then your plug is getting too hot (burning away the
electrode), you need a cooler heat-range plug (higher number in NGK range).
You may have noticed some misfiring and engine starting problems.
If the gap has not widened but the electrode is covered in thick black or
dark-grey carbon/ash/varnish/caramel coating, then the plug is not getting
hot enough to burn the crap off, you need a "hotter" plug, lower number in
the NGK range. You may have noticed some rough running, and perhaps cold-
The latter scenario can also be caused by too rich fuel-mixture and to a
lesser degree, by too much oil in the fuel.
We cannot easily find in South Africa, the B10ES or BR10ES which is supplied on many new paramotors, so we mostly use the
BR9ES which does gradually burn away the electrode, however we just re-set
the gap every 10 hours or so (or whenever it starts misfiring) and keep
using the plug until the electrode gets too short (Maybe 60 hours or more).
By the way, for your info. In the NGK plug model names depict:
B - is the thread size (Diameter and pitch)
R - denotes the presence of a Resistor, or spark-noise supressor, which is
essential if you fly with a radio
1 or 2 digits is the heat-range as discussed above.
E - denotes Extended Length thread.
S - Denotes Standard Tip
If you want a better plug, get the Iridium-tipped Plug. It does not burn
away even at extreme temperatures, so they can sharpen the electrode to a point, which makes for a stronger and more reliable spark, easier
starting, better fuel-economy and cleaner-burning. Iridium melts at the
2450 C, considerably hotter than Copper or Platinum.
Iridium is 8 times stronger and 6 times harder than Platinum. A single
Iridium plug could conceivably last your entire flying life!
To order: S (standard tip) is replaced with IX (Iridium). So you ask for
BR9EIX or similar.
They are available from some specialized auto-spares dealers, but some pilots have ordered them from abroad.
Denzo make an even smaller diameter Iridium electrode than NGK (0.4mm vs
0.6mm) with higher performance, though shorter service life. These are
considerably more expensive and not easy to find commercially.
PS. Warning! Do not fit a suppressed-plug as well as a supressor-cap!
The R plugs have a 5 kilo-Ohm supressor.
Suppressed Caps can be 5 kilo-Ohm, or 10.
You need max 5 kilo-Ohm, or risk overloading the ignition coil.
So use EITHER a 5 kilo-Ohm cap, or a R-plug, never both and never the 10k
cap (even if used without a R-plug)
Address comment to Keith Pickersgill at email@example.com