by John Logan in the January & February 2008 RootesReview:

This article is based on a technical presentation given in November 2006 by a spark plug engineer at the Robert Bosch Corporation (“Bosch”) North American Technical Center in Michigan. Steve White of Bosch assisted in obtaining slides and permission. The information has been edited in the interest of brevity and to remove proprietary information. I will therefore, only cover the air gap type plugs in detail.

Some Ignition History

Internal combustion engines have had some sort of ignition system since 1807. As an example, there were “constant flame” systems where a small window in the block opened at the top of the compression cycle exposing the mixture to an open flame to ignite it for the power stroke. By 1858, there were electric spark ignition engines that had electrical contacts within the combustion chamber connected to a low voltage battery and usually actuated by the camshaft. When the contacts touched, sparks were produced igniting the mixture.

Reliability of engines used in automobiles was greatly improved when relatively high voltage magnetos were introduced in 1902 that could produce a spark across electrodes. This improvement lead to removable “plugs” screwed into the combustion chamber. Some of the first plugs used mica as the insulator and could be taken apart for cleaning.

The Ford Model T used four coils with vibrating points that fired each plug of the four-cylinder engine. The plugs were screwed into the head with a ¾” pipe thread, but since then, all plugs have had metric threads.

Bosch introduced their first plug in 1902 that led to the modern designs. The picture below shows the similarities and most of the improvements of the Air Gap plugs that they and their competitors have made since then and there are also several optional electrode designs now on the market. It has not been what you would call a fast moving technology.

Spark Plug EvolutionHot and Cold spark

Answers to Myths and Aftermarket Claims

Engine horsepower will increase just by the changing spark plug design:

If the ignition system is in good shape already, you won’t see any improvement. In racing typically differences of no more than 5 hp from spark plug type to spark plug type are found. A daily driver will not feel this difference. Horsepower gains can be noticeable when changing to a new spark plug design, when worn plug wires and the distributor cap are replaced due to their already deteriorated performance. You may also see an improvement by changing out spark plugs and going to a different design if your wires and/or distributor are worn out. By going to the new spark plug you reduce the required ignition voltage significantly thus making the problems you might have with your distributor and or wires go away. So in fact you can experience a significant horsepower gain by eliminating misfires that you attribute to the spark plug but you are only masking other potential problems.

A change in spark plug design will provide Fuel Economy benefits:

It is not automatic that fuel economy improvements will be noticed and they may even decrease. A spark plug manufacturer only needs to show an improvement in one vehicle in order to be allowed to make a claim such as: “Up to 25 % improved fuel economy”.

Only Motorcraft spark plugs work on Ford engines:

Motorcraft uses a non-ISO standard insulator head diameter of 10.9 mm vs. 10.5 mm for the ISO standard. If plug wires have formerly been used on Motorcraft plugs, the boot stretch caused by the larger diameter Motorcraft plugs can allow a ground path through the boot gap, causing misfires.

Multi Spark Discharge will have no effect on spark plugs:

The increased number of sparks with MSD Ignition will decrease plug life. Consider plug life as determined by number of sparks consumed.

Multiple electrode spark plugs generate multiple simultaneous sparks:

Spark will only jump to one ground electrode per ignition event, the path of least resistance. There are no big flashes of fire as seen in ads. Multi electrodes do provide increased life of the plug. There is no way to adjust the plug gaps.

V-groove & U-groove electrode designs improve combustion efficiency:

Combustion advantages cannot be substantiated. Service life is reduced in these designs due to reduced amount of center or ground electrode material. It has been noted that they may improve extreme cold temperature start ability.

Precious metal electrodes (platinum, iridium, etc.) improve combustion:

Special precious metals provide increased service life, not combustion efficiency. Improvement in combustion stability is due to reduced quenching of the flame kernel if you are using a lead additive, the lead will cause wear to precious metal electrodes.

Fine Wire electrode spark plugs are just a gimmick:

Fine wire designs do improve combustion characteristics and can provide smoother idle. The fact that Iridium is used has no effect on combustion.

Aftermarket Plugs and Helpful Tips:

Delphi does not produce spark plugs anymore; Currently Bosch, Denso and NGK produce ACDelco branded spark plugs for OEM. Honeywell builds aftermarket plugs for Motorcraft, Autolite, and ACDelco brands.

Beware of any spark plugs with black oxide coating. There is a risk of seizing in cylinder head. Black oxide does not have as good of resistance to corrosion as do other types of coatings or plating.

The China manufactured spark plugs with “Cage’ type ground electrodes are not recommended in higher performance engines. The thermal path is too long to remove heat from the ground electrode so there is a high risk of pre-Ignition.

A large air gap improves idle up to a degree. A large air gap has a larger energy transfer but is limited by the voltage to the plug. Also the tendency of rim firing, i.e. the spark jumping from the center electrode directly to the shell increases when the spark plug gap is increased. Rim firing has a negative influence on idle stability. Typical gap settings in today’s cars are .030” to .055” or 0.8 mm to 1.37 mm.

Voltage demands go up when the fuel/ air ratio is high or low i.e. off stoichiometric. Indexing plugs so that the ground electrode is perpendicular to the mixture flow in the combustion chamber can allow a significant reduction in air fuel ratio. This should improve fuel economy. However, it is difficult to know the direction of mixture flow and it’s likely that a computer-controlled engine won’t adjust for the difference.

If you are using high voltage coils, the wires will have a shorter life, so maintaining wires and distributor caps is highly recommended.

Use anti-seize when installing plugs, especially in aluminum heads, to reduce the possibility of head damage or the inability to remove the plug. If you have a new car with aluminum heads, consider removing the plugs and applying anti-seize so that they can be removed at the recommended mileage.

Recommended Tightening Torques

Spark Plug TypeCast Iron HeadsAluminum Heads
14mm thread, conical seat11 - 18 lb ft
15 - 25 Nm
8 - 15 lb ft
10 - 20 Nm
14mm thread, gasket seat15 - 30 lb ft
20 - 40 Nm
15 - 22 lb ft
20 - 30 Nm
18mm thread, conical seat15 - 22 lb ft
20 - 30 Nm
11 - 17 lb ft
15 - 23 Nm
18mm thread, gasket seat22 - 33 lb ft
30 - 45 Nm
15 - 26 lb ft
20 - 35 Nm

Reduce tightening torque by 25% when using anti-seize. Always use a torque wrench. Consult a manual for other spark plug types.

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