Introducing the ultimate evolution in spark plug technology and performance.

Iridium – Superior to Platinum


The motive behind tipping spark plug electrodes with precious metals was purely performance driven. The smaller the centre electrode, the better the spark plug performs.

Reducing the centre electrode diameter of a standard nickel alloy spark plug, although easily done, would result in a very poor and unacceptable service life.

Using precious metals, NGK were able to decrease the electrode size whilst actually increasing service life. In doing so the spark plug was able to achieve performance benefits previously not possible.

Electrode wear or erosion is primarily caused by the high temperatures and pressures inside the cylinder.

The higher the melting point of the electrodes, the greater the wear resistance and life.

Another factor in electrode wear is oxidation. Depending on the metal, when a certain temperature is reached within the cylinder the electrodes will begin to oxidize.

If a metal is subjected to a temperature higher than its oxidation point, the erosion rate rapidly increases.


Iridium has a high melting point combined with a higher oxidation starting temperature than both Tungsten and RutheniumFollowing the introduction of Platinum, NGK researched the application of other precious metals such as Tungsten, Ruthenium and Iridium.

While conducting the research consideration needed to be given to both the metal melting temperature and the temperature at which oxidation first starts to occur.

The graph shows Tungsten has by far the highest melting temperature, however it has a much lower oxidation temperature.

Iridium has a high melting point combined with a higher oxidation starting temperature than both Tungsten and Ruthenium.


To further test the suitability of these metals, NGK Engineers made sample spark plugs with electrodes tipped with each metal.

Melting PointsThe first test saw the plugs placed in a pressurised cylinder at room temperature with sparks continuously sent through them for 200 hours.

The resulting increase in spark gap was measured – the smaller the gap the more durable the spark plug – Tungsten showed the least amount of gap growth or erosion while Platinum had the most.

During this test no combustion took place and the operating temperature was just above room temperature – resulting in most durable to least durable being in the same order as highest to lowest melting point.


Next, another set of spark plugs tipped with the same metals were placed in an engine simulating extreme driving conditions and temperatures.

Tungsten, which performed the best at room temperature, due to its high melting point, was the least durable in this test and performed poorly.

Melting PointsThis is attributed to the relatively low temperature at which it begins to oxidise and highlights the importance oxidation temperature has on the service life of components.

For the same reason Ruthenium, although better than Tungsten also demonstrated unacceptable durability.

Iridium delivered the best performance and was chosen as the new precious metal for NGK's premium performance spark plugs.


Using Iridium has allowed NGK to reduce the centre electrode diameter down to a mere 0.6mm, increasing the performance of the plug, while at the same time maintaining an excellent service life.

In comparison Platinum centre electrode diameters are 0.8mm.

Iridium IX plugs increase performance by increasing the spread of flame in the combustion chamber.

Unique Two Stepped Electrode


NGK's Iridium IX plugs have been designed with a patented Two Stepped Centre Electrode which improves anti-fouling performance in all driving conditions.

Standard PlugFouling occurs when carbon deposits build-up along the insulator nose, down to the point where the insulator meets the metal shell.

Carbon is conductive and as carbon builds up it can form a leakage path on the insulator nose. This can then cause misfire.

Carbon build-up occurs when an engine has been running rich or has been not been given a chance to reach normal operating temperature, e.g. prolonged idling, driving in heavy traffic, or as a result of aftermarket modifications which act to keep the car relatively cool.

Previously the most common way to prevent carbon fouling was to burn the carbon off.

Spark plugs have been designed with a self-cleaning ability, where above certain engine speeds, the temperature of the plug is high enough to burn these carbon deposits away (approx. 500°C).


NGK's new Iridium IX spark plugs overcome fouling problems with a patented two stepped centre electrode.

This new design makes room for a very small air gap between the centre electrode and insulator nose. During operation, in conjunction with the main spark a secondary micro discharge is generated in this air gap.

Iridium IX Plug< A secondary micro discharge effectively burns off the carbon deposits as they form – this is often refered to as active burning of the carbon

Since the carbon deposits do not reach the centre electrode, the plug continues to spark correctly at the gap, eliminating misfires and rough idle/running problems.

