Facts about Spark Plugs You Should Know


May 3, 2016
Since all gasoline engines use spark plugs of one type or another, one would think that all of the information about spark plugs found on the internet today would be accurate. However, while some information is accurate and useable, there is at least an equal amount of information that is not, so how do you, a non-professional mechanic, know whom or what to believe?
If you do your own car repairs, including routine servicing, and have been offered conflicting advice about which spark plug is right for your engine, read on, and we will explain some of the issues. In this guide, we will look at how spark plugs work, why your engine uses a particular spark plug design, and clear up some myths and misconceptions about spark plugs, starting with-
How does a spark plug work?
Regardless of design specifics of the spark plug and the ignition system, all spark plugs “produce” a spark in the same way. In simple terms, the ignition system converts a low voltage current into a high voltage current that can exceed 40 000 volts and even more when it emerges from an ignition coil.
This current is then passed to the spark plug via a special lead; the current then accumulates in the sparkplug’s centre electrode. Once the current reaches a critical strength, it “jumps” across the gap between the tip of the centre electrode and the piece of bent metal that is attached to the metal casing that forms the body of the spark plug, since the spark plug is grounded on the engine. If the both the ignition system and the spark plugs are in good condition, the entire process of creating and delivering a spark to ignite the air/fuel mixture is measured in nano-seconds.
Why do spark plugs wear out?
Spark plugs wear out in the sense that the gap between the centre and side electrodes widens to the point where the ignition system cannot supply enough current for the available current to jump the gap between the electrodes.
This is caused by a process called “spark erosion”, in which each spark removes a few molecules’ worth of metal from both electrodes each time a sparks jumps across the gap. Over time, the gap not only widens, but the continuous erosion of metal from the electrodes also changes the shape of the electrodes. Since the spark prefers to jump across the gap between the two points with the smallest diameters (cross-sections), the rounded shapes of the electrodes make it even more difficult for the spark to jump across the gap.
This process of erosion happens fastest in spark plugs with soft, copper electrodes, and even on old applications that used relatively low-voltage, non-electronic ignition systems, the average useful life of a spark plug was only about 6000 miles or so.
What does “heat grade” mean?
A sparkplug’s heat grade refers to the time it takes for the “nose” of the spark plug to reach a temperature that is high enough for any carbon deposits that may have formed on the plug to burn off. Essentially, this is the temperature at which the spark plug becomes self-cleaning, and while spark plugs with a different heat grade than the grade recommended by the engine manufacturer will work in almost any other engine, it will do only do so for a short while, since in practice, a sparkplug’s heat grading is a function of its design. Thus, as a practical matter, a “cold” spark plug will quickly burn away in an engine that requires “hot” spark plugs, while a “hot” spark plug will quickly become fouled with carbon deposits in an engine that requires “cold” spark plugs, since the “hot” spark plug will never reach the temperature at which it becomes self-cleaning.
All spark plug manufacturers supply spark plugs in all possible heat ranges, but the trick when buying spark plugs is to insist that the parts store supply spark plugs that are the exact equivalents of the OEM spark plugs if you decide to switch to another brand. Most spark plug manufacturers also supply charts with which to convert their spark plug identifying numbers to those of most other major manufacturers, so insist that the parts store use such a chart when you switch between spark plug brands.
What are the different types of spark plugs available today?
While there is a bewildering range of spark plugs available today, the manufacturer of your vehicle will always recommend a specific design that delivers predictable performance over the useful life of the spark plug. This is to ensure that the recommended spark plug provides good fuel economy because the air/fuel mixture is ignited reliably, which is the basis of compliance with ever more stringent emissions regulations.
Nonetheless, apart from the centre electrode, spark plugs can have either a single, or multiple side electrodes depending on the ignition system, as well as on the design and configuration of the combustion chamber, which ultimately determines how efficiently the detonation flame propagates. Below are some details of the various types of “modern” spark plugs-
Platinum spark plugs
The primary motivation behind using platinum electrodes involves the fact that softer materials are unable to withstand the high rates of spark erosion that occurs with ultra-high voltage ignition systems.
Moreover, the tips of centre electrodes have been reduced to an almost needlepoint to further improve the performance of these spark plugs. Additionally, on some modern platinum spark plugs, a small platinum disc is welded onto the “receiving” face of the side electrode, which means that the spark jumps between two platinum surfaces. In practice, this greatly reduces the effects of spark erosion, thus increasing the useful lives of platinum spark plugs to at least 40 000 miles.
Iridium spark plugs
Modern ignition systems are using ever-increasing voltages, which means that with ultra-efficient engines designs, even platinum spark plugs are not durable enough to provide performance that is fully predictable over extended periods.
