Spark Plug Overview

The spark plug is quite simple in theory: It forces electricity to arc across a gap, just like a bolt of lightning. The electricity must be at a very high voltage in order to travel across the gap and create a good spark. Voltage at the spark plug can be anywhere from 40,000 to 100,000 volts.

The spark plug must have an insulated passageway for this high voltage to travel down to the electrode, where it can jump the gap and, from there, be conducted into the engine block and grounded. The plug also has to withstand the extreme heat and pressure inside the cylinder, and must be designed so that deposits from fuel additives do not build up on the plug.

Spark plugs use a ceramic insert to isolate the high voltage at the electrode, ensuring that the spark happens at the tip of the electrode and not anywhere else on the plug; this insert does double-duty by helping to burn off deposits. Ceramic is a fairly poor heat conductor, so the material gets quite hot during operation. This heat helps to burn off deposits from the electrode.

Some cars require a hot plug. This type of plug is designed with a ceramic insert that has a smaller contact area with the metal part of the plug. This reduces the heat transfer from the ceramic, making it run hotter and thus burn away more deposits. Cold plugs are designed with more contact area, so they run cooler.

Electrode Construction

There are many different types of electrode construction but there are only about 3 basic types of materials used. Center Electrodes can be made from

  • Copper
  • Platinum
  • Iridium

Grounds will usually be the traditional nickel construction or it may be have a platinum tip.

Copper is widely used in many basic sparkplug designs.
Advantages are

  • Excellent conductor of electricity
  • Durable and reliable
  • Inexpensive

Disadvantages are

  • Shorter service life compared to some of the precious metals used today

Platinum is used by many auto manufactures today as their Original Equipment sparkplug.
Advantages are

  • Long lasting
  • Reaches its self-cleaning temperature faster

Disadvantages are

  • Cost

Iridium is a fairly new to the market.
Advantages are

  • Stronger than platinum
  • Higher melting point that platinum
  • Conducts electricity better than platinum


  • More expensive than platinum

Center Electrodes and Ground Designs

There are many different styles of the center electrodes that different manufactures use.
Center Electrode Designs

  • Fine wire
  • Ultra-fine
  • Tapered point
  • Taper cut
  • Necked down
  • V-Power

There are even more different styles of the ground electrodes that are used.
Ground Electrodes Designs

  • Trapezoid cut ground
  • Cut back ground
  • Fine wire ground
  • Tapered cut
  • Trimmed side electrode
  • Wedge shaped ground
  • Inverted V-tip
  • Angled ground strap
  • U-groove
  • Multi-ground
  • Supplementary gap
  • Intermittent discharge
  • Semi-surface discharge
  • Surface discharge

Resistor Plugs

When the spark jumps the sparkplug gap it causes a high frequency burst of energy, that is known as radio frequency interference (RFI). Placing a resistor within the spark plug suppresses the RFI. Without the resistor plugs in your car you cannot only have static on your radio but also the RFI could cause interference with the automobile's electronics.

The most basic function of the spark plug is to ignite the air/fuel mixture. Voltage must be supplied by the ignition system to cause a spark to jump across the spark plug's gap and ignite the air/fuel mixture.
To survive the combustion chambers high temperatures and function properly, the spark plug must dissipate the heat that it absorbs. The temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling.
It is important to remember that spark plugs do not create heat, but instead they must remove heat. The heat range determines the plug's ability to dissipate the heat from the spark plug to the cylinder head where it is absorbed by the coolant system.

How quickly this heat is transferred is determined by:

  • The insulator nose length
  • insulator nose surface area exposed to the air/fuel mixture
  • The construction of the electrode and the porcelain insulator

Heat flow path of a Spark Plug
The insulator nose length is the distance from the 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.
The spark plug tip temperature must remain between 500C-850°C. If the tip temperature is lower than 500°C, the insulator area around the center electrode will not be hot enough to burn off combustion chamber deposits. 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. 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.

Tip Temperature and Firing End Appearance

There are three basic spark plug operating conditions:

  • Fouled operating under 500 C
  • Overheated operating over 800 C
  • Good operating between 500 C and 800 C

The insulator nose length is a large factor in determining 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. The heat range of a plug has no relationship to spark quality or intensity. Many people mistakenly associate a hotter plug with a spark plug that has a hotter spark.
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 rpms 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.

What affects the spark plugs operating temperatures
Below is a list of some of 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. They do not affect the heat range but how the plug is performing under the current conditions. The engine manufacture would consider these parameters when determining the characteristics of the sparkplug they would require.
The spark plug cannot create these conditions but it must be able to cope with them. Air/Fuel Mixtures seriously affect engine performance and spark plug operating temperatures.

