Understanding Spark Plugs [for Beginners]

Spark plugs are one of the biggest “Gimmicks” on the market. What I mean by that is there are dozens of bullshit marketing reasons to get you to spend more money on something you don’t need.

Spark plugs are one of the most misunderstood components of an engine. Numerous questions have surfaced over the years, leaving many folks confused.

Spark plugs are the “window” into your engine (your only eyewitness to the combustion chamber) and can be used as a valuable diagnostic tool. The spark plug displays symptoms and conditions of the engine’s performance.

The experienced tuner can analyze these symptoms to track down the root cause of many problems or to determine air/fuel ratios (to a degree).


The spark plug has two primary functions:

To ignite the air/fuel mixture

To remove 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 it to spark across the spark plug’s gap.

This is called “Electrical Performance.”

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 selected.

It is important to remember that spark plugs do not create heat, they can only remove heat. The spark plug works as a heat exchanger by 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 transfer is determined by:

  • The insulator nose length
  • Gas volume around the insulator nose
  • The materials/construction of the center electrode and porcelain insulator

Understanding what a heat range is:

A spark plug’s heat range has no relationship to the actual voltage transferred through the spark plug. It will not increase engine torque or horsepower.

The heat range is a measure of the spark plug’s ability to remove heat from the combustion chamber in order to burn carbon deposits off of its electrode (clean itself).

The heat range measurement is determined by several factors; the length of the ceramic center insulator nose and its’ ability to absorb and transfer combustion heat, the material composition of the insulator, and center electrode material.


Hot Type

  • It has a larger surface exposed to combustion gasses.
  • It dissipates heat slowly.
  • Its firing end heats up quickly

Cold Type

  • It has a smaller surface exposed to combustion gasses.
  • It dissipates heat quickly.
  • It’s firing end heats up slowly.

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.

Whether the spark plugs are fitted in a lawnmower, boat, or race car, the spark plug tip temperature must remain between 932°F-1562°F.

If the tip temperature is lower than 932°F, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits.

These accumulated deposits can result in spark plug fouling leading to misfire. If the tip temperature is higher than 1562°F the spark plug will overheat which may 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 plug types, the difference from one heat range to the next is the ability to remove approximately 158°F to 212°F from the combustion chamber.

Normal: Grey to Light Golden-Brown Color

  • This condition is ideal, the spark plug and engine air/fuel mixture are operating properly.

Dry: Black Soot Buildup

  • Air/fuel mixture is too rich, the carburetor settings are incorrect, or choke was left out too long.
  • Spark plug heat range is too cold for the operating conditions.
  • Ignition system problems causing a weak or intermittent spark.

Oil Fouling: Shiny, Black Appearance

  • Excessive wear or damage to valves or valve guides and seals.
  • Excessive wear or damage to piston and/or piston rings.
  • Excessive wear or damage to cylinder walls.

Overheated: White, Blistered, Melted Electrode

  • Lean air/fuel mixture due to incorrect carburetor settings or intake leak.
  • Spark plug heat range is too hot for operating condition of the engine.
  • Plug is not properly gapped and/or torqued onto head.
  • Overly advanced timing.

Lead Fouling

  • Lead fouling usually appears as yellowish brown deposits on the insulator nose.
  • Caused by using an incorrect mixture of lead additive to fuel (lead is rarely found in modern pump fuel).

Excess Deposits: Bumpy, Chalky Buildup

  • Poor fuel quality.
  • Poor oil quality.
  • Contamination to fuel or oil.

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 is called the spark plug self-cleaning temperature.

The temperature at this point is where the accumulated carbon and combustion deposits are burned off.

Keeping 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 and to the cooling fins.

This means the plug has a higher internal temperature and is said to be a hotplug. 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 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. (Engine temperature can affect the spark plug’s operating temperature, but not the spark plugs heat range).

Spark Plug Gap

All electronic ignition systems should have a spark plug gap of 0.038″-0.042″.
All points ignition systems should have a spark plug gap of 0.022″-0.025″.

Harley-Davidson brand Spark Plugs

Harley-Davidson Spark plugs are manufactured by ‘Champion Spark Plugs’. H-D has a contract through Champion for the optimum heat range for H-D street motors, this heat range is only available on H-D plugs and is not sold in stores. Many other brands of spark plugs will work in your Harley, some of them will work very well, but I don’t believe any of them will measure up to stock H-D plugs.

Harley-Davidson Gold and Platinum Spark Plugs?

More money for the same stock plug

Screamin’ Eagle Split-fire Spark Plugs?

Impossible to accurately gap

Electrodes break off causing serious engine damage

Little known fact:

2005 and later Harley-Davidson EFI systems incorporate an “Anti-knock” feature that operates by reading the current resistance through the spark plugs.

The use of any other spark plug other than ‘Stock’ (6R12) disables the anti-knock feature.
(Yes, that means the Gold, Platinum, and SE plugs will also disable this function)


Leave a Reply

Your email address will not be published.