Frequently asked questions

For four-wheel vehicle owners

What is the “iridium” in the IRIDIUM POWER series?

Like platinum, iridium is a noble metal, and is characterized by its extremely high melting point and strength. Because of these properties, it is widely used in cutting-edge technologies in various industries such as the aerospace, medical, and automobile industries. Using a new iridium alloy developed independently by DENSO, the IRIDIUM series has achieved an extremely thin center electrode of just 0.4 mm in diameter. Just as it is easier for lightning to strike pointed objects, the thinner the center electrode of a spark plug, the more reliable its sparking will be. It maximizes engine performance and improves mileage and acceleration.

What is the “iridium” in the IRIDIUM POWER series?

Melting point: Compared with platinum, the melting point is approximately 680°C higher and it has the property of being hard to melt even under high temperature from electrical discharge or combustion.
Strength: Compared with other materials, it has greater material strength and is stable under various operating conditions.
Electricity: Because it has low resistance, even compared with platinum or nickel, it is suitable for spark plug electrodes.
Hardness: It is a tough material that is harder and stronger than other materials.

How does the IRIDIUM POWER series differ from the genuine iridium spark plugs of vehicle manufacturers?

It has improved ignitability.

How does improved ignitability change things?

It has a wide range of effects, including improved mileage.

Stable idling engine speed
Because the combustion conditions are stable, there is less rotational fluctuation and interruption during idling.

Improved combustion
There is less ignition failure due to misfiring in various operating areas, and the combustion conditions are vastly improved. For this reason, combustion is often improved.

Improved engine performance
Because engine performance is enhanced, acceleration is better than with normal spark plugs.

Cleaner exhaust
CO and CO2 emissions are reduced, producing cleaner exhaust. For those with a high concern for the environment, we especially recommend IRIDIUM POWER.

Can I use the IRIDIUM POWER series in my own car?

These plugs are compatible with most Japanese and foreign vehicles.

They can be used to replace most spark plugs commonly fitted in Japanese and foreign vehicles. Search the vehicle model compatibility column of the spark plug item number compatibility chart and select an iridium plug suitable for your vehicle. For a normal engine, select a spark plug with the same thermal value as a standard plug, and for a tuned engine, select a spark plug with a thermal value that matches the level of tuning.

What is the “thermal value” of a spark plug?

What can you tell from the condition of a burnt plug?

