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 Spark Plug Selection

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PostSubject: Spark Plug Selection   Thu Jul 10, 2008 2:39 pm

The primary function of the internal combustion engine spark plug is to ‘ignite’ the incoming air-fuel mixture (nominal ratio of 14.7:1) under extreme engine operating conditions. To perform this task efficiently and effectively the spark plug’s firing end must maintain a certain temperature range before and after the actual arcing of the spark. The temperature of the spark plug’s firing tip (or end) must be kept lower than the ‘pre-ignition’ temperature and high enough for a ‘self-cleaning’ temperature. (Refer to Spark Plugs Technical Paper on this web site for full explanation.)

The secondary (and most overlooked) function of the spark plug is to provide a reliable path for the heat generated by the explosion of the air-fuel mixture to be dissipated. Average exhaust temperatures occur in the 1,100 to 1,350 degrees Fahrenheit range. Taking into consideration that at the point of explosion of the air-fuel mixture, temperatures of 3,800 to 4,500 degrees Fahrenheit can be generated, the difference is dissipated into the engine cooling system. Since the melting point of nickel alloy is 2,200 to 2,400 degrees Fahrenheit, it can be readily seen that temperature can be a major factor in the performance of the spark plug. The major path of this heat dissipation (in a water cooled engine) is through the spark plug (from spark plug center electrode tip, to the ceramic insulator, to the shell, to the engine head, to the water jacket). The faster the heat is dissipated, the spark plug is considered a ‘cold plug’ whereas the slower the heat is dissipated, the spark plug is considered a ‘hot plug’. (Refer to Spark Plugs Technical Paper on this web site for full explanation of heat range.)

A spark plug has a big job to perform under extreme conditions. The following are some circumstances that need to be considered when the selection process of the right spark plug is at hand:

Operational factors affecting the required voltage


The required voltage decreases with the increase in richness of the air-fuel ratio. However, if the air-fuel ratio is too rich, the required voltage increases due to the cooling down of the electrode. The ideal air-fuel ratio is considered to be 14.7:1.


The required voltage increases proportionally to the increase of cylinder pressure. Higher voltages are required to cause sparking under higher cylinder pressure. It may be wise to consider a ‘colder plug’ when the compression ratio is increased. The gap, too, needs to be adjusted to a smaller size to accommodate higher cylinder pressures.


As the compression pressure reaches a maximum in what is known as top-dead-center (T.D.C.), the required voltage also reaches a maximum. The required voltage decreases in accordance with the advancing of the spark’s timing. This occurs when the compression pressure is lowered and the spark plug’s firing-end temperature rises.


Different types of fuel affect the required voltage. The bonding of the gas particles differs from one fuel to the next and each fuel has a different required voltage. When ‘nitrous’ is introduced into the cylinder, typically lower spark plug gap is recommended to reduce the possibility of ‘blowing out’ the spark itself. This lowered gap is relative to other specifications of the engine and is not an exact figure.


The required voltage is reduced in direct relationship to a narrow spark plug gap and higher temperatures as well. In a four-stroke engine the normal rate of gap growth (created by erosion of metals) is 0.01mm to 0.02mm per 1,000 miles. In a two-stroke engine the normal rate of gap growth is 0.02mm to 0.04mm per 1,000 miles. In newer spark plug designs, the more exotic metals assist in a significantly reduced rate.


The required voltage is lower when a NEGATIVE polarity is used. On some newer engine models, the coil fires a cylinder from both the positive side and alternatively the negative side of the coil. This required voltage irregularity seen on a scope should NOT be construed as a defective ignition system or defective spark plug.


In a standard center electrode spark plug, worn and/or rounded center and ground electrodes become increasingly more difficult to fire and require higher voltages to produce a spark compared to new ‘sharp-edged’ center electrode. In newer vehicle engines, the trend is to have a smaller, more pointed center electrode which require more exotic metals (higher resistant to erosion over time) such as Platinum, Platinum alloy, Iridium or Iridium alloy). This ‘pointed’ center electrode reduces the required firing voltages thereby reducing the overall demand from the entire ignition system. In a broad statement, should the engine have a need for a new set of spark plugs, a two to three percent better fuel burn will be noticed as a result of the new spark plugs, no matter the manufacturer.


