What is a spark
plug? What does it does?
people think, the spark plug doesn't
"makes" a spark. It does offer the gap
necessary for the electric current produced by the
coil jumps as a spark. This spark starts the
ignition of the air/gas mixture compressed at the
top of the piston, running the engine.
A spark plug has dimensions and
features that should be taken in account when time
arrives to a substitution, to prevent damage to
a. thread diameter: diameter of
the plug's threaded part, the one which goes into
thread reach: length of the threaded part;
heat range: the velocity which the plug is able to
get rid of excess heat.
Let's talk a
little about each one of those features:
Diameter: the engines used in
R/C boating use spark plugs with 14 mm thread.
This is the feature that causes no worry, because
of the obvious reason that you can not put a
14 mm plug on a 12 mm cylinder or v.v.
reach: the spark plug with adequate reach doesn't
leave any portion of the thread exposed into the
cylinder, on the other hand doesn't leave any
portion of the cylinder thread non covered. A
short plug allows carbon deposits at the
unprotected thread, demanding a careful cleaning
before installing a new plug. A long plug,
however, leaves a portion of the thread exposed
into the combustion chamber. On the exposed
portion carbon deposits are formed, that may difficult
taking the plug off and even damage the cylinder
thread. Besides, a too long plug may touch the
head of the piston on it's upper stroke, with
disastrous consequences to the engine. As another
detrimental effect, the plug with wrong length
doesn't reach the correct working temperature:
cooled by the incoming new mixture being
compressed, heated by the ignition of the mixture,
the correct plug maintain the adequate
range: To work correctly, the electrode
demands a working temperature between
752º/1652º F. Below 752º non burned carbon/oil
deposits will foul the plug, above 1652º the
electrode tip becomes white-hot and cause
pre-ignition/detonation (ignition before the
correct time/uncontrolled and fast burning of the
to their heat range, plugs are classified in
"cold" or "hot". This
classification has not to do with the spark
temperature - a "hot" plug doesn't have
a spark hotter than the one of a "cold"
plug and v.v. . The heat range of a plug indicates
it's capacity of getting rid of excess heat. The
picture bellow shows how a plug dissipates this
heat. As we can see, 58% of this heat is
dissipated trough the cylinder, being this the
more important way of cooling.
a "cold" plug dissipates heat quicker
that a "hot" one. Bellow, you can see
the difference between a hot and a cold plug. The
last one has a shorter porcelain isolator, offering
a quicker path to heat transfer to the
that the spark plug needs to work between certain
limits of temperature, it should be understood
that different conditions of demand from the
engine varies this temperature - on high RPM the
engine comes hotter and the plug needs to
dissipate heat more quickly. This is the reason
why hoped up engines may need colder plugs than
the stock one.
how to read the plug: this will show you if you is
using the plug with the correct heat range.
Dark brown to black isolator:
too cold plug
brown isolator: correct plug
White to light gray isolator:
too hot plug - change immediately
in doubt, choose the colder one. The worst that
can happens is an engine that easily fouls the
plug and runs erratic. Nothing very dangerous..
Resistor plug: as a path for the
spark, the plug is an excellent interference
producer. On your car you exchange you contagiros
for a set of non resistor spark plugs. Tune your
radio on a not to strong broadcast station and
have fun with the noise of the sparks as you run
your engine. With a little practice you can
estimate the engine rotation by the noise on your
your boat the problem is more serious. Your
receiver doesn't receive music but the signals of
your transmitter. The non resistor plug creates a
interference field that may even suppress the transmitter
signal. If you are using a Shark Racing FailSafe,
prepare to swim/paddle to recover the stalled boat
in the middle of the lake. Se you are not using a
fail safe, pray for your boat runs off gas before hitting
something or somebody.
So, always use a resistor spark
plug. It cuts the interference caused by the
jumping of the spark and allows that the receiver
only tunes the transmitter.
what means that array of numbers and letters on
the spark plug?
inform all the features of the spark, the ones we
already talked and others that we didn't, like the
type of electrode, it's material, dimensions for
the outer body and even the metal of the body.
A "R" letter on the
code identifies a resistor spark plug. The reach
is also indicated by a letter. The heat range has
a numeric indication - on some manufactures
the higher the number, the colder the plug, on
others, the opposite is true.
always follow the indication of the manufacturer
of your engine. If you need more information, use
the links bellow.
How to choose and
read spark plugs: http://link.sandiego.com/scripts/wheelbase/message.idc?passin=458
How you can read
spark plugs and select them - by Gordon Jennings:
spark plugs: http://www.classictruckshop.com/clubs/earlyburbs/projects/spark/plugs.htm
Look at your
Plugs and resistor spark plugs caps: http://www.ultralightnews.com/enginetroublshooting/resistorcapsandplugs.htm
Spark plugs: http://home.clara.net/dave.moore/plugs.html
Spark plugs: http://www.dansmc.com/sparkplugs1.htm
Spark plugs: http://www.picknowl.com.au/homepages/harrals/tech/spark.htm
Spark plugs and
what they say: http://www.motocross.com/motoprof/moto/mcycle/plug2/plug2.htm
Spark plugs overview
by NGK: http://www.sentra.net/tech/sparkplugs.shtml