Appearance Condition or type of burning Engine or plug condition Causes Countermeasures
Normal The insulator nose turns an ash or reddish-brown color. Electrode wear is minimal. The engine runs smoothly on starting and during both high-speed and low-speed driving.   (Normal condition)
Corona stain Brown deposits on the insulator just above the housing No influence on spark plug performance Caused by electrical stress in the air around the ceramic insulator (not leakage of the spark plug gas, as it is often mistaken for)  
Carbon fouling The insulator nose and electrode parts are covered in dry carbon. Engine starting worsens, engine turnover is unstable at low speeds, and this can eventually lead to stalling and poor acceleration.
(Nearly 90% of engine trouble from spark plugs is caused by carbon fouling or oil fouling.)
  1. Inappropriate thermal value
  2. long-time engine idling or low-speed driving
  3. Excessively rich air-fuel mixture
  4. Clogged air filter
  5. Delayed ignition
1 & 2: Using a spark plug with a slightly lower thermal value or adjusting engine idling
3 & 4: Performing a complete readjustment
Oil fouling The insulator nose and electrode parts are shiny black from gasoline or oil. Engine starting worsens, engine turnover is unstable at low speeds, and this can eventually lead to stalling and poor acceleration.
(Nearly 90% of engine trouble from spark plugs is caused by carbon fouling or oil fouling.)
  1. Oil leakage through the piston due to friction on the piston rings, valve guides, cylinder wall, etc.
  2. Excessively rich air-fuel mixture
  1. When running-in a new engine or an engine immediately after an overhaul, spark plugs may become wet until the oil flow control becomes normal, but this can easily be remedied by simply cleaning and reinstalling the plugs. In any other cases, essentially, a complete engine overhaul is required.
  2. Adjusting the carburetor
Overheating The insulator nose is burnt to a bleached white appearance, and the electrodes are also burnt white or to “pale purple color”. Electrodes also rapidly become worn. On occasions such as when engaged in continuous high-speed driving, long-time hill climbing, or carrying a lot of luggage, the engine loses power and the vehicle cannot pick up speed.
  1. Inappropriate spark plug thermal value
  2. Use of low octane gasoline
  3. Ignition timing too early
  4. Insufficient cooling
  5. Poor air-fuel mixture
  1. Using a spark plug with a slightly higher thermal value
  2. Using high octane gasoline
  3. Adjusting the ignition timing
  4. Checking the cooling system
  5. Adjusting the carburetor
Pre-ignition The electrodes are melted, and in extreme cases, even the insulator nose may be melted. As overheating continues, the temperature of the combustion chamber rises rapidly, which can cause damage not only to the spark plug, but also the piston.
  1. Inappropriate spark plug thermal value
  2. Use of low octane gasoline
  3. Ignition timing too early
  4. Insufficient cooling
  5. Poor air-fuel mixture
  1. Using a spark plug with a slightly higher thermal value
  2. Using high octane gasoline
  3. Adjusting the ignition timing
  4. Checking the cooling system
  5. Adjusting the carburetor
Insulator breakage The insulator nose has lengthwise cracking and the insulator shows overheating or has a somewhat large amount of deposits on it. On occasions such as when engaged in continuous high-speed driving, long-time hill climbing, or carrying a lot of luggage, the engine loses power and the vehicle cannot pick up speed.
  1. Inappropriate spark plug thermal value
  2. Ignition timing too early
  3. Malfunctioning cooling system
  4. Poor air-fuel mixture
  1. Using a spark plug with a slightly higher thermal value
  2. Adjusting the ignition timing
  3. Checking the cooling system
  4. Adjusting the carburetor
Insulator breakage The insulator nose is cracked in a ring shape or may have fallen off due to lengthwise cracking. Often occurs in tuned cars, usually due to poor setting. Occurs due to sudden acceleration and deceleration or continuous high-speed driving and can lead to piston erosion and turbo damage.
  1. Poor setting during tuning (bad A/F ratio, over-supercharging, knocking, computer modification, etc.)
  2. Inappropriate spark plug thermal value
  3. Insufficient cooling
  1. Performing proper tuning
  2. Selecting a spark plug with thermal value that matches the tuning
  3. Checking the cooling system
・Ground electrode is broken in the middle or at the base.
・Much of the cross section shows fatigue fracturing. ・Ground electrode is broken in the middle or at the base.
・Much of the cross section shows fatigue fracturing.
・Ignition failure
・Damage to the engine or auxiliaries from the grounding terminal fragment
  1. Increased stress on the ground electrode due to a high-output, high-torque engine or tuning, modification, etc.
  2. Excessive vibration due to poor maintenance, severe operating environment, etc.
  3. Insufficient fastening ⇒ excessive vibration due to loose plug
  1. Spark plug selection
    As a countermeasure to increased stress on the ground electrode, changing to a spark plug with a small ground electrode is an effective way to handle the problem from the plug side. However, fundamentally, it is necessary to select a spark plug that matches the vehicle’s level of tuning.
    Examples: 2-ground electrode plug, small ground electrode plug (taper cut), racing plug, surface discharge plug, etc.
  2. Engine and vehicle body maintenance
    ・Reduction of excessive vibration through engine and vehicle body maintenance
    ・Tightening plugs with the recommended torque

How do you differentiate the uses of IRIDIUM POWER and IRIDIUM RACING?

IRIDIUM POWER is designed for response and IRIDIUM RACING is designed for high engine speed.

IRIDIUM RACING plugs have a platinum ground electrode and a strengthened insulator designed for racing. However, due to the sparking point being further away from the combustion chamber, ignitability at medium and low speeds is sacrificed to some extent. On the other hand, because its users can feel an extension in high engine revolution at high speed, it is favored among racers. If you place importance on response, choose IRIDIUM POWER; if high engine speed is your main focus, choose IRIDIUM RACING. IRIDIUM RACING is particularly popular among circuit racers.

What kind of vehicles are surface discharge plugs used for?

Understand the advantages & disadvantages and select the right spark plug for you.

Surface discharge plugs are designed for racing, and are widely used in the All-Japan Road Race Championship JSB1000 class and in drag racing.
Eliminating the ground electrode has the following advantages and disadvantages.

