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Technical Information - Spark Plugs
Spark plugs are one of the most
misunderstood components of an engine. Numerous questions have surfaced
over the years, leaving many people confused.
This guide was designed to assist the technician, hobbyist, or race
mechanic in understanding, using, and troubleshooting spark plugs. The
information contained in this guide applies to all types of internal
combustion engines: two stroke engines, rotary engines, high
performance/racing engines and street vehicles.
Spark plugs are the "window" into your engine (your only eyewitness to
the combustion chamber), and can be used as a valuable diagnostic tool.
Like a patient's thermometer, the spark plug displays symptoms and
conditions of the engine's performance. The experienced tuner can analyze
these symptoms to track down the root cause of many problems, or to
determine air/fuel ratios.
SPARK PLUG BASICS:
The spark plug has two primary functions:
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To ignite the air/fuel mixture |
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To remove heat from the combustion chamber
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Spark plugs transmit electrical energy that turns fuel into working
energy. A sufficient amount of voltage must be supplied by the ignition
system to cause it to spark across the spark plug's gap.
This is called "Electrical Performance."
The temperature of the spark plug's firing end must be kept low enough
to prevent pre-ignition, but high enough to prevent fouling. This is
called "Thermal Performance", and is determined by the heat range
selected.
It is important to remember that spark plugs do not create heat,
they can only remove heat. The spark plug works as a heat
exchanger by pulling unwanted thermal energy away from the
combustion chamber, and transferring the heat to the engine's cooling
system. The heat range is defined as a plug's ability to dissipate heat.
The rate of heat transfer is determined by:
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The insulator nose length |
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Gas volume around the insulator nose |
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The materials/construction of the center
electrode and porcelain insulator |
HEAT RANGE
A spark plug's heat range has no relationship to the actual voltage
transferred though the spark plug. Rather, the heat range is a measure of
the spark plug's ability to remove heat from the combustion chamber. The
heat range measurement is determined by several factors; the length of the
ceramic center insulator nose and its' ability to absorb and transfer
combustion heat, the material composition of the insulator and center
electrode material.
Heat rating and heat flow path of NGK Spark Plugs
The insulator nose length is the distance from
the firing tip of the insulator to the point where insulator meets the
metal shell. Since the insulator tip is the hottest part of the spark
plug, the tip temperature is a primary factor in pre-ignition and
fouling.
Whether the spark plugs are fitted in a lawnmower, boat, or a race car,
the spark plug tip temperature must remain between 500C-850°C.
If the tip temperature is lower than 500°C, the insulator area
surrounding the center electrode will not be hot enough to burn off carbon
and combustion chamber deposits. These accumulated deposits can result in
spark plug fouling leading to misfire.
If the tip temperature is higher than 850°C the spark plug will
overheat which may cause the ceramic around the center electrode to
blister and the electrodes to melt. This may lead to
pre-ignition/detonation and expensive engine damage.
In identical spark plug types, the difference from one heat range to
the next is the ability to remove approximately 70°C to 100°C from the
combustion chamber.
A projected style spark plug firing tip temperature is increased by 10°C
to 20°C.
Tip Temperature and Firing End
Appearance
The firing end appearance also depends on the
spark plug tip temperature. There are three basic diagnostic criteria for
spark plugs: good, fouled and overheated. The borderline between
the fouling and optimum operating regions (500&800 deg;C) is called
the spark plug self-cleaning temperature. The temperature at this
point is where the accumulated carbon and combustion deposits are burned
off.
Bearing in mind that the insulator nose length
is a determining factor in the heat range of a spark plug, the longer the
insulator nose, the less heat is absorbed, and the further the heat must
travel into the cylinder head water journals. This means the plug has a
higher internal temperature, and is said to be a hot plug.
A hot spark plug maintains a higher internal operating temperature to burn
off oil and carbon deposits, and has no relationship to spark quality
or intensity.
Conversely, a cold spark plug has a shorter
insulator nose and absorbs more combustion chamber heat. This heat travels
a shorter distance, and allows the plug to operate at a lower internal
temperature. A colder heat range is necessary when the engine is modified
for performance, subjected to heavy loads, or is run at high rpm for a
significant period of time.
The colder type removes heat more quickly, and will reduce the chance of
pre-ignition/detonation and melting or damage to the firing end. (Engine
temperature can affect the spark plug's operating temperature, but not the
spark plugs heat range).
Below is a list of some of the possible external influences on a spark
plug's operating temperatures. The following symptoms or conditions may
have an effect on the actual temperature of the spark plug.
The spark plug cannot create these conditions, but it must be able
to cope with the levels of heat...if not, the performance will suffer and
engine damage can occur!!
Air/Fuel Mixtures seriously affect engine
performance and spark plug operating temperatures.
