The spark plug's location exposes it to extreme temperature
variations, chemicals, fuels and oils. It is also attacked by cylinder
pressures produced by the piston as well as cam timing, then it is also
assaulted by high-output ignition units. As a result of all of this, one
can effectively learn what the engine is doing by reading the firing end
of the spark plugs.
By careful examination of the plug's color, gap, and any
deposits that reside on it, you will be shown the efficiencies as well as
deficiencies of what is going on in the engine. Spark plugs should be
checked at least yearly, and replaced as often as necessary. In most cases
you can follow the manufacturers recommendations, but in a race car, our
replacement intervals are quite frequent.
The basics of a spark plug is that it must perform two
- To Ignite the Air/Fuel mixture
- To REMOVE the heat
out of the combustion chamber
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 the spark to jump the across the spark plug gap,
thus creating what is called Electrical Performance.
Additionally, 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 of the spark plug.
It is important to understand that spark plugs CANNOT create
heat, only remove it! The spark plug works as a heat exchanger, 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 is determined by:
- The insulator nose length
- Gas volume around the insulator nose
- Materials and/or construction of the center electrode and
Now to the actual function:
As the Ignition is triggered and sends the spark through the rotor, to the
cap, down the wire and then it jumps the gap of the spark plug, a spark
kernel is created that ignites the air/fuel mixture in the combustion
chamber. Proper timing of this spark is not the only concern as described
above. You must have the proper heat range (described later) as well as
the correct gap.
On weaker or stock ignitions, opening up the gap CAN
increase the spark kernel size, thereby creating a more efficient burn.
The problem lies in that any added gap creates more strain on the other
- Coils may not have enough stored energy to fire, or in the
least case, not enough energy to cross the gap, creating a miss-fire.
- Plug wires will break down due to the added resistance as the
spark tries to reach ground.
- Rotor and Cap, as well as points (if you still have an
interest in prehistoric ignitions), and the carbon bushing in the center
of the distributor cap will show early failures.
All of this is because the greater the gap, the higher the
voltage requirement to jump the gap. Do not forget the gap between the
rotor arm to the distributor cap too. A high performance rotor is a bit
longer, allowing less spark loss or chance of spark scatter in the cap as
the spark attempts to jump the plug gap.
As many of us know that race, it is also possible to slow
down a car if the gap is too big. I will get into this later when I
describe proper spark plug gaps.
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A spark plug's heat range has no relationship on the actual
voltage transferred through 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
- The insulator nose's ability to absorb and
transfer combustion heat
- The material composition of the insulator
- The material composition of the center
The longer the insulator nose, gives a larger surface area
exposed to combustion gasses and heat is dissipated slowly. This also
means the firing end heats up more quickly. We are talking about exposed
ceramic length, not extended tip length.
The insulator nose length is the distance from the firing
tip of the insulator to the point where the 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. No matter
what the plugs are installed in, be it a lawnmower, a boat, your daily
driver or your race car, the spark plug tip temperature must remain
between 450°C to 850°C. If the tip temperature is lower than 450°C, the
insulator area surrounding the center electrode will not be hot enough to
deter fouling and carbon deposit build-ups, thus causing misfires. If the
tip temperature exceeds 850°C, the spark plug will overheat which can
cause the ceramic around the the center electrode to blister as well as
the electrodes will begin to melt. This may lead to
pre-ignition/detonation and expensive engine damage. (see the plug
In identical spark plugs, the differences 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
temperature is increased by 10°C to 20°C.
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 (450°C) is called the spark plug self-cleaning
temperature. This is the temperature point where the accumulated carbon
and combustion deposits are burned off automatically.
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 that 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 can be necessary when an engine is modified for
performance, subjected to heavy loads, or it is run at high RPMs for
significant periods of time. The higher cylinder pressures developed by
high compression, large camshafts, blowers and nitrous oxide, not to
mention the RPM ranges we run our engines at while racing, make colder
plugs mandatory to eliminate plug overheating and engine damage. The
colder type plug removes heat more quickly, and will reduce the chance of
pre-ignition/detonation and burn-out of the firing end. (Engine
temperatures can affect the spark plug's operating temperature, but not
the spark plug's heat range).
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on Spark Plug Temp and Performance:
Below is a list of possible external influences
on a spark plug's operating temperatures. The following symptoms or
conditions may have an affect on the actual temperature of the spark plug.