■ Advantages
(1) Interference with the piston can be avoided in an engine that has been tuned to increase compression ratio.
(2) With no superfluous projections into the combustion chamber, the combustion efficiency at high rotation speed is improved.
(3) Problems relating to the ground electrode are prevented.

■ Disadvantages
(1) Because ignitability is not as good, feeling and response at low and medium rotation speeds are less favorable.
(2) The engine becomes “peaky” (sudden speed increase), making it tougher to handle. In the popular ST600 class, many riders use a plug with a ground electrode.

Is it possible to switch from 2, 3, or 4-electrode plugs to IRIDIUM POWER or IRIDIUM TOUGH?

Yes, it is.

This is because both IRIDIUM POWER and IRIDIUM TOUGH have thin center electrodes, giving them lower sparking voltage and superior ignitability compared with 2, 3, and 4-electrode plugs. See the STANDARD, RESISTOR PLUG CHART to determine what models are 2, 3, and 4-electrode plugs.

■ Sparking voltage
Because the field intensity affecting sparking voltage becomes stronger as the electrodes become thinner, the sparking voltage becomes lower. The diagram below illustrates each type of electrode shape for various plug models, and shows how IRIDIUM TOUGH has lower voltage than a 2-electrode plug.

■ Sparking voltage

■ Ignitability
The smaller the contact area between the flame kernel and the electrodes, the smaller the cooling effect of the electrodes, and the better the ignitability. Thanks to IRIDIUM TOUGH’s thin center electrode, the contact area between the flame kernel and the electrodes is smaller than with a 2-electrode spark plug, giving it superior ignitability.

■ Ignitability

What is the merit of using an iridium spark plug rather than a normal spark plug or platinum spark plug?

Stable sparking at low voltage.

In older vehicles, the ignition system itself is outmoded, and so in most cases the ignition voltage is low. Thus, the merit in using an iridium spark plug that enables stable sparking at a low voltage.

Why has the idling engine speed increased?

IRIDIUM POWER may be unable to have its full effect due to deterioration of the ignition system.

We have previously received the following type of inquiry:

I drive a Rover Mini automatic (H2). After changing to IRIDIUM POWER, the idling engine speed in neutral has increased by around 400 rpm (before, it was 1,000 rpm in neutral, and after changing the plugs it is 1,400 rpm). And when shifting from neutral to drive, the engine speed used to be 550 rpm, but now it is less than 550 rpm and the engine sometimes stalls. The engine speed increases smoothly when driving, and the acceleration is good, . . . so what’s going on?

In older cars especially, the ignition system is often deteriorated, which can prevent IRIDIUM POWER from having its full effect. As a countermeasure to this kind of problem, adjust the spark gap from 1.1 mm to 0.8 mm. The VK series also includes the VK20Y model, which has the gap set at 0.8 mm from the start.

Is IRIDIUM POWER suitable for older vehicles?

Yes, it is. You will notice improved performance.

In fact, improved performance is even more noticeable in older vehicles than in the latest models. In the latest vehicles, the ECU collects numerous data to keep engine combustion near the optimal state; but because older vehicles use outmoded types of ECU, such as carburetor control, combustion conditions are not optimal. Because older vehicles have greater room for improving performance, it is easier to notice the benefit in switching to IRIDIUM POWER. Also, older vehicles may well be suffering such problems as reduced ignition coil performance. An iridium spark plug with low required voltage uses the energy of the coil effectively for efficient combustion of the air-fuel mixture.

What kind of maintenance is effective?

Avoid cleaning with a wire brush.

Compared with modern vehicles, the combustion conditions of old vehicle engines are not so good, which can often lead to oil fouling of the spark plugs. Also, as the engine itself ages, oil leakage through the piston or valve guide can occur, which can lead to a buildup of carbon deposits.
In such cases, please avoid cleaning iridium spark plugs with a wire brush. Because the electrodes of an iridium plug are very thin, care must be taken not to damage them. If you have access to such equipment, cleaning with a sandblaster is effective. If you are a regular owner, please clean off oil and carbon fouling using a parts cleaner, engine conditioner, or gasket remover, etc.

The vehicle service manual recommends replacing spark plugs every 200,000 km, but is it alright to not replace them after 200,000 km?

It is possible to continue driving without replacing your spark plugs after 200,000 km, but because the plugs are continually sparking in the combustion chamber, where the temperature is several thousand degrees, they naturally suffer degradation and wear. Degradation of the spark plugs leads to poor startability, poor mileage and poor acceleration. To maintain the performance of a new vehicle over a long period, we recommend that you regularly inspect the spark plugs.

