![Heat Rating and Heat Flow Path of Spark Plugs](images/DailyArticles/February2010/ModernQuestForFire/Modern-Quest-For-Fire-LeadT.gif)
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You can learn a lot about what’s going on inside your motor by simply taking a look at your plugs. Tip temperature will determine whether your plug is good, fouled or overheated. The happy spot between fouling and optimum operating temperature is called the spark plug self-cleaning temperature and this is where accumulated carbon and combustion deposits are burned off. Too high of a temp and the plug itself will burn, too low and it will foul and reduce efficiency.
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(left) The traditional center electrode is a copper/nickel alloy and generally 2.0 to 2.5mm across. Standard in most V-twins, they are efficient, durable and inexpensive to make.
(right) The use of a fine wire center electrode in the 0.7mm to 1.1mm range lowers the voltage necessary for an efficient spark. Electricity will seek the path of least resistance and the smaller the center electrode, the smaller the path the electrical charge will have before it jumps to the ground electrode. The result is reduced misfires and higher horsepower.
The Modern Quest for Fire
Story by David Zemla
Surging from zero to 40,000 volts in under 100 microseconds, electrons stack up on the center wire before arcing to ground and igniting a volatile mixture of air and fuel into an explosive release of energy. Sounds like some sort of science fiction weaponry, but if you ride a bike, this process occurs thousands of times per minute inches from your crotch. Yes, we speak of the venerable sparkplug and the perilous task it must complete in the name of internal combustion.
From the beginning, sparkplugs have been a bit of a puzzle. Their origin is even a little fuzzy, although most sources credit Robert Bosch and company with the first successful production over a century ago (Bosch owns the first sparkplug patent from 1902). During the preceding 100+ years, the basic concept has remained the same, while the technology surrounding it has become increasingly complex. The need for maximum efficiency and longevity has turned a simple electrical short into a modern quest for fire.
To better understand what a sparkplug does, one must first realize what it does not do. No matter the marketing hype, a plug cannot generate more voltage or a hotter spark. This is the function of the coil. Neither does a plug produce heat, when in fact it does just the opposite. These and other myths have caused more then their fair share of confusion. The very nomenclature used to classify sparkplugs (hot or cold) is in part to blame for some of the mystery. In reality, a plug must operate at a set temperature of approximately 900 ~ 1500 degrees Fahrenheit, weather it is in a stock bike, a race bike or your lawnmower. Hot or cold in this case is actually a reference to the plugs ability to transfer heat from its firing end into the cylinder head surrounding it and has absolutely nothing to do with the spark temperature.
Lets first look at the components of your basic sparkplug. Starting at the top is the terminal stud (sometimes capped with a terminal nut) that extends into a ceramic insert, which sits in a hex metal shell. The bottom of the metal shell consists of a seat, a threaded “reach” and a captured sealing washer. The bottom and business end of the plug reveals the center electrode surrounded by the ceramic insulator. A ground electrode extends off of the threaded section and the gap between the two is where all the real action happens. Each of these elements has its own relevant technology and configuration and only one combination of all of them is genuinely right for your bike.
The ceramic insert (the white part of the plug) isolates high voltage and ensures the spark occurs at the tip and not anywhere else within the plug. This insert also functions as a thermal storehouse as ceramic is a relatively inferior heat conductor and will tend to stay hot. The more contact it has with the metal part of the plug, the cooler the plug will be, as the surrounding metals will pull heat from the ceramic. The inverse is also true, as a plug with less contact between the insert and the metal around it will not have as much of a chance to shed heat and will remain hotter. The ceramic compound, as well as the center electrode material, are also factors in the temperature of a given plug. So why is the plugs physical temperature so important? We’ll need to explore what your plugs are really doing to illustrate that.
