What does a spark plug actually do?

By: Terry Buddell, 2006


A spark plug has two functions. One, it ignites the fuel/air mix (its “electrical” performance), and two, it removes heat from the combustion chamber (its “thermal” performance).

It works as follows: the ignition system produces a voltage high enough (between 40,000 and 100,000 volts) to jump the small gap from the plug’s central electrode to the "earth stop" – the small bent piece of metal at the tip of the plug.

The spark plug performs under extreme heat and pressure inside the cylinder. It must be designed to withstand deposits from combustion by-products (soot and carbon) and from oil (in two-stroke motors) and the residue from additives.

As a "heat exchanger", spark plugs pull unwanted heat from the combustion chamber and transfer it via the porcelain insulation nose to the metal cylinder cooling jacket and the coolant. The rate of heat transfer is determined by the insulator "nose" length, gas volume around the insulator nose, and the construction of the spark plug itself.

The porcelain insulator’s length determines if the plug is a "hot" or "cold" type. "Hot" plugs dissipate the heat quicker. The temperature of a spark plug firing must be kept low enough to prevent a phenomenon called "pre-ignition", but high enough to prevent fouling of the plug. 

How long should plugs last?

It depends on engine use, but some basic rules apply:

A hard-driven, high-revving engine will be tough on its plugs, while a "cruising" engine with mulitple cylinders will see plugs lasting much longer. Outboards have their own peculiarities. Undersized engines on large, heavy hulls cause a motor to work hot and hard. The more cylinders, the easier on the plugs. Remember that optimum performance falls when the plugs are at their life expectancy, so changing them beforehand is recommended.

Why do plugs fail?

There are many reasons… poor general engine health is one. Spark plugs withtoo small a gap can cause erosion of the electrode, but abnormal combustion is probably the number one enemy for spark plugs.

Common causes for abnormal combustion are:

Pre-ignition – the ignition of the air/fuel mixture before the pre-set ignition timing mark. Pre-ignition results in detonation which can break electrodes and spark plugs. It’s caused by hot areas in the combustion chamber. 

  • These can be caused by over-advanced timing
  • Too "hot" a spark plug type (try a "colder" plug)
  • Lean air/fuel mixture
  • Engine compression too high
  • Insufficient cooling
  • Low Octane fuel

Misfires – caused by insufficient voltage delivered to burn all the fuel in the mixture. Misfires cause lack of engine power and poor acceleration. Severe "misfires" will cause poor fuel economy and can also lead to engine damage. The spark plug can deliver a weak spark for a variety of reasons:

  • A defective coil
  • Too much compression with incorrect spark plug gap
  • Dry or wet fouled plugs
  • Poor electronic ignition performance from power packs
  • Shorting and high-tension tracking problems, shorting leads caused by salt encrustation, wet leads, damp leads, failing leads, cracked or broken leads or connections.
  • Broken or cracked distributor caps

Note: All of the above are electrically induced misfires – they’re not fuel or carburettor related.

Plug fouling – occurs when spark plug tip temperature is insufficient to burn off carbon or oil or engine additives, and when the fuel/oil mixture (two-stroke engines) is incorrect. Wet, fouled spark plugs can be cured by running engine to normal operating temperature. When changing fouled plugs, be sure to find the cause of the problem.

It certainly pays to keep a sharp eye on your spark plugs and associated leads, ignition and so on when maintaining your engines.

As Mark Twain said: "Thunder is impressive, thunder is great, but it’s lightning that does the work." Couldn’t have put it better myself.

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