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Proper engine operation requires the following three conditions.
Even when a good fuel and air mixture is supplied to an engine and good compression exists, the engine will not start without good spark production.
A quality spark plug is a critical part in the production of good sparks; this Q & A Booklet is designed to provide you with technical information about spark plugs and instructions on how to use them.

  • What is the function of a spark plug?

    A It serves as a lighter to ignite the air/fuel mixture.

    It operates under extreme conditions.


  • Why are TORCH spark plugs superior?

    A They are designed to extract maximum performance from the engine throughout its range.


    1.It is an "ultra wide heat range" spark plug. It has a center electrode with copper deeply inserted in the tip to quickly dissipate large amounts of heat. This makes for a spark plug with an "ultra wide heat range" that resists both overheating and fouling.

    When the thermal characteristics of spark plugs with and without copper cores are compared, spark plugs with copper cores prove to be superior in heat and fouling resistance and provide a wider thermal operating range.

    2.This spark plug incorporates an insulator made of state-of-the-art high-alumina ceramics.

    • It has superior insulation at high temperatures and offers continuously sparking.

    • It provides superior thermal conductivity and prevents overheating.

    • It resists thermal shock and provides superior mechanical strength.

    3.It has solid construction to ensure reliable gas-tightness.

    Special hot crimping process to make sure superior gas-tightness and strong construction.

    4.The electrode tip made of special nickel alloy ensures superior durability.

    Special nickel alloy ensures superior heat resistance and durability.

  • What is the heat rating of a spark plug?

    A spark plug must dissipate the heat produced by the combustion gases. The heat rating is a measure of the amount of heat dissipation.

    It is essential to use a spark plug with a heat rating that matches a specific engine and its condition of use.

    When a wrong heat rating is selected,

    When the heat rating is too high,

    The spark plug temperature remains too low and causes deposits to build up on the firing end; the deposits provide an electrical leakage path that possibly cause misfire.

    When the heat rating is too low,

    The spark plug temperature rises too high and induces abnormal combustion (pre-ignition); this leads to melting of the spark plug electrodes that could cause problems such as piston damage.

  • What do TORCH part numbers signify?

    A The TORCH part numbers are the basis for selecting the spark plug specified by the manufacturer.

  • Why is the firing end appearance of a spark plug important?

    A Because the firing end appearance reflects the suitability of the spark plug as well as the condition of the engine.

    These are the three basic standards for evaluating a spark plug.
    Spark plug tip temperature and firing end appearance
    The border-line between the fouling and optimum operating regions (500 °C) is called the spark plug self-cleaning temperature.
    It is at this temperature that accumulated carbon deposits are burnt off.

  • What is the function of the ribs on the insulator?

    A They ensure insulation and prevent flash-ove


    What is flash-over?

    Flash-over is when there is a spark between the terminal and the metal shell, as shown in the picture.

    Flash-over can be prevented by the following.

    Ribs are provided on the insulator to extend the surface distance of the insulator between the terminal and the metal shell. This ensures the insulation needed for preventing flash-over.

    The correct spark can be maintained by the spark gap.

    During sparking:

    High voltage is constantly applied between the terminal and metal shell.

    This high voltage tries to leak along the surface of the insulator.

    If the voltage required by the spark gap is high, flash-over can easily occur.

    Flash-over resistance voltage

    NOTE : Always ensure that SPARK PLUG COVERS/CAPS are clean. Old or dirty SPARK PLUG COVERS/CAPS increase chances of flash-over.

  • What is a resistor spark plug?

    A It incorporates a ceramic resistor to suppress ignition noise generated during sparking.


    It incorporates a ceramic resistor.

    It suppresses ignition noises from sparks.

    Resistor spark plugs prevent electrical interference that can disrupt car radio reception, two-way radio and cellular phone operation.

    This type of spark plug also prevents electrical noise from interfering with the operation of the computer in the engine.

    Noise suppression effects of a resistor spark plug

    (Compared with one of our conventional types)

    As shown above, noise is reduced in all frequency zones by installing resistor spark plugs.
    Example of part number of a resistor spark plug
    As the resistor spark plug has an incorporated resistor, some people think that it has adverse effects on start-up, acceleration, fuel economy and emissions. However, this is wrong. It does not affect engine performance so don't hesitate to use it.

    Resistance vs. power

  • What is a V-grooved type spark plug?

    A It has a 90°V-groove in the tip of the center electrode to enhance ignitability.



    It has a 90°V-groove in the tip of the center electrode.


    The V-groove ensures that the spark is directed to either left or right edge of the electrodes.

    The flame core is generated near the perimeter of the electrodes and grows.

    Ignitability is improved because the electrodes are interfering less when the flame core growing.

  • What is the Iridium spark plug?

    A Iridium has very high melting point at 2545 centigrade much higher than that of platinum at 1772. This allows the electrode to be thinner than the platinum type, improving ignitability. In addition, the thermo edge designed in the Iridium spark plug makes it highly resistant to carbon fouling.


    The diameter of the center electrode is very fine.

    The tip of ground electrode is taper cut.

    It produces sparks very easily and has excellent ignitability.

    Better performance is achieved with improved starting, acceleration, and idling stability, as well as fuel-efficiency.