A sparkplug has two primary duties. The first is obviously to trigger combustion, but the equally critical second responsibility is to dissipate heat. The nose (or tip) of the plug resides in the hottest environment of your engine (sometimes exceeding 4000 F) and shedding this heat is paramount to continued service. A narrow temperature window must be maintained and correct heat range selection is the primary method of accomplishing this. Too hot of a plug (meaning, one that disperses heat too slowly) will glow and can induce pre-ignition. An overheated plug will cause the air/fuel mixture to be ignited prematurely, resulting in your engine potentially trying to compress a scorching blast of expanding gas as the piston rises in the cylinder. The ensuing heat, load and stress on your engine will quickly destroy it. Although not as harmful, too cold of a plug (meaning, one that sheds heat too quickly) can be a problem as well. A function of optimum plug tip temperature is to burn away residual carbon and a cold plug will have less ability to do this. The subsequent build up will reduce the plug’s ability to fire, creating an inefficient and very likely non-running engine. As important as the actual spark is, the correct heat range for your motor is equally critical.
A sparkplugs principal function is to transmit electrical energy, which then turns fuel into working energy by igniting an air/fuel mixture at precisely the correct time and as efficiently as possible. Seems simple enough, the ignition and coil handle the timing and power generation, a plug is essentially a controlled electrical short. If all engines were the same, the story would end here, but with wide variations in voltage, temperature and combustion pressure, comes a wide variation in plug styles. Our beloved V-Twin for the most part, uses a traditional projected nose plug, meaning the nose of the plug and the electrodes extend into the combustion chamber. There are a number of other designs, including surface gap, retracted gap, and multi ground. These exist primarily to compensate for valve or piston clearance problems, or where boost pressures cause excessive combustion temperatures and are generally considered a compromise. Even with a traditional projected nose plug, the electrode material and size can vary. The core (the part of the center electrode hidden in the ceramic body) is generally copper. Offering superior heat conductivity, meaning heat is removed from the firing end of the plug, copper helps prevent hot spots that can lead to pre-ignition. Since copper itself would not survive for long in the hostile environment of combustion, a center electrode of another material is bonded to the copper core and this is the part you see. The most common material for a center electrode is a copper/nickel alloy. Nickel is a decent conductor of both electricity and heat and this plug type is comparatively inexpensive to produce.
More recently on the sparkplug timeline, higher performance and longevity demands have prompted the use of precious metal electrodes. Materials such as Platinum and Iridium (the most corrosion resistant metal known to man!) are now commonly used. These materials have a significantly higher melting point and allow spark plug manufactures to use smaller center electrodes without the risk of the electrode overheating and self-destructing. Electricity will seek the path of least resistance and the smaller the center electrode, the smaller the path the electrical charge will have before it jumps to the ground electrode. The advantage of these plugs is less voltage is required for an effective spark, thereby reducing misfires. Being stronger, the exotic metal electrodes also create a longer lasting plug. Ground electrodes have not seen as many “technological” advances, and in some cases have taken a step backwards. Because of their need to be welded to the spark plugs shell, which is steel, most exotic materials cannot be used. Clever shaping and tapering has led to a more efficient spark much like the advances seen in the center electrode, but several designs utilizing split or multiple grounds actually block the flame kernel growth and offer no performance gain.
So why does all of this matter to you? If your motor is stock, it may not, if like many of us you are looking for maximum performance and have changed a few things it could be critical. Modifying the carburetion, ignition timing, camshaft, and compression ratio could also necessitate a plug change. Raising compression for instance also increases the temperature within the combustion chamber. Since part of the responsibility of the sparkplug is to remove heat and a modified motor makes more heat; the plug must now remove more heat. The result is the need for a colder heat range plug to avoid potential pre ignition problems. Swapping in a one heat range cooler plug can remove approximately 160 F to 200 F from the combustion chamber. The beauty of a sparkplug is it will actually tell you if it is not working to maximum effectiveness. After any major engine mod, it is important to inspect your sparkplugs, looking for tell tale signs of misfires, pre ignition or fouling. A sparkplug correctly matched to your motor can easily make the difference between a quest for fire and a quest for new pistons.