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Reading Spark Plugs

As we hold a spark plugs the first thing that we all notice is that Spark Plugs have numbers or identifiers from the manufacturer. These designations are placed upon the plug so that we cam learn about the plug from its identifier.
You should rely upon the plug manufacturer to provide the decoder which tell you about the plug and do not rely upon the cross reference charts that you find in automotive books as the accuracy is often lacking especially in plug heat range.
The numbers tell the diameter of the spark plug, the reach of the plug, how far is goes into the motor, the heat range of the spark plug, usually if it is a resistor of non resistor type ans some times the definition of the electrode is included. Some manufacturers have as many as eight characters on a spark plug to identify the characteristics of a spark plug.

The item that you have to pay attention to when reading a spark plug are: size: most plugs' thread diameter is a nominal 14 millimeters. There are some other sizes ( 10MM. !2MM and 18MM) but they are rarely found in any karting applications. Because differences in thread diameters are so large yery few people get into trouble through trying to apply a l4mm plug into a 12mm hole or vice versa. The reach or the length of threads which exist on the spark plug are a source of many issues. There are four normal reach lengths or dimensions: 9.5MM or 3/8-inch, 11.2MM or 7/16-inch, 12.7MM or 1/2-inch and 19MM or 3/4-inch.

Plugs which are to short cause issues in two areas you have just effectively enlarged the area and there by lowered the compression and you have exposed the lower threads of the cylinder head to heat and contamination which may make it hard to install the correct spark plug.

The worst problem created by an over length plug in an engine is that the exposed threads absorb heat from the combustion process. This raises the plug temperatures and may take them up high enough to make the side electrode glow white hot. If this happens you have the white-hot electrode firing the mixture far too early and this causes per ignition or detonation. We have even seen overheated plugs come apart and destroy pistons and valves.

A single exposed thread in an engine's combustion chamber will raise electrode temperatures . Most karters who are following the recommendations of their engine builder do not get into these situations but we often find novices who are relying upon some local experts having spark plug length issues.

There are also resistor and non resistor spark plugs. For most karting applications it is necessary to use a resistor type plug. The resistor plug uses a 5K ohm ceramic resistor to surpress ignition noise generated during sparking. Without the resistor plug it is very likely tat you will experience interference with your on board instrumentation especially if you are using any of the newer on board data acquisition systems.

The next area is called "heat range." A spark plugs have to stay hot enough to burn away deposits (oil, carbon, etc.) that otherwise would short-circuit the plug. In most four cycle engines we see plug temperatures from a low of 500+ degrees to a high of 1000+. degrees. In two cycle motors we see the temperatures slightly higher with lows being in the 700+ degree range and high temperatures in excess of 1200+ degrees.

Some of the spark plugs using the more exotic metals such as Iridium Platinum or Premium Platinum can run with temperatures in excess of 1500 degrees. This will all depend upon the fuel air mixture and the compression ratio which was selected by the engine builder.

Heat range is determined by altering the length of the path the heat travels from the center electrode and insulator nose cone to the plug shell and the plugs threads. A plug with a long insulator nose, which leads heat into the plug body before it reaches the cooler cylinder head, are "hot" plugs. Plugs with a shorter heat path, are "cold." Terms "hot:" and "cold" for plugs are very misleading, The engine puts heat into the plug, the plug does increase or decrease engine temperatures. A "hot" plug does not make an engine run hotter; a "cold" plug make if run cooler.

Nearly all of the spark plug manufacturers use a number-based code to designate heat range: Europe and Japan follow a system in which higher numbers mean colder plugs; American companies do just the opposite, assigning hotter plugs higher numbers. Racers should always review the manufacturers specifications prior to installing any plugs.

Do not let the price of the spark plug be the determining factor in the plug that you select. We have tested plugs which were in excess of $10 each and have selected NGK or ND plugs which are available for a few dollars each and often found better dyno results with the less expensive spark plugs. The expensive plugs where developed to solve a problem which we did not have such as issues caused by superchargers or some altitude extremen which we were not going to find in karting.

