Spark Plugs

The spark plug's location exposes it to extreme temperature variations, chemicals, fuel and oil. It is also attacked by cylinder pressures produced by the piston, and 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 annually, and replaced as often as necessary. In most cases you can follow the manufacturer’s recommendations, but in a race car, replacement intervals are quite frequent.

How a Spark Plug Works

The basics of a spark plug is that it must perform two primary functions.

1. To ignite the Air/Fuel mixture

2. To REMOVE the heat from 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 spark plug’s actual function: As the ignition is triggered it sends the spark through the rotor, to the cap, down the wire and then it jumps across the gap of the spark plug, where 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. However, 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 misfire.
• Plug wires will break down due to the added resistance as the spark tries to reach ground.
• The rotor, cap and points (of old ignition systems), and the carbon bushing in the center of the distributor cap will fail early.

These complications arise because the greater the gap, the higher the voltage required to jump the gap. Do not forget the gap between the rotor arm and the distributor cap as well. 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 attemperaturets to jump the plug gap. It is also possible to slow down a car if the gap is too big.

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

A long insulator nose means a larger surface area is exposed to combustion gases 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 and 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 and the electrodes 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 revolutions for significant periods of time. The higher cylinder pressures developed by high compression, large camshafts, blowers and nitrous oxide, not to mention the rev 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 burnout 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 Temperature 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 temperatures.

• 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 ECU.
• High compression ratios and forced induction will elevate spark plug tip and in-cylinder temperatures.
• As compression increases, a colder heat range plug is required.

Advancing ignition timing: A 10° advance in ignition timing 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 and motor homes, etc.), the more critical this becomes.

Ambient Air Temperature

• As air temperature falls, its density 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.
• As temperature increases, its density decreases, as does intake volume.

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.

Barometric Pressure and Altitude

• The higher the altitude, the lower the cylinder pressure becomes. As the cylinder temperature decreases, so does the spark plug 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 if you can use the extended tip, do so. 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 an optimum location. 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 the burn, the better the 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 its 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. Choose the ones for your car carefully.

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 plug’s ability to ground properly.

The NGK V-Power Plugs, the Champion Premium Fine-Wire Gold plugs, and the NipponDenso U-Groove plugs are good spark plugs for Indian cars. For 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), and of course the Autolite plug. 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.

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.

Race-designed ignition units 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° (crank degrees) of spark duration as well as much higher spark energy output. A high-performance coil helps, 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 engine speeds more safely and efficiently. 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.

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.

The Cause of RFI

If we were to slow down the spark, we’d see that it 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. If you have a flow of electric current you will have a magnetic field. Coils, relays, switches, solenoids, generators, servo motors 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.

Adding an Aftermarket Ignition Unit (Amplifier)

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 injection, forced induction, etc)?
• Am I planning on racing the vehicle?
• Am I raising the redline 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.
• 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.

Proper Plug Gapping

• Proper gapping of the spark plug is necessary to get maximum spark energy, lowest RFI release and 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 use 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 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