For this reason, iridium is increasingly being used in modern spark plug designs, since it offers all of the advantages of platinum spark plugs, but with significantly increased service lives.
Multiple electrode spark plugs
Some spark plug designs feature two, three, or sometimes even four side electrodes and while it might be tempting to fit these spark plugs to an engine that requires only single electrode plugs to improve combustion, doing so is a really bad idea.
For one thing, multiple-electrode spark plugs require really high voltages to create multiple sparks, which means that ignition systems that were not designed to work with these plugs simply cannot supply the required voltages without suffering harm. Moreover, if a low-voltage ignition system does manage to create multiple sparks, each spark will be weaker than it would have been if the proper plugs had been used, which defeats the purpose of having multiple sparks in the first place.
However, the main reason why some spark plugs have multiple electrodes has to do with how these plugs initiate the combustion process, and the subsequent propagation of the detonation flame front. On some applications, the ignition system delivers two sparks in rapid succession, which improves combustion, but only on engines that are designed to work with two ignition events. Thus, fitting multiple-electrode spark plugs on an engine that does not require them is not only a waste of money- it can also cause fatal damage to the ignition system.
Channelled vs. un-channelled spark plugs
The word “channelled” refers to the notch or indentation in the receiving face of the side electrode. While this creates two sparks, the primary purpose of the notch or indentation is to prevent the side electrode from quenching the detonation flame, so by increasing the surface area of the side electrode, quenching is reduced, which in turn, improves the propagation speed of the detonation flame.
Note however, that there is still some debate among engine designers about the effectiveness (or otherwise) of channelled spark plugs. As a practical matter, any advantage (or otherwise) these spark plugs offer over un-channelled spark plugs seems to be too small to have a definitive and measureable effect on engine performance and/or fuel economy. Thus, if the fancy takes you to fit one type instead of the other, you are unlikely to notice any differences in performance or fuel economy.
Spark plugs for “waste spark” ignition systems
On waste spark ignition systems, a spark plug delivers a spark at the right moment during the compression stroke, but unlike other systems, a spark is also delivered on the adjacent cylinder’s exhaust stroke. However, the spark on the cylinder that is on its exhaust stroke jumps from the side electrode to the centre electrode as a result of the ignition system’s need to return the ignition pulse to an ignition coil.
For this reason, spark plugs that are designed to work with waste spark ignition systems are protected with small platinum discs on the side electrode to prevent excessive spark erosion of the side electrode when the spark jumps “backwards”. Therefore, only spark plugs that are specifically designed to work with waste spark systems should ever be used with such systems. Note however that while non-suitable spark plugs will generally work on waste spark ignition systems, doing so will result in severely reducing the useful life of the spark plugs, and may even damage the ignition system.
Why do some spark plugs have built-in resistors?
This has nothing to do with improving the sparkplug’s performance. The primary purpose of the resistor is to act as a suppressor to prevent electromagnetic pulses that originate in and around spark plugs from interfering with electronic equipment such as radio, navigation, and telephony systems.
Why do some spark plugs have tapered seats?
As compression ratios rose in newer engine designs, combustion temperatures also rose, which made some engine designers think that the high thermal expansion rate of modern aluminum cylinder heads would cause the normal sealing washers on conventional spark plugs to start leaking.
As a means to prevent this, engine designers introduced tapered sealing surfaces on spark plugs and cylinder heads to increase the contact area between the cylinder head and the spark plug when the spark plug is tightened properly. However, and as it turned out, modern cylinder heads did not move as much as expected when they expand and contract, and previously tapered seats have now largely been replaced with flat surfaces that can be adequately sealed with conventional sealing washers.
It is worth noting though that many mechanics often over tightened tapered spark plugs, which caused a sort of “welding” to take place on the sealing surfaces between the plug and the cylinder head. As a result, many of these spark plugs broke off just above the threaded part upon removal, which almost always required removal of the cylinder head in order to remove the stubs.
Should spark plugs be torqued down?
As the number of valves per cylinder increased from two to four, it became necessary to reduce the physical dimensions of spark plugs merely to be able to fit them into the space available. In some engine designs, the available space into which spark plugs must fit has decreased so severely that some spark plugs have threaded bodies that are just 10 mm in diameter.
However, “normal” 14mm and slightly smaller 12mm thread sizes are still in common use, but these tend to be over tightened by inexperienced mechanics, which damages the threads in cylinder heads. So to avoid stripping the threads in the cylinder head, or breaking small-diameter plugs on tightening, the following torque setting are recommended-
14mm spark plugs – 28 lb.ft.
12 mm spark plugs – 15 lb.ft.
10 mm spark plugs – 10 lb.ft.