    Combustion pressure test The Iridium spark plug has less dispersion than a conventional spark plug, allowing it to maintain stable performance.

    Acceleration test Increased power in the middle rpm range provides improved acceleration performance.
    Since the center electrode is very fine, the flame core grows larger.
    Moreover, the tip of ground electrode is taper cut.

  • What kinds of spark plugs offer good resistance to carbon build-up?

    A A spark plug designed for the spark to jump along the insulator surface of firing end, in order to burn off accumulated carbon deposits which may lead to fouling.

    The intermittent discharge plug, the spark plug with a supplementary gap, and the semi surface gap type.



  • What is spark plug fouling?

    A It is a phenomenon by which carbon accumulated at the firing end causes electrical leakage that leads to misfiring.

    As the high voltage generated by the ignition coil leaks away through the carbon, misfiring may occur and cause running and starting difficulties.


    Insulation resistance vs. voltage generated by ignition coil


    As carbon builds up, the insulation resistance of the spark plug drops and the voltage generated by the ignition coil is reduced. When the generated voltage becomes lower than the required voltage of a spark plug (the voltage needed to cause sparks at the spark gap), sparking is suppressed and misfiring occurs.

    Causes of fouling and corrective actions


  • What are the effects of spark plug overheating?

    A Prolonged overheating may induce abnormal combustion such as pre-ignition and detonation resulting in melting of the spark plug electrodes and damage of engine.

    When there is overheating, the insulator surface of the spark plug becomes pure white and the burning gases deposit spots. Electrode melting is a more advanced type of overheating and when spark plug temperatures reach 800 °C or more, the tip of the plug may glow red hot and become a source of ignition prior to sparking, causing abnormal combustion that can damage the engine.


    Causes of Overheating and corrective actions

  • What is good ignitability?

    A "Ignition performance" refers to an engine's ability to successfully and effectively, burn a wide range of air/fuel mixtures. A good spark plug can improve "Ignition performance"

    There are four stages from the time the spark is generated at the gap, to the combustion of the air/fuel mixture.


    The quenching action is where the cooler center and ground electrodes drain the energy of the flame core by way of heat transfer. If quenching is severe, the flame core can be extinguished, causing ignition to fail. Therefore, spark plugs designed to reduce the quenching effect have better "Ignition performance".

  • Is a stain between the insulator and metal shell caused by gas leakage?

    A It is not a stain caused by gas leakage but by corona discharge (corona stain).

    When a spark plug is removed, a brownish stain that looks like a sign of combustion gas flow is often seen at the crimping portion of the metal shell.

    This stain is the result of oil particles suspended in air adhering to the surface of the insulator due to the high voltage. It does not affect spark plug performance.

    Corona stain

    Mechanism of corona discharge The high voltage applied to the spark gap is also applied to the area between the center electrode and the metal shell, causing an insulation breakdown of the air at the gap (a) between the insulator and the metal shell. The phenomenon is called a corona discharge. The generated corona discharge develops toward the terminal nut. This last process is the pale blue corona discharge that may be observed in dark conditions.

    Corona discharge


  • Is there a specified tightening torque for the spark plug

    Q15 Is there a specified tightening torque for the spark plug

    A The tightening torque for a spark plug changes with the diameter of the spark plug.

    The following are the recommended torque values.

    When installing the spark plug, first screw it in by hand. Once the gasket has made contact with the cylinder head, use the torque wrench to tighten it to the tightening torque shown below.

    A rubber pipe as shown in the figure to the right is a good substitute for hand

    tightening/removing spark plugs.



    Conical seat type For (spark plugs without gaskets)


    The tightening angle for both new and reused spark plugs is 1/16th of a turn.


    Examples of common installation problems and advice to avoid them.


  • In addition to correct heat rating, what other precautions should be taken durin

    A Ensure the spark plug has the correct thread reach.

    The engine will not operate properly without the proper thread reach.

    If a spark plug with the wrong thread length is erroneously used, the piston or valve could strike it and damage the engine. Moreover, there is also the concern that the electrode could overheat and melt.


    A Ensure the spark plug has the correct projection dimensions.

    There are several types of spark plug igniter shapes.

    Sometimes, even when the length of the thread is correct, installing a spark plug with long metal shell projection dimensions may cause it to be struck by the piston or valve and result in engine trouble. Only use projection type plugs as specified by the manufacturer.


    When trying to remove a spark plug that is tight and will not come out easily, removing it by force may damage the neck of the threaded section and break the seal section of the spark plug, causing the threaded section to remain in the cylinder head.

    To remove such a spark plug without excessive force, first operate the engine and allow it to warm up the cylinder head, then apply penetrating oil to the threaded section.

    After a short while, the spark plug can be removed.

  • How long will a spark plug last?

    A Even when a spark plug is correctly used, periodic replacement is required as it is a consumable item.

    Examples of problems caused by excessively long periods of use: Worn-out electrodes will have difficulty in sparking.

    Deposits accumulated on the firing end may induce abnormal combustion, causing problems that include melting of the electrodes or damaging engine.


    These spark plugs require replacement.

    It must be noted that these phenomena may also be caused by inadequate engine servicing (fuel systems and ignition system) and incorrect spark plug selection.

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