Field ResultsActual field results:

The effectiveness of the two-stepped centre electrode to burn off carbon deposits can be seen by the white ring surrounding the centre electrode. >

Taper Cut Electrode


NGK's Iridium IX Spark Plugs are designed with a tapered cut ground electrode. This new shaped electrode serves two purposes. Firstly, it allows for a better spread of flame during combustion. Secondly, in conjunction with the 0.6mm diameter centre electrode, it helps reduce the electrode quenching effect.

Tapering< Tapering the ground electrode reduces the volume of metal mass at the plugs firing end.

CombustionThe flame kernal is therefore less inhibited by metal while igniting the fuel mixture, allowing the flame to have a better spread and achieve more thorough combustion.

This increase in the flame spread means the fuel mixture is burnt faster and more completely, resulting in the engine delivering more power and acceleration, better responsiveness, and increased fuel efficiency.

To demonstrate, these images were taken during combustion by a high speed camera. >


As well as excellent flame spread, the tapered cut ground electrode helps reduce the electrode quenching effect.

Quenching begins just after sparking occurs, when the small flame kernel begins to expand.

Quenching Standard Plug
Quenching is the loss of heat energy from the flame to the electrodes of the spark plug that would otherwise contribute to the combustion of the air fuel mixture.

Quenching Iridium IX Plug

NGK's Iridium IX minimises quenching by reducing the size of the ground electrode and by using a very fine 0.6mm diameter centre electrode, i.e. less than one tenth the volume of a standard plug electrode.

Minimising the amount of metal mass surrounding the flame kernel means less energy is transferred or lost through the metal during combustion.


If the mixture is sufficiently lean, or the heat transfer to the electrodes sufficiently large, the electrodes can take enough energy away from the flame to extinguish it. The result of this loss of energy is a misfire.

Misfire Standard Plug

In effect the electrodes act to quench the flame, hence the term "quenching effect".

The less heat energy lost to the electrodes, means the leaner the mixture can be made before misfire occurs.

Electrode quenching has two consequences. Firstly, during idle it limits how lean the mixture can be set by vehicle manufacturers to increase idling efficiency.

Secondly, as energy is lost to the electrodes, it follows if this lost energy actually worked towards igniting the mixture greater combustion efficiency would be achieved, resulting in faster and better burning of the mixture.

Super Fine Centre Electrode

The rationale behind tipping spark plug electrodes with precious metals such as Iridium is purely performance driven. By using durable metals the electrodes can be made smaller and still last a long time.

NGK have achieved this with Iridium by producing a spark plug with a centre electrode diameter of just 0.6mm.

Power TestAcceleration Test

The super fine centre electrode coupled with the tapered cut ground electrode enables the plug to ignite fuel mixtures faster and with a more complete combustion. This helps your engine deliver more power and improves acceleration.



Improving ignitability is key to achieving greater performance. To test ignitability we measure how lean the air/fuel mixture can be before misfire occurs. Lean mixtures by definition are difficult to ignite but as can be seen from the graph reducing the size of the electrode enables the spark plug to ignite leaner mixtures.

Lean LimitThe better a spark plug performs in the lean limit test, the better the ignitability performance.

The higher the Lean Limit reading – the leaner the mixture before misfire.

This graph indicates the results using spark plugs with different centre electrode diameters.



The Iridium IX spark plugs, with their 0.6mm centre electrodes, offer excellent ignitability performance.

This can be seen by their ability to successfully ignite lean air fuel ratios up to 17.6 parts air to one part fuel. The ignitability characteristic of the Iridium IX has the added benefit of increasing fuel economy, through increased combustion efficiency. In turn this reduces the harmful emissions from vehicles, which is good for the environment.

As well as excellent ignitability performance, a premium quality spark plug must be able to demonstrate good durability, have resilience to fouling and finally, not require a tremendous amount of voltage to perform its intended task.

Required VoltageLower required voltage;

  • puts less strain on the ignition system;
  • allows for improved cold starting;
  • decreases the chances of misfire due to flashover – offers better responsiveness during sudden acceleration.



Field StrengthThe fine centre electrodes of the NGK Iridium IX plugs offer the additional benefit of a reduction in required voltage by concentrating the electric field strength between the gap. The standard plug's field strength is spread over a wider area, requiring a higher voltage for sparking to occur.

This diagram compares the electric field strength surrounding a standard plug (2.5mm centre electrode) and an Iridium IX plug (centre electrode of only 0.6mm).

The Iridium IX's field strength has a greater concentration across the gap, resulting in sparking occurring more easily (or at a lower voltage).