Will installing racing plugs in a normal engine improve its performance?

You should not expect improved performance. Racing plugs are designed to resist breaking when an engine is tuned up or when used under severe conditions (such as sustained high engine speed on a race circuit). For this reason, the spark gap is narrower than normal, and the sparking point is moved away from the combustion chamber, which would negatively affect startability, idling stability and mileage if used in normal road driving.

I’ve heard that not replacing spark plugs can cause stress to the ignition system. Is this true?

Yes. When spark plugs are used over a long period, the gap that the spark must cross becomes wider. If the gap is too wide, the voltage needed to produce a spark (required voltage) becomes higher. Continuing to use spark plugs in this condition causes the coil to be constantly subjected to high voltage, which places a great burden on it. If we compare it to the human body, it is like the strain placed on the heart of a person with high blood pressure.

I want to coat the spark plug thread with burn-prevention grease. Is there anything I should beware of?

If you apply grease to the spark plug thread before inserting it, you could easily end up overtightening it due to the reduced resistance. If applying grease, please be careful to only tighten it to the recommended angle of rotation. (Example: a half turn for an M14 plug)
For details, refer to the information in “SPARK PLUG Installation”.

When spark plugs deteriorate, what symptoms occur in the engine?

(1) Engine starting worsens. (2) Idling becomes unstable. (3) Hesitation, stumble or sag during rapid acceleration. If you experience symptoms such as these, please replace the spark plugs.

Does carbon fouling still occur even with the mainstream EFI (electronic fuel injection) of today?

Compared to the age of carburetors, carbon fouling of spark plugs is much rarer, but there are still cases of carbon fouling caused by low engine speed from drivers engaging in extreme eco-driving due to rising gas prices in recent years, or frequent driving in heavy traffic.

Please explain the methods for spark plug maintenance.

In recent years, iridium plugs with a thin center electrode have become mainstream, and so people used to performing maintenance on conventional spark plugs may find that using a brush to clean off carbon deposits will bend the electrode. Also, adjusting the spark gap when the edges of the electrode have become rounded will not restore the sparking performance to the same level as a new plug. We recommend that you think of the spark plugs as expendable parts and replace them with new ones.

Why am I unable to rev the engine immediately after replacing the spark plugs?

We sometimes hear of such cases, but in most cases when we have recalled and inspected such plugs, we have found nothing wrong with them. We assume that the cause is incorrect installation or some other problem relating to the vehicle, but if you have any concerns, please consult with the store where you purchased the product.

Is there any difference between spark plugs for turbo cars and those for regular cars?

Spark plugs for turbo cars usually have a narrower spark gap and a higher thermal value. This is because a turbo car has a high combustion temperature and a high sparking voltage.

Does using iridium plugs in an older vehicle make carbon fouling more likely?

This is because iridium spark plugs have superior ignitability to normal spark plugs. In terms of achieving sure combustion, iridium plugs could be said to be more resistant to carbon fouling. However, the predominant causes of carbon fouling are poor fuel adjustment and mismatching of the thermal value and the operating environment, so it is considered important to take countermeasures against these causes.

How compatible are these spark plugs with hybrid and stop-start vehicles?

Iridium spark plugs are indispensable to hybrid and stop-start vehicles. In particular, they are an essential item in stop-start vehicles, in which startability is especially important.

If both genuine platinum spark plugs and IRIDIUM TOUGH can last 100,000 km before replacement, what is the difference?

Because their electrodes are thinner, IRIDIUM TOUGH spark plugs have superior ignitability to genuine platinum spark plugs.

Is it necessary to run-in the engine after changing spark plugs in a passenger vehicle?

This is unnecessary.

Can I use some old plugs I had lying around?

If the thread or mounting parts are rusted, please clean off all rust before installing them in the engine.

Which is more economical: using an iridium plug over a long period, or replacing normal nickel plugs frequently?

For this answer, we will assume an engine with a standard installation of normal nickel spark plugs. Replacing your spark plugs with long-life plugs like IRIDIUM TOUGH will provide a lifespan approximately 5 times that of a normal nickel spark plug and will also improve mileage. The thermal value of an iridium spark plug is higher than that of a normal nickel spark plug, which can provide a benefit in running cost.