New vehicles have installed therein both oxygen sensors and knock sensors. (Oxygen sensors can be investigated at the web site http://ngksparkplugs.home.att.net.) The overall function of each oxygen sensor (once it has gone from ‘open loop’ to ‘closed loop’) is to bring the air-fuel mixture back to a rich side when it sees too lean of a mixture (and vice-versa). The knock sensor will change the timing to facilitate reduction of engine knock. Under certain extreme engine conditions (or engine load) a noise may be heard coming from the engine. This “noise” (more commonly referred to as pinging or knock) may be caused by insufficient octane rating of the fuel or engine timing too far advanced. “Pre-ignition” occurs when the firing end of the spark plug is the incorrect heat-range for the overall engine operating conditions and the tip retains too much heat thereby igniting the air-fuel mixture prior to the piston being on the power stroke. The resulting ‘noise’ is the detonation of the air-fuel mixture meeting up with the upward compression of the piston. Should this be allowed to occur on a frequent basis, major engine damage can result. Engine knock occurs when the gas film covering the cylinder walls becomes thinner and is broken up by the vibrations. Hot gas touches the wall surface directly and the wall is heated up. Usually, this phenomenon disappears by retarding the ignition’s timing.

Cross-referencing spark plugs from one manufacturer to another

Most OEM engine manufacturers spend hundreds of hours of testing different designs of spark plugs to determine just the right design to allow the engine to perform at it’s peak over a wide range of conditions (within acceptable engine performance limits). Once the design is determined, it is recommended that the consumer replace the spark plugs with as close a design (and heat-range) as was originally placed in the engine. In some instances, a certain new design was NOT available at the time the engine was designed and produced. NEW aftermarket designs (produced after the engine was originally placed in service) at times are, in fact, better designs than the original spark plugs. (Example: Iridium-tipped vs. standard nickel electrode; Platinum tipped vs. standard nickel electrode; Multiple-ground vs. single-ground electrode, etc.)

Every spark plug has what is referred to in the automotive industry as a thermal load indicated by an IMEP rating. IMEP (Indicated Mean Effective Pressure in pounds per square inch) is determined by a SAE (Society of Automotive Engineers) standard. This in simple terms is the ‘heat-range’ of the particular spark plug. The resulting confidential number is determined by having a spark plug which has its thermal load increased by a test engine being super charged and increasing the intake manifold pressure until pre-ignition occurs. That thermal load indicated as an IMEP requirement to cause a spark plug to pre-ignite the air-fuel mixture is the heat rating of a specific spark plug. When going from one spark plug manufacturer to another simply by ‘cross-referencing’ the product code number, there is a danger that the IMEP rating of the design is NOT exactly as the cross-referenced spark plug manufacturer. External features of the spark plug may have the appearance of being the same, but the internal functionality of the spark plug design may cause the engine to perform differently (at times causing undesirable results). It is considered best to go by application only when considering a particular spark plug manufacturer, thereby reducing the possibility of undesirable results in selection of the incorrect spark plug heat range or design.

Torque Specifications

Insufficient torque on the spark plug can lead to poor engine performance since gas leakage can occur. Poor heat dissipation may occur as well. Excess torque spark plugs can lead to immediate failure of the spark plugs. (I.e. ceramic breakage, threads stripped and/or threads separating from spark plug shell.)

Suggested torque specifications for Cast Iron cylinder head:


Gasket 18mm 25.3 to 32.5 ft. lb.
Gasket 14mm 18.0 to 25.3 ft. lb.
Gasket 12mm 10.8 to 18.0 ft. lb.
Gasket 10mm 7.2 to 10.8 ft. lb.

Tapered 18mm 14.5 to 21.6 ft. lb.
Tapered 14mm 10.8 to 18.0 ft. lb.

Suggested torque specifications for Aluminum cylinder head:


Gasket 8mm 5.8 to 7.2 ft. lb.

Tapered 18mm 14.5 to 21.6 ft. lb.
Tapered 14mm 7.2 to 14.5 ft. lb

Mechanical Shock

Mechanical shock can occur in certain racing engines wherein resonant frequencies (occurring upon explosion of the air-fuel mixture) are such that the cylinder head allows high frequencies to resonate within itself in sufficient degree to ‘shatter’ the ceramic of the spark plug. Typically, this problem can be resolved by removing the ceramic from the cylinder (selection of a non-projected tip) or redesigning the cylinder to remove the allowance of the resonate frequencies. Typically, this problem occurs more frequently in two-cycle engines with the revving capability of in excess of 12,000 RPM.
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