Briggs 4 cycle engines have large intake ports angled toward the head and the spark plug. The Briggs plug is positioned in the path of the intake flow and the alcohol fuel and the large volume of air tend to cool the spark plug tip. Many engine builders use hot plugs to overcome this issue and to keep the plug from fouling. Racers use surface-fire plugs in the Briggs motors especially in SuperStock, Limited and Modified classes where the stock carburetor is replaced with a carburetor and a high volume fuel pump. We have run these motores where we could deplete a one gallon fuel tank in fifteen laps on a mile oval track. Surface-fire plugs don't have a specific heat range, they run at about the same temperature as the combustion chamber's walls are immune to overheating and run very well in these high performance motors.

Now that you can read the outside of the plug and you can determine its relative sizes it is time to move to reading the internals of the spark plug.

The first rule to understand is don't try to read old spark plugs; even the experts find that difficult to impossible. New plugs are necessary to gain information about what's happening inside an engine. New plugs can give you a complete picture after just a few minutes of hard running or a couple of really good laps.

The first step is to teach the driver how to help us. We want the driver to kill the motor on the track. We usually ask the driver to pull the spark plug boot off of the plus so that the motor stops and we do not idle the motor down pit road. When the kart comes to a complete stop remove and inspect the electrode's tip with a plug scope which is a magnifying glass with a light. Examine the edges of the electrode, if it is rounded from melting, then you know there's overheating of the spark plug from weither incorrect fuel air mixture or incorrect ignition timing. . You should also have a close look at the tip of the ground electrode, checking for overheating symptoms,you may find that it got so hot that material has been eroded by the spark. Inspect the condition of the insulator, which should be white. A porous, grainy appearance is evidence of overheating. If the signs of overheating are confined mostly to the center electrode you have too much ignition timing.

If an engine has its timing only slightly advanced then a fuel buildup will be visible along part of the center electrode and end close to the tip. The reason for no build up at the tip is that the tip is sufficiently hot to burn off any fuel buildup. It is also possible that the timing is correct and the issue is that the heat range of the plug is multiple heat ranges to cold. It takes careful analysis to determine the cure so always get your engine builder involved before making wholesale timing or plug changes.

When you look at a freshly removed plug from a two-stroke engine and make decisions based on the color of the oil deposited on the insulator nose you are likely passing a meaningless or incorrect decision. We all see plugs which are just wet and know that the mixture is far to rich but the real analysis takes place by reading the insulator deep inside the plug body. The insulator is coolest where it contacts the metal shell, which is where you "read" your mixture stetting. Look far inside the plug, where the insulator joins the shell,if your engine's mixture is too rich is a colored ring will be present. If
this ring continues outward along the insulator to a width of even a millimeter you can be sure the mixture is rich enough to be safe, and too rich for maximum performance.

In most engines best performance is achieved when the mixture contains only enough excess fuel to make just a wisp of a "mixture ring" on the plug insulator.

Karting two-stroke air cooled motors like a slightly richer mixture, which provides internal cooling and lubrication. Briggs four-stroke engines give their best power when the mixture is leaned to a point that the last trace of color deep inside the plug completely disappears.

An air/fuel mixture that yields maximum power is only slightly richer than the one that causes detonation. The plug will tell you when there has been even slight detonation inside your engine. The signs to look for are pepper-like black specks on the insulator nose, and tiny balls of aluminum concentrated mostly around the center electrode's tip.

Severe detonation will blast a lot of aluminum off the top of the piston and give the plug a gray coating. If you find examples of detonation get you engine builder involved an determine if the motor can be run again or if it is in need of a rebuild.

The trick in all this is to know enough about spark plugs to be able to choose the right basic type, and to understand what the plug has to say about conditions inside your motor. Reading plugs takes a fair amount of experience and the only way to gain the experience is to become diligent about reading the plugs after each outing. When you have doubts seek advice from your motor builder of one of the top tuners at the track. You will find that the guys are ready to offer more advice when they find a person who is really making an effort to learn. You will soon learn how to get a lot more performance from your motors as you will have mastered the techniques necessary for the spark plug to talk to you.

Bob Chiras
Crew Chief
Allkart International
603-432-4766 evenings
781-442-3045 days