The following information has been taken off the Web. This author does not
care for Autolite plugs. This in no way reflects my opinion on them. I
think his article on spark plugs is an excellent one for those wanting to
understand the principles behind them.
Introduction:
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.
How a Spark Plug Works:
The basics of a spark plug is that it must perform two primary
functions.
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 porcelain
insulator
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 ignition parts.
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.
Spark Plug Heat Range:
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 nose
The insulator nose's ability to absorb and transfer combustion heat
The material composition of the insulator
The material composition of the center electrode
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 center
electrode to blister as well as the electrodes will begin to melt. This may
lead to pre-ignition/detonation and expensive engine damage.
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).
Influences 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:
Air/Fuel Mixtures seriously affect engine performance and spark plug temps.
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 in-cylinder temperatures.
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, Turbocharging, Supercharging)
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 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.
Humidity: 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 Altitude:
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.
Spark Plug Types and Designs:
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 ...
RFI or "Radio Frequency Interference":
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. (as seen in the illustration at right in position A to B).
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
(as seen in position B to C above). The spark continues to arc across the
gap at more or less constant voltage until the arc is extinguished (at
position D above). 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
(positions D to E above) 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.
Proper Plug Gapping:
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, plugs).
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.
When increasing the gap size for our high performance applications
utilizing advanced ignition systems such as Mallory, Accel, Jacobs, Crane
and Holley ... it is important never to go more than plus or minus .008".
This is to maintain parallel surfaces between ground and the center
electrodes.
Something many do not know, is that with Higher Compression ratios and
Superchargers as well as Nitrous, in many cases smaller spark plug gaps must
be used as well as the use of a much hotter ignition system. These higher
cylinder pressures require more energy to jump the spark plug gap.
The rule of thumb on plug gaps is to open them up in .002" increments at
a time. When the car (race vehicle) begins to lose power or slow down then
go back .001-.002" and this in most cases is the optimum 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 opt
imum spark plug gap settings.
Lastly, NEVER use the porcelain insulator as a fulcrum point when
setting these gaps, this can cause damage to the spark plug.
Ron
00 SLT QC 4X2 5.9 46RE 3.92 LSD
-----Original Message-----
From: owner-dakota-truck@buffnet.net
[mailto:owner-dakota-truck@buffnet.net]On Behalf Of John Neff
Sent: Tuesday, August 29, 2000 6:51 PM
To: Dakota Mailing List
Subject: DML: Colder plugs. Why do they work?
How to colder plugs allow the engine to make more power?
What exactly do they do differently than a hotter plug?
What are the differences between plugs that makes one hotter than another?
What are the ramifications of running colder plugs? Obviously, if they were
the perfect answer, auto manufs would install them from the factory.
John
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