  • Rich air/fuel mixtures cause tip temperature to drop, causing fouling and poor drivability
  • 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
  • Higher Compression Ratios/Forced Induction will elevate spark plug tip and in-cylinder temperatures
  • Compression can be increased by performing any one of the following modifications:
    • Reducing combustion chamber volume (i.e.: domed pistons, smaller chamber heads, mill ing heads, etc.)
    • Adding forced induction (Nitrous, Turbo charging or Supercharging)
    • Camshaft change
  • As compression increases, a colder heat range plug, higher fuel octane, and careful attention to ignition timing and air/fuel ratios are necessary
  • Advancing Ignition Timing
  • Advancing ignition timing by 10° causes tip temperature to increase by approx. 70°-100° C
  • Engine Speed and Load
  • Increases in firing-end temperature are proportional to engine speed and load. When traveling at a consistent high rate of speed, or under heavy loads, a colder heat range spark plug should be used.


  • As humidity increases, air intake volume decreases
  • 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 upon ambient temperature

Barometric Pressure/Altitude

  • Also affects the spark plug's tip temperature
  • The higher the altitude, the lower cylinder pressure becomes. As the cylinder temperature de-creases, so does the plug tip temperature
  • Many mechanics attempt to "chase" tuning by changing spark plug heat ranges
  • The real answer is to adjust jetting or air/fuel mixtures in an effort to put more air back into the engine

Types of Abnormal Combustion

  • Defined as: ignition of the air/fuel mixture before the pre-set ignition timing mark
  • 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
  • A change to a higher octane fuel, a colder plug, richer fuel mixture
  • or lower compression may be in order
  • You may also need to retard ignition timing, and check vehicle's cooling system
  • Pre-ignition usually leads to detonation; pre-ignition an detonation are two separate events


  • The spark plug's worst enemy!
  • Can break insulators or break off ground electrodes
  • Pre-ignition most often leads to detonation
  • Plug tip temperatures can spike to over 3000°F during the combustion process
  • 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.

  • Most of the damage than an engine sustains when "detonating" is from excessive heat
  • The spark plug is damaged by both the elevated temperatures and the accompanying shock wave, or concussion


  • 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 spark plug can deliver a weak spark 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.
  • Slight misfires can cause a loss of performance for obvious reasons (if fuel is not lit, no energy is being created)
  • Severe misfires will cause poor fuel economy, poor drivability, and can lead to engine damage


  • Will occur when spark plug tip temperature is insufficient to burn off carbon, fuel, oil or other deposits
  • Will cause spark to leach to metal shell...no spark across plug gap will cause a misfire
  • Wet-fouled spark plugs must be changed
  • Dry-fouled spark plugs can sometimes be cleaned by bringing engine up to operating temperature

Stock 2001 4.6 plugs

The Stock plug for the Bullitt is "AWSF-32P". The AWSF is the family type. The "32" is for the heat range. The "P" is for a Platinum center electrode.
They also list a "AWSF-32PP" which is platinum for the center electrode and the ground electrode.

The main purpose of the platinum plugs is to extend the life of the plug. The stock platinum plugs are scheduled to be changed at 100,000 miles! If for some reason you do not leave your plugs in that long then maybe you could save money by buying a standard Copper electrode plug instead of the more expensive Platinum plugs.

The copper plug is cheaper and actually will conduct electricity better than the platinum plugs will. The Motorcraft number is "AWSF-32C".

If for some reason you need a plug of a different heat range the Motorcraft plugs range number goes up with the higher operating temperatures.
To go down One heat range in the Motorcraft plug would be a "AWSF-22P" plug.
Two heat ranges colder would be a "AWSF-12P".

Some plug manufactures "Heat range" may be wider than others.
If you were to ask for the "AWSF-22P" you may get the same plug as a "AWSF-32P".

A few years ago when I had my Thunderbird with a 4.6 engine, I was changing the spark plug and for the heck of it I checked the resistance on the plugs. Even though the stock Motorcraft plugs had almost 40,000 miles on them the difference between the highest plug resistance and the lowest was within 30%. That may sound like a lot until you start looking at some other manufactures. The worse ones that I had were the brand new Autolites. They varied as much as 600% from the lowest to the highest. Granted in the over all scheme of things this will probably not make that much of a difference but to me it speaks of the quality of their manufacturing process. I have not tried some of the more exotic plugs but of the popular platinum and copper plugs the Motorcraft were by far the most consistent.

Below are general torque settings for new sparkplugs. Please check with your plug manufacture for your application.

If you do not have a torque wrench then you can install the plug finger tight, until the shoulder or the gasket makes contact and then tighten the plug according to the angle column.

NOTE: When reinstalling a plug with the washer, that has already been crushed, you only need to turn the plug 1/5th to 1/4 turn after it is finger tight.

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