The Grand Tour

How Spark Plugs Work (Ultimate Guide)

Spark plugs are tiny parts of an engine that fit into the cylinder head. Without them, there is no chance the engine would start, let alone run. On this page we’ll go over how spark plugs work, what they’re made of, and how to choose the best for your car. So let’s get stuck in.

How do spark plugs work?

Very simply, spark plugs are able to produce a spark of electricity across a gap which enables the petrol and air being pumped into the cylinder to combust. Now you know the outline, let’s start at the beginning.

The ignition coil carries electrical energy to the spark plug which is pushed into the ignition chamber. As you know, this is so it can produce a spark. The timing of the spark has to be very precise however, as it needs to align with the air fuel mixture and when they are fed into the engine, as well as the position of the piston. This timing is called ignition timing.

The current travels down the centre of the spark plug which is called the central electrode. This is insulated by a ceramic surround and houses a resistor too which removes the electrical interference produced when the spark plug fires. Without it, the electrical systems in your car could very easily malfunction.

Once the electricity gets to the end of the central electrode, it ionises the air/fuel mixture in the gap, thusly producing a spark. This ignites the mixture and the electricity then travels back up the outside of the spark plug via the grounding electrode. This is connected to the block of your engine which of course is grounded.

This all seems pretty simple, but what’s daunting about buying them for your car is all the different options available. We’ll go into the different types below, but always consult your owner’s manual before buying a new set.

What are the different types of spark plug?

Now you know how spark plugs work, let’s talk about the different types of spark plug.

There are four main different types of spark plug. Each have different characteristics which will be needed by your specific car. When replacing your spark plugs, make sure to replace the existing plugs with the right type. Consult your owners manual or a professional if you don’t know which to choose.


  • Usually the cheapest type of spark plug.
  • Copper tip – copper is a good conductor but very soft meaning it needs to be replaced sooner. In fact, it’s so soft that is has to be coated in a nickel alloy.


  • More expensive.
  • Platinum tip has lower isn’t as conductive as copper.
  • Platinum tip is harder, and therefore more resistant to wear.
  • Will last longer.

Double platinum / iridium

  • Both sides of the gap have a platinum or iridium electrode.
  • This allows a spark to come from either the centre electrode or the ground electrode.
  • Because of this, it resists wear much more easily.


  • This has a fine iridium tip which improves its ignitability.
  • Iridium is much harder and therefore resists wear and lasts the longest of all the listed plugs.

Make sure you consult your owner’s manual when replacing your spark plugs so you know exactly which type to replace them with.

Why do spark plugs have to be replaced?

There are four reasons why a spark plug will have to be replaced as it gets older.


The first being that deposits build up on the end of the electrodes and make it harder for the spark to jump across. These deposits come about due to the interaction with the air fuel mixture and the temperature they run at. It is said that running your car to redline can clear these deposits, but after testing, this is simply not true on the majority of vehicles.


Some plugs can start to rust at the end of the terminal nut making the movement of electricity difficult between the ignition coils and the plug. You can simply sand the rust off with sandpaper, but it’s easier to just replace them.

Cracked ceramic insulator

If the insulator gets damaged, it could allow a spark to jump from the central electrode to the ground electrode before it sparks at the end. This would make the car misfire. If the ceramic is cracked, it must be replaced.

Wider gap

If the gap at the end of the spark plug has widened, it’s a good sign that it’s wearing out and needs to be replaced. A wide gap means sometimes the spark is unable to jump from one electrode to the other, making it useless and leading to misfires.

Our Preferred Makes

If you check your spark plugs and find that they’re suffering from any of the above, look for replacements. The following are some of the best brands to look for:

Alex Harrington

Alex started racing at a young age so certainly knows his way around a car and a track. He can just about put a sentence together too, which helps. He has a great interest in the latest models, but would throw all of his money at a rusty old French classic and a 300ZX. Contact: [email protected]

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