Spark Plug Tech courtesy of Kennedy’s Dynotune

Spark Plug Tech

Spark Plug Tech

In an internal combustion spark ignition engine, the spark plug serves two purposes. The primary function is to ignite the air/fuel mixture. Secondarily, the plug transfers a significant amount of heat from the combustion chamber to the head and hence to the coolant.

The characteristics of plug design which accomplish these functions differs for different engines and according to the operational use of the engine. The first requirement for a plug is that it fits! The “reach” of the plug, as diagrammed above, is typically selected to be long enough so that the threaded portion ends flush with the surface of the combustion chamber. Obviously, the diameter of the threaded portion must also be correct for the head as well. Thread diameters can typically be 10, 12 or 14mm. Most automotive plugs are 14mm. Also, the size of the hex may be an issue on some heads – there must be room for a wrench. Some heads are designed to use a plug with a gasket, as pictured above. Others have a tapered seat, but most use a flat seat and gasket. Short plugs are also available for tight clearance situations.

Besides needing to fit, the major variable to consider when selecting a plug is heat range. “Heat range” describes the capacity of the plug to conduct heat from the combustion chamber to the cylinder head. If a plug that is too hot is selected, the plug functions like a glow-plug. The air-fuel mixture ignites on the hot parts of the spark plug projecting into the combustion chamber before the spark is applied. This is called pre-ignition. The effect is similar to detonation – it costs power and can break a motor. If the plug is too cold (less than ~900 degrees), the tip will be fouled by combustion deposits. The result is misfiring.

To ensure that the plug runs between the desired temperatures, plugs with different heat ranges are available. The heat range assigned to each spark plug is used to quantify the heat dissipation capacity. With a properly selected heat range, the plug will operate between 1000-1600 degrees F. In this range, the spark plug will not foul yet will not be hot enough to pre-ignite the air/fuel mixture.

The design characteristics of plugs that determine heat range are the insulator nose length, gas volume around the insulator nose, and the materials/construction of the center electrode and porcelain insulator.

Spark Plug Flow Path

This diagram from NGK illustrates some of these points.
Note that the colder plug has a shorter exposed insulator nose.

Anything which raises cylinder pressure will in turn increase plug temperatures and require a colder plug. Since every motor head should know that the way for a given engine to make more power is either to spin faster or to increase cylinder pressure, it is easy to see why modified motors often need a colder plug. A good rule of thumb: go one heat range colder than stock for each additional 50-100hp. Even a mildly modded motor can use a plug reach one range colder than the stock recommendation. With a modern ignition system, misfiring should not be a problem and the colder plug will be less detonation prone under high load.

Factors That Necessitate a Colder Plug

  •     Increased compression ratio.
  •     Forced induction (turbo or supercharging)
  •     Nitrous oxide
  •     Anything which raises volumetric efficiency (VE): bigger camshaft, better heads, intake mods, etc.
  •     Advancing ignition timing
  •     High engine load factor
  •     Leaner air fuel ratios will also raise chamber temps
  •     Very high operating temps
  •     Evidence of detonation not otherwise explained

Factors That Necessitate a Hotter Plug

  •     Otherwise unexplained plug fouling

Installation Tips

  •     Install in a cold engine when possible
  •     Gap the plug to the desired clearance
  •     Be sure the threads (both in the head and the plug) are clean
  •     Apply a small amount of “anti-seize” to the plug threads
  •     Tighten finger tight
  •     Tighten ~1/3-1/2 turn more with a wrench or torque to ~20lbs or per head manufacturers specification.

Brand of Plug

There are a number of manufacturers to choose from. We use more NGK than any other brand as they are of high quality and available in a wide variety of designs and heat ranges for various applications. We can supply them at a competitive price.

Design and Material

In an attempt to gain a real, or perceived advantage over the competition, manufacturers make a bewildering array of tip/electrode designs. These often have sexy names. But in the end, it would be hard to choose between them on any objective basis.

The are a number of different materials used for center electrodes. For street use, long life type plugs using platinum, palladium, or iridium are an excellent choice. In general, these are not our choice for a race engine or for a very high cylinder pressure (blown or nitrous) street car. Here, we prefer a “conventional” type plug and expect to change them frequently. Under these conditions, plug wires also need frequent changes. This is because higher voltage is required to generate a spark across the gap with higher cylinder pressures. The higher voltage leads to more rapid breakdown of the plug wires.

Plug Wire Tech

Click here to read about plug wire tech.

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