What Do Spark Plugs Do?

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AMSOIL P.i.® Performance ImproverSignature Series Synthetic Motor OilAMSOIL Antifreeze & Coolant
Brad Nelson
by Brad Nelson
September 7, 2022

Spark plugs are responsible for igniting the air/fuel mixture inside an engine. They do their job by creating a miniature bolt of lightning, or spark, within the combustion chamber as the piston reaches the top of the compression stroke. As such, spark plugs are a critical component of gasoline engines.


Spark plugs ignite a mixture of air and fuel, creating an explosion that is converted to mechanical power.

How Spark Plugs Work

Two metal electrodes on the tip of the spark plug create a gap that electric current must cross. And it’s no small act. In fact, it takes a lot of voltage to make it happen. Spark plugs usually require 12,000–25,000 volts or more to fire – and can be as high as 45,000 volts.

Spark plugs draw their power from the high-voltage current generated by an ignition coil or magneto. As electricity flows from the coil, a voltage difference develops between the center electrode and ground electrode on the spark plug. At first, nothing happens. That’s because the distance between the electrodes and the insulating properties of the air/fuel mixture creates a lot of resistance for the electric current.

But as the voltage continues to rise it begins to alter the molecular structure of the gases between the electrodes, causing them to become ionized. The ionized gas allows current to flow across the gap and the spark plug fires.

The intense heat created by the spark creates a small fireball within the gap. That combustion kernel rapidly expands until the entire air/fuel mixture within the cylinder chamber combusts. The resulting explosion creates power that the engine can harness.

Heat Range

Spark plugs work properly within a certain heat range. Temperature on the firing end of a spark plug must be kept high enough to prevent fouling but low enough to prevent pre-ignition of the air/fuel mixture. The exact operating temperature range for spark plugs varies depending on the application and spark plug design, but they generally operate with an electrode temperature normally between 930°F (500°C) and 1,470°F (800°C).

To avoid overheating and causing pre-ignition, spark plugs disperse excess heat through the plug into the water jacket of the cylinder head. How much heat the spark plug can disperse is known as its heat range or thermal performance.

Cold vs. Hot Spark Plugs

Cold spark plugs have a short heat flow path for a rapid rate of heat transfer. They also use a short insulator nose to limit heat absorption.

Hot spark plugs are just the opposite. They feature a longer insulator nose and longer heat flow path. The design slows the rate of heat transfer to the water jacket.


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Anatomy of a Spark Plug

Here are the common components of a spark plug.

Insulator Body

The insulator body is molded from aluminum oxide ceramic. The body has exceptional dielectric strength, high thermal conductivity and excellent resistance to shock.


Ribs on the outside of the insulator body provide a reliable seat for the rubber spark plug boot. They help protect against secondary voltage or spark flashover.


The hex is the contact point for a socket wrench.


The steel shell is designed for exact tolerances using a special cold extrusion process.


The shell is plated for durability and rust and corrosion resistance.


Most spark plugs for a seal with the engine block using a folded steel gasket. Gasketless spark plugs form a seal with a tapered seat and tight tolerances.


Spark plug threads hold the spark plug securely in the engine block.

Ground Electrode

There are different ground electrode shapes and configurations, but most are manufactured from nickel-alloy steel. The ground electrode must be resistant to both spark erosion and chemical corrosion under extreme temperatures.

Center Electrode

Center electrodes must be manufactured from a special alloy that is resistant to both spark erosion and chemical corrosion.

Electrode Gap

The area between the ground electrode and the center electrode is called the gap. This is the gap the spark jumps when firing.

Insulator Nose

The insulator nose must be capable of shedding carbon, oil and fuel deposits at low speeds. At higher engine speeds, the insulator nose is cooled to avoid pre-ignition.


AMSOIL synthetic lubricants help your spark plugs last longer and engine run cleaner and more efficiently, by reducing carbon and sludge buildup that can lead to engine knock caused by pre-ignition or engine detonation.


by Brad Nelson

Brad Nelson is a staff writer for AMSOIL. Outside of work he enjoys family adventures, wilderness exploration and riding/wrenching on vintage metric motorcycles.

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