The spark plug cannot create these conditions, but it must be able to deal
with all the levels of heat, otherwise performance will suffer and engine
damage can occur:
seriously affect engine performance and spark
- Rich air/fuel mixtures cause tip temperature to drop, causing
fouling and poor drivability.
- Lean air/fuel mixtures cause plug tip and cylinder
temperatures to increase resulting in pre-ignition, detonation, and
possibly serious spark plug and internal engine damage.
- It is important to read spark plugs many times during the
tuning process to achieve optimum air/fuel mixture.
Higher Compression Ratios and
Forced Induction will elevate spark plug tip and
- Compression can be increased by performing any one of the
- a) reducing combustion chamber volume (i.e.: domed
pistons, smaller chamber heads, milling heads, etc.)
- b) adding forced induction (Nitrous, Turbocharging,
- c) camshaft change
- As compression increases, a colder heat range plug is
required, as well as higher octane fuel and paying careful attention to
ignition timing and air/fuel ratios are also necessary.
Advanced Ignition Timing:
Advancing timing by 10° causes plug
temperature to increase by approximately 70°C to 100°C.
Engine Speed and Load:
Increases in firing-end temperatures and are
proportional to engine speed and load. When traveling at a constant high
rate of speed, or carrying/pushing very heavy loads, a colder heat range
spark plug should be installed.
Ambient Air Temperature:
- As air temperature falls, air density volume
increases, resulting in leaner air/fuel mixtures. This creates
higher cylinder pressures and temperatures that causes an an
increase in the spark plug's tip temperature. Fuel delivery
should be increased.
- As temperature increases, air density decreases,
as does intake volume, and fuel delivery should be decreased.
As humidity increases, air volume decreases. The result is lower
combustion pressures and temperatures, causing a decrease in the spark
plug's temperature and a reduction in available power. Air/Fuel mixture
should be leaner, depending on ambient air temperature.
Barometric Pressure and
- Affects the spark plug's temperature
- The higher the altitude, the lower the cylinder
pressure becomes. As the cylinder temperature decreases, so does
the tip temperature.
- Many tuners attempt to "chase" tuning by changing spark
plug heat ranges.
- The real answer is to play with the jetting or air/fuel
mixtures in an effort to put more air back in the engine.
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There are hundreds of different spark plug types designed
for different engines as well as specific applications. The two main ones
we use in racing are the standard tip and extended tip. It is my opinion
that any time you can use the extended tip, do it! The longer tip gets the
spark kernel started further into the center of the bore for a more
complete combustion process.
But, there are also specially designed spark plugs that are
supposed to increase the spark kernel size. You see, the larger the spark
kernel that is generated by the spark jumping the electrode gap, the more
complete burn, the better power and efficiency and the smoother the car
will run for longer periods of time. These specially designed plugs will
have multiple angles that allow the spark to find the easiest path as well
as getting the spark kernel out from under the ground electrode which can
quench the kernel size limiting it's size and expansive growth.
Of these plugs, there are those that work, those that are
hype, and those that are a great idea but they are manufactured by
companies whose only purpose is profit instead of quality.
One thing to be wary of is plug material. Of the plugs I've
tested, the fine-wire gold plugs have made the best power. The Platinum
plugs are only good for longevity, though most imports run better with a
platinum plug. This has to do with the material of the cylinder head and
engine block and the plugs ability to properly ground.
People always ask my plug recommendations. I must say that I
like the NGK V-Power Plugs, the Champion Premium Fine-Wire Gold plugs, and
the Nippondenso U-Groove plugs for domestics. In the Imports, the Bosch or
NGK Platinum plugs usually work best. I have been testing the AC RapidFire
plugs currently and I must say I am impressed by the results. Plugs that
are useless to me are the Split-Fire plugs (poor materials -- actually an
Autolite plug with the "V" tip), and of course the Autolite plug itself. I
have seen the most failures and poor performance out of these plugs. This
is not a bash session, so those of you that love your Split-Fires or
Autolites, I'll wait for you at the finish line ...
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Many people believe that spark plugs fire instantly. This is
partly true because they fire in milliseconds, although if one looks at an
oscilloscope pattern you will see much more than a single instantaneous
firing event. Many things also occur that you cannot see even with the
oscilloscope. Part of what you cannot see, but can in many cases hear, is
the noise that is picked up in the speakers of your car stereo. This is
called RFI, or Radio Frequency Interference.
Spark Plug Firing Voltages:
When the breaker points or solid state ignition unit
(switching device) interrupts current flow in the primary ignition
circuit and induces current flow into the secondary windings of the
coil, there is an instantaneous voltage spike.. This represents the
voltage required to overcome the spark plug and distributor rotor gaps.
