Dr Jay's Tech Topic - Spark Plugs

Spark Plugs. A necessary part of any non-Diesel internal combustion engine, but probably one of the most misunderstood components. The engine won't run without one, yet through all the mystery, they are a very simple device. So what's the big deal with spark plugs? We'll try to dispel some of the Spark Plug "myths” with this month's discussion. Throughout this article, we are of course discussing our Yamaha production Motorcycles.

Q: First off, what exactly does a spark plug do?

Simply put, a spark plug initiates the combustion cycle. The plug is connected to the high voltage generated by the ignition coil. As the electrons flow from the coil, a voltage difference develops between the center electrode and side electrode. No current can flow because the fuel and air in the gap is an insulator, but as the voltage rises further, it begins to change the structure of the gases between the electrodes. Once the voltage exceeds the dielectric strength of the gases, the gases become ionized. The ionized gas becomes a conductor and allows electrons to flow across the gap. This is the "click" heard when observing a spark, similar to lightning and thunder. Spark plugs usually require voltage in excess of 20,000 volts to 'fire' properly.

As the current of electrons surges across the gap, it raises the temperature of the spark channel to 60,000 K ! The intense heat in the spark channel causes the ionized gas to expand very quickly, like a chain reaction the fuel begins to burn- one fuel molecule igniting the next.  The heat and pressure force the gases to react with each other, and at the end of the spark event there should be a small ball of fire in the spark gap as the gases burn on their own. The size of this fireball or kernel depends on the exact composition of the mixture between the electrodes and the level of combustion chamber turbulence at the time of the spark. In Ideal conditions, the fuel being burned is consumed in an orderly fashion, not really an explosion, but a controlled propagation of the flame front as it moves out form the spark channel.


Q: My Friend Says he changed to a "Racing” Spark Plug and gained allot more power and better gas mileage. Is this possible?

In a word, NO. In a normal stock internal combustion engine, there is no increase in power to be had from a Spark Plug. The Spark Plug Simply Initiates the combustion process. Your friend probably had a bad spark plug and replacing it simply corrected a problem.

A Racing Spark plug is designed with a more expensive core electrode material and design that can actually change the cores resistance thereby increasing the amount of voltage available at the gap. This is assuming there is more voltage to be had. In a normal ignition system, the spark voltage is enough to create the spark kernel with a standard spark plug and there is no real benefit to a more expensive plug in an otherwise stock environment. The center, or core electrode is usually the one designed to eject the electrons because it is normally the hottest part of the plug; it is easier to emit electrons from a hot surface, because of the same physical laws that increase emissions of vapor from hot surfaces. In addition, electrons are emitted where the electrical field strength is greatest; this is from wherever the radius of curvature of the surface is smallest, i.e. from a sharp point or edge rather than a flat surface. It would be easiest to pull electrons from a pointed electrode but a pointed electrode would erode after only a few seconds. Instead, the electrons emit from the sharp edges of the end of the electrode; as these edges erode, the spark becomes weaker and less reliable.

Q: If I install a Hotter Spark Plug will I increase power and gas mileage?

Again the answer is no. The operating temperature of a spark plug is the actual physical temperature at the tip of the spark plug within the running engine. This is determined by a number of factors, but primarily the actual temperature within the combustion chamber. There is no direct relationship between the actual operating temperature of the spark plug and spark voltage. However, the level of torque currently being produced by the engine will strongly influence spark plug operating temperature because the maximum temperature and pressure occurs when the engine is operating near peak torque output (torque and RPM directly determine the power output). The temperature of the insulator responds to the thermal conditions it is exposed to in the combustion chamber but not vice versa. If the tip of the spark plug is too hot it can cause pre-ignition leading to detonation/knocking and damage may occur. If it is too cold, electrically conductive deposits may form on the insulator causing a loss of spark energy or the actual shorting-out of the spark current. A spark plug is said to be "hot" if it is a better heat insulator, keeping more heat in the tip of the spark plug. A spark plug is said to be "cold" if it can conduct more heat out of the spark plug tip and lower the tip's temperature. Whether a spark plug is "hot" or "cold" is known as the heat range of the spark plug. The heat range of a spark plug is typically specified as a number, with NGK spark plugs, a lower number indicates a hotter plug and a higher number indicates colder plugs. An NGK "B8ES” indicates a cooler plug than a "B7ES”.

So, what have we learned?

1.   Use the correct spark plug for your application- both in design and heat range

2.       In a stock Yamaha production Motorcycle, there is no power or gas mileage improvement from a new "Standard" spark plug versus a new "Racing" spark plug

3.       When replacing plugs, always check the gap. (The actual gap spec is not as important as being sure the gap isn't smashed closed)

4.    ALWAYS put a little lubricant on the threads when installing a spark plug (oil or grease is ok, anti sieze (Moly) is better)

5.     Ignore the advertising hype about "special" plugs

6.    Change them at the mileage recommended in your owners manual

Ride Safe!

Dr Jay