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Analyzing Spark Plugs
Analyzing Spark  Plugs


Product Code: 017



By Bob Chiras

Learning to read sparkplugs is one of the best ways to understand exactly what is happening inside your engine. Whether it is “Carburetion Day” at Indyor “Happy Hour” in Winston Cup, anyone who has spent time in the pits of a professional motorsports event will attest to the numbers of crew chiefs and tuners who are looking through plug scopes at their spark plugs trying to determine what carburetion and ignition changes need to be made.

The first step in understanding spark plugs is learning how to read the number code on the side of the plug. Each manufacturer has a different formula for their numbers, which you can usually get, from the dealer where you buy the plugs. It is best to rely on these charts as we have found automotive cross-reference charts to be inaccurate, especially in the plug heat range.

The numbers tell the diameter of the spark plug, the reach of the plug –(how far it goes into the motor), the heat range of the spark plug, if it is a resistor or non resistor type and sometimes the definition of the electrode is included. Some manufacturers have as many as eight digits on a spark plug to identify the characteristics of a spark plug.

The first thing to look at is the size of the spark plug. Most sparkplugs’ thread diameter is 14 millimeters. There are some other sizes (10MM. 12MM and 18MM), but they are rarely found in any karting application. Because differences in thread diameters are so large very few people get into trouble through trying to apply a l4mm plug into a 12mm hole or vice versa.

Plug Length:

The reach or the length of threads often trip racers up. There are four normal reach lengths; 9.5MM or 3/8-inch, 11.2MM or 7/16-inch, 12.7MM or 1/2-inch and 19MM or 3/4-inch. Having either too short or too long of plugs usually results in some type of engine malfunction.

There are two potential problems that can arise if plugs are too short. First, by shortening the length, a tuner has effectively enlarged the head area and lowered the compression. Secondly, a short plug leaves the lower threads of the cylinder head exposed to heat and contamination making it difficult later on to install a plug with the proper length.

If the plugs are too long, then the threads extend into the combustion chamber and are no longer protected by the walls of the head. The exposed threads then 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 it will begin firing the mixture far too early causing pre-ignition or detonation. Overheated plugs also have the potential to come apart and destroy pistons and valves, ruining the engine. 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.

Resistor or Non-resistor

There are also resistor and non-resistor spark plugs. At the moment the spark jumps the gap it causes a high frequency burst of energy, this is known as RFI (radio frequency interference). Placing a resistor within the spark plug suppresses the RFI. Without resistor plugs in your car you can experience static on your radio as well as interfere with other sensitive electronic equipment. Some later model vehicles as well as newer Powersport engines must use resistor plugs for a proper “talkback” to the electronic ignition. Outboard marine Capacitive Discharge Ignition (CDI) such as used on some Johnson and Evinrude marine engines require a special inductive type resistor. Use of non-inductive resistor type plugs on these motors can cause misfire and poor performance.

For most karting applications, it is necessary to use a resistor type plug. The resistor plug uses a 5K ohm ceramic resistor to suppress ignition noise generated during sparking. Without the resistor plug it is very likely that a racer will experience interference with any electric instrumentation on the kart, especially any of the newer data acquisition systems.

Heat Range:

Before you venture into heat range one lesson is a must. Spark plugs are not thermostats. You do not change plugs to make a motor run hotter or cooler.

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 that is 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, and those with a shorter heat path, are “cold.” plugs. The terms “hot:” and “cold” for plugs can be very misleading, the engine puts heat into the plug, the plug does not increase or decrease engine temperatures. A “hot” plug does not make an engine run hotter nor does a “cold” plug make it 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.

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 motors where we could deplete a one-gallon fuel tank in fifteen laps on a 1/4 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.

Do not let the price be the determining factor in the plug that you select. We have tested plugs that cost 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 problems like those caused by superchargers or altitude extremes that really do not apply to karting.

Reading the Plug:

Once a racer or tuner has an understanding of the dimensions of a plug and how they effect the engines performance, they can move on to reading the various surfaces of the plug to understand how it is performing in the engine. The best way to look at a plug is to use a plug scope. A plug scope is a device that is basically a magnifying glass with a flashlight attached used to closely inspect the surfaces of the spark plug.

The first rule to understand is; do not try to read old spark plugs. Even the experts find it difficult or impossible to get accurate results from doing so. It is best to start with a clean slate and new plugs are the only way to gain information about what is currently happening inside the engine. New plugs can give you a complete picture after just a few minutes of hard running or a couple of really good laps.

It is important to get a clear picture of what is happening when the kart is under race conditions. Therefore it is important for the driver to kill the motor while they are still on the track. We usually ask the driver to pull the spark plug boot off of the plug so that the motor stops and we do not idle the motor down pit road. When the kart comes to a complete stop, we remove and inspect the electrode’s tip with a plug scope.

First the edges of the electrode should be examined for any rounding that has occurred from melting. Rounding is an indication that the spark plug is overheating. This can be caused from either incorrect fuel air mixture or incorrect ignition timing.

The tip of the ground electrode may also show signs of eroding, another sign of overheating.

Analyzing Plugs To Improve Performance

Next, 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, then the engine has 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. If you are sure that the engine timing is correct, the issue may be that the heat range of the spark plug is several ranges too 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 see plugs that are just wet and know that the mixture is far too 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 setting. Look far inside the plug, where the insulator joins the shell. If the engine’s mixture is too rich 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 2 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 indicate when there has been even slight detonation inside an 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 your engine builder involved and determine if the motor can be run again or if it needs 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 a 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 or 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 by mastering the techniques necessary for you to read spark plugs and understand what they are saying.


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