This section of
the Tech Zone will hopefully educate you in the how the Spark Plug works,
and also introduce you to the technique of proper plug reading.
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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 and 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 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 it 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.
Opening The Plug 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 and 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 at the tip, 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.
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 you 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
pictures that are part of this article)
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).
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.
Computer-controlled engine applications do a pretty good job of this
with the various sensors that report back to the ECM.
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:
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.
The heavier your vehicle or greater the
amount of work the engine sees (racing applications, construction
trucks, vans, RVs & Motor homes, etc.), the more critical this
becomes.
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.
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.
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. This
is also achieved with the better aftermarket cylinder heads on many
available engine applications. The newer heads position the spark plug
location closer to a more optimum location.
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. Use common
sense and don't believe all the hype that a marketing company bombards you
with.
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, but at the price of a short life, which requires
short replacement intervals. 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 ... <G>
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. The coil is only the pawn of the ignition trigger or
control unit. The coil is the real workhorse and takes most of the abuse
... make sure you use a good coil.
(Read
below about aftermarket ignition amplifiers and
controls)
The Cause of RFI:
If we were to slow down
the oscilloscope to perhaps 0.00000025 seconds and greatly expand the
pattern (as pictured at right), 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 radio transmitter.
RFI
Standards:
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 center
electrode.
Other ways that
control RFI include:
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.
OK, I felt I had better address
this now. There are numerous ignition units available that say they give
you "X" horsepower gain, or do this or that for your engine. In most
cases the ignition unit will benefit your engine, but is it needed? To
decide if adding an ignition unit to your vehicle is going to be
worthwhile you need to ask yourself a few simple
questions:
Am I trying to get maximum spark energy,
engine efficiency, lowered emissions, and power output from my
engine?
Am I modifying the engine (cams, head
porting, exhaust system, larger carbs/injectors, compression, or any
other assortment of aftermarket performance parts) so that cylinder
pressures will be increased?
Am I adding a serious power enhancer
(Nitrous, Supercharger, Turbocharger, etc)?
Am I planning on racing the
vehicle?
Am I increasing the RPM range of the
engine?
If you answered YES to even one of the questions above you could
consider the addition of one of the available ignition units. If you
answered YES to 2 or more of the above questions, you'd better install a
quality ignition amplifier (Mallory Hyfire, Holley Annihilator, Crane
Hi-6, MSD, etc).
Of course there is going to be
B.S, hype from many ignition manufacturers, and the facts surrounding
what an ignition unit can really do.
Some
Facts:
A small power increase and strides in
efficiency will be seen in most cases. Note that if you are racing,
not having a good ignition amplifier can cause serious power loss and
engine damage if the air/fuel mixture is not ignited
properly
Without the correct or adequate coil the
ignition unit cannot do its job properly
Analog ignition units are slow and not as
good as digital units
Opening up your plug gaps just because you
have added an amplifier CAN SLOW YOU DOWN and cost your horsepower.
(read below)
Not all ignition units are the same
(Inductive amplifier and Capacitive Discharge amplifier, not to
mention brands)
If it takes 10k volts to fire your spark
plug, that is what you will get. Just because you have a bazillion
volt system does not mean the box will give that to your plugs and
combustion process. Don't believe that just because the ignition has
the highest "millijoule" rating that it is the best.
Using the wrong coil can have serious
implications (we have seen coils overheat, boil over, and catch the
vehicle on fire because the coil could not handle what the box was
telling it to do).
Not all ignition triggers are equal
(breaker point, magnetic, magnetic breakerless, hall effect, photo
optic, etc). Each can affect the efficiency of your ignition
unit.
One of the main benefits of an ignition
amplifier is the spark duration of 20° crank degrees per spark.
(this is the B to C distance on the chart above under
RFI)
Some
Hype:
"Guaranteed horsepower
claims"
"Our amplifiers will help ALL
vehicles"
"It will work with any coil" (this usually
means that their unit does "squat"). Remember, the coil is the
workhorse and the box is managing the coil.
"You can now increase the spark gap for
better performance". This usually means again that their box does
"squat". If you "have to" open the gaps up to get spark energy, the
box is not supplying what they claim. Increasing plug gaps should be
decided on an individual engine basis, not by the box.
The lowest plug wire resistance is not
always the best either. A coil has to build energy (resistance) and
then release it. This resistance is a combined part of the coil, plug
wires, plugs, etc. The more efficient units can operate with slightly
higher plug wire resistances to eliminate outside interferences.
Weaker units require extremely low resistance plug wires and ignition
coils, virtually just blowing the spark through the coil and wires to
get to the plugs.
"All these controls will help you achieve
more performance". Many of the available "gadget boxes" are just that.
If you have a real need for timing controls, high speed retards, etc,
then get a box that uses these features. Just because the box has
these controls does not mean you will get more performance. Computer
programmable ignitions (where you hook your laptop or PC to the
ignition) are for dyno rooms. Once you have a setting that works for
your specific engine, it probably never needs to be tweaked again. You
can play with those settings for months and never get a single HP gain
from them. When I see these on street cars and/or many bracket cars I
have to laugh.
Sure, we have our preference of
what brand(s) of ignition units we like and those we don't. There are
reasons for these decisions (reliability, factual claims, performance
output, consistency). We have tested numerous ignition brands/styles and
know what works for an individual's application. The term "Ignition
System" fits. If you want your ignition to work correctly it needs to be
a compatible system for your specific application.
(example
of an excellent ignition unit ... and one I have on my personal
vehicle)
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 (see above). 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 optimum 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.