Once the spark gaps have been bridged, the secondary voltage required to
sustain the spark across the gap is much less and drops . The spark
continues to arc across the gap at more or less constant voltage until
the arc is extinguished . This is due to
coil energy drop in that it can not sustain the spark any longer. During
this arc duration (Spark Duration), the plug actually fires several
times. This is caused by high frequency oscillations in the primary and
secondary windings of the coil, which continues to induce voltage
spikes. They continue and slowly diminish even after they are no longer
strong enough to sustain spark. All of this takes place in roughly one
thousandth of a second.
With our race designed ignition units, they concentrate
their efforts on sustaining spark duration as well as limiting the
voltage drop after the gaps have been bridged. Most aftermarket
ignitions concentrate on giving us 20° of spark duration (crank degrees)
as well as much higher spark energy output. A high performance coil
helps this out, but the Capacitive discharge and digital ignition units
assist in storing and delivering this power through the coil more
efficiently, faster and give the ability to achieve higher RPMs more
safely and efficiently in fuel mixture burning.
The Cause of RFI:
If we were to slow down the oscilloscope to perhaps
0.00000025 seconds and greatly expand the pattern,
we would see that what appears to be consistent from position C to D in
the the first illustration above is actually a series of extremely high
bursts of energy. These energy bursts are discharged at the same
frequency band as radio and TV frequencies. It is these bursts that make
your car radio snap - crackle - and pop ... as well as just about
anything electronic including telephones, aircraft control towers and
heart pace makers by causing static and interference.
Sources of RFI:
Automotive ignition systems are not the only things that
spew RFI into the atmosphere. Lawn mowers, snowmobiles, ATV's, tractors,
power lines, traffic control devices, etc. all do it. One publication
refers it to "electronic air pollution". As many of us know, we live in
a sea of constant electromagnetic waves.
Any time you have a flow of electric current you will have
a magnetic field. Coils, relays, switches, solenoids, generators,
servomotors all affect communication equipment, electronic circuits and
computers. The higher the voltages, the more critical this becomes.
Anytime you have the spark jump a gap or a contact, you have a miniature
Back in the 1930's, engineers recognized that RFI could be
a nuisance. As the years, testing and technology advancements went by,
it turned into an even greater problem. Especially with the advent of
high-tech communications systems, computers and electronic engine
control devices. The Society of Automotive Engineers (SAE) decided to
set up standards for measuring as well as the control of RFI. These are
called "EMI Standards" or the more technical name for radio static of
Electromagnetic Interference (EMI).
The current standard for EMI was adopted in 1961 and is
known as J551. It limits RFI at frequencies between 20-1,000 MHz. All
spark plug manufacturers must adhere to it. The most common method used
to suppress RFI is to install a resistor in series with the spark plug's
Other ways that control RFI
- The metal fenders, grille and hood of your car. These provide
a shielding affect which absorbs much of the RFI emitted from your
ignition components. Plastic and composite body panels are basically
transparent to RFI and provide little to no shielding.
- The use of capacitors, silicone grease at connections,
proper grounding of all circuits and routing wires to reduce
electromagnetic interference are all helpful in reducing RFI.
- The use of carbon impregnated secondary wiring (plug
wires) and resistor spark plugs have the most impact when reducing and
controlling RFI. In our race cars with our high output ignitions, it is
best to use a specifically designed plug wire for our applications.
These are usually the what is called "Spiral Wound" style plug wires.
The construction of these wires starts with a Ferro0-Magnetic
impregnated inner core, helical wrapped copper alloy conductor, a high
dielectric insulator then a heavy fiberglass braid. Wrapping this is a
8mm to 10mm silicone jacket. Also, secure connections of the plug wire's
terminal ends are mandatory along with secure fitting boots.
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- Proper gapping of the spark plug is necessary to get
maximum spark energy, lowest RFI release as well as what is best for the
longevity of the secondary ignition components (coil, cap, rotor, wires,
- When checking plug gaps, the correct way is to use ONLY
wire gauges, though many of us are using the slider style gapping tools.
These flat or feeler gauge style gauges do not accurately measure true
width of spark plug gap.
Remembering that the
Ignition Unit, plug brand as well as heat range, cap and/or rotor styles
and in many cases fuel type or brand will change the optimum spark plug
Lastly, NEVER use the porcelain insulator as
a fulcrum point when setting these gaps.