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Rich air/fuel mixtures cause tip temperature
to drop, causing fouling and poor drivability |
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Lean air/fuel mixtures cause plug tip and
cylinder temperature to increase, resulting in pre-ignition, detonation,
and possibly serious spark plug and engine damage |
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It is important to read spark plugs many times
during the tuning process to achieve the optimum air/ fuel mixture |
Higher Compression Ratios/Forced Induction
will elevate spark plug tip and in-cylinder temperatures!
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Compression can be increased by performing any
one of the following modifications:
a) reducing combustion chamber volume (i.e.: domed pistons, smaller
chamber heads, milling heads, etc.)
b) adding forced induction (Nitrous, Turbo charging or Supercharging)
c) camshaft change |
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As compression increases, a colder heat
range plug, higher fuel octane, and careful attention to ignition timing
and air/fuel ratios are necessary. Failure to select a colder spark plug
can lead to spark plug/engine damage! |
Advancing Ignition Timing
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Advancing ignition timing by 10° causes tip
temperature to increase by approx. 70°-100° C |
Engine Speed and Load
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Increases in firing-end temperature are
proportional to engine speed and load. When traveling at a consistent high
rate of speed, or carrying/pushing very heavy loads, a colder heat
range spark plug should be installed |
Ambient Air Temperature
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As air temperature falls, air density/air
volume becomes greater, resulting in leaner air/fuel mixtures. |
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This creates higher cylinder
pressures/temperatures and causes an increase in the spark plug's tip
temperature. So, fuel delivery should be increased. |
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As temperature increases, air density
decreases, as does intake volume, and fuel delivery should be decreased
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Humidity
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As humidity increases, air intake volume
decreases |
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Result is lower combustion pressures and
temperatures, causing a decrease in the spark plug's temperature and a
reduction in available power. |
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Air/fuel mixture should be leaner, depending
upon ambient temperature. |
Barometric Pressure/Altitude
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Also affects the spark plug's tip temperature
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The higher the altitude, the lower cylinder
pressure becomes. As the cylinder temperature de-creases, so does the plug
tip temperature |
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Many mechanics attempt to "chase" tuning by
changing spark plug heat ranges |
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The real answer is to adjust jetting or
air/fuel mixtures in an effort to put more air back into the engine
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Types of Abnormal Combustion
Pre-ignition
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Defined as:
"..ignition of the air/fuel mixture before the pre-set ignition timing
mark" |
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Caused by hot spots in the combustion
chamber...can be caused (or amplified) by over advanced timing, too hot
a spark plug, low octane fuel, lean air/fuel mixture, too high
compression, or insufficient engine cooling |
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A change to a higher octane fuel, a colder
plug, richer fuel mixture, or lower compression may be in order |
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You may also need to retard ignition timing,
and check vehicle's cooling system |
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Pre-ignition usually leads to detonation;
pre-ignition an detonation are two separate events |
Detonation
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The spark plug's worst enemy! (Besides
fouling) |
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Can break insulators or break off ground
electrodes |
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Pre-ignition most often leads to detonation
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Plug tip temperatures can spike to over
3000°F during the combustion process (in a racing engine) |
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Most frequently caused by hot spots in the
combustion chamber. Hot spots will allow the air/fuel mixture to
pre-ignite. As the piston is being forced upward by mechanical action of
the connecting rod, the pre-ignited explosion will try to force the piston
downward. If the piston can't go up (because of the force of the premature
explosion) and it can't go down (because of the upward motion of the
connecting rod), the piston will rattle from side to side. The resulting
shock wave causes an audible pinging sound. This is detonation. |
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Most of the damage than an engine sustains
when "detonating" is from excessive heat |
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The spark plug is damaged by both the elevated
temperatures and the accompanying shock wave, or concussion |
Misfires
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A spark plug is said to have misfired when
enough voltage has not been delivered to light off all fuel present in the
combustion chamber at the proper moment of the power stroke (a few degrees
before top dead center) |
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A spark plug can deliver a weak spark (or no
spark at all) for a variety of reasons...defective coil, too much
compression with incorrect plug gap, dry fouled or wet fouled spark plugs,
insufficient ignition timing, etc. |
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Slight misfires can cause a loss of
performance for obvious reasons (if fuel is not lit, no energy is being
created) |
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Severe misfires will cause poor fuel economy,
poor drivability, and can lead to engine damage |
Fouling
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Will occur when spark plug tip temperature is
insufficient to burn off carbon, fuel, oil or other deposits |
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Will cause spark to leach to metal shell...no
spark across plug gap will cause a misfire |
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Wet-fouled spark plugs must be changed...spark
plugs will not fire |
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Dry-fouled spark plugs can sometimes be
cleaned by bringing engine up to operating temperature |
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Before changing fouled spark plugs, be sure to
eliminate root cause of fouling |
THIS INFORMATION IS PROVIDED TO YOU
BY NGK, AND IS NOT INTENDED
TO BE TUNING SUGGESTION FROM WAKULA RACING.
TUNE AT YOUR OWN RISK!!
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