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Racing Technology
Creating the right spark: selecting, gapping, and installing spark plugs
by Wayne Scraba
Different spark plugs do different things, but all spark plugs have one common function: ignite the air-fuel mixture in the combustion chamber and, at the same time, ignite the air-fuel charge with efficiency. There is no perfect spark plug for a particular application. Some powerplants, like nitro-burning race cars, run extremely hot; because of this, a very cold spark plug is required. On the flip side, your daily driven crew cab might have a very cold-running powerplant, in which the type of spark plug utilized is just the opposite — it must be relatively hot.
The thermal characteristics, or heat rating, of a given spark plug reflect the component’s ability to transfer combustion-chamber heat from the firing end of the spark plug into the cylinder head. The range of temperatures from idle to maximum rpm determines the heat range of the spark plug. From a design and construction point of view, the length of the insulator nose determines the heat range of the spark plug.
Cold spark plugs normally have a short heat-flow path. This results in a very quick rate of heat transfer. The short insulator nose found on cold spark plugs also has a small surface area, which does not allow for a massive amount of heat absorption. On the other hand, hot spark plugs feature a longer insulator nose as well as a longer heat-transfer path. This results in a much slower rate of heat transfer to the surrounding cylinder head and to the water jacket.
The combustion-chamber temperature of your engine will dictate the heat range of the spark plug. If you use spark plugs that are too cold for your powerplant, the engine will load up, carbon will form, and the spark plugs will need constant attention. If the spark plugs are too hot, it will begin to glow and eventually detonate, which can easily split a cylinder wall and ruin a cylinder along with the piston. These are extreme cases, but even a spark plug that is slightly too hot will inevitably fail over a period of time.
Making a selection
To select the right heat range for your powerplant, start with a set of spark plugs that are too cold and work your way up to hotter spark plugs until fouling stops. Use a spark plug that is several heat ranges colder than specified in a common factory manual for your vehicle. According to the folks at Autotronic Controls Corporation (manufacturers of MSD ignition controls), the tip temperature of the spark plug must remain lower than the normal preignition temperature of approximately 1,400 degrees Fahrenheit. In addition, the tip temperature must remain hot enough to keep from oil or carbon fouling, which requires a temperature of approximately 800 degrees F. MSD states that you should try to use the hottest tip temperature that will live in the powerplant without creating any spark-plug-induced problems, like detonation. A projected-nose spark plug is preferred, if it will clear the piston dome.
Projected-nose spark plugs are preferred for several reasons — most important is the plug's capability of acting like a hot spark plug at low rpm levels, and then cooling off and acting like a cold spark plug at high rpm levels. The projected-nose spark plug is cooled by the incoming air-fuel charge, which effectively blows by the extended spark plug tip. At slow engine speeds, the increased insulator length improves the spark-plug temperature characteristics, allowing the plug to burn cleanly without fouling. The projected nose also helps place the spark plug in a well-suited position to initiate the ignition process.
The use of high-powered, modern ignition systems, such as computer ignitions, multiple-spark ignitions, and many electronic systems, also creates problems with spark-plug selection. High-quality, high-power ignition systems produce relatively large amounts of coil current. Because of this, the spark-plug condition is not as critical as it once was in the days of single- and dual-point distributors. While the spark-plug condition is less critical, it doesn't mean that the spark plugs are set-and-forget components. With these ignition systems, reading the plugs is certainly more difficult, but it isn't impossible.
Many racers who have made the switch from a conventional ignition system to a high-powered system of some sort have claimed that the spark plugs are reading lean. The ignition system does not directly affect the air-fuel mixture, but with the better-burning, more efficient properties of the modern ignition setup, changes may be required to be made in the air-fuel mixture.
To read spark plugs used in conjunction with a modern ignition system, MSD offers the following suggestions.
All major spark-plug manufacturers offer a high-performance-tuning guide booklet with information on spark-plug tuning. Examine the correct color and appearance of your spark plugs and compare them to the guide. This should give you some insight on things like detonation, overheating, coloration, and the like. This tech tip is not intended to replace the spark-plug manufacturer's guide but to supplement it.
To read the spark plugs in a modified engine, the engine must be shut off clean. Accelerate your car for approximately an eight-mile, then click off the engine, allowing the car to coast to a stop. Remove the plugs and inspect them. This type of clean shutoff will be easier with manual-transmission cars than with automatics.
When it fires, the spark-plug insulator transfers heat from the tip to the threads. If you look closely at a spark plug, you'll see the point of effective heat transfer, which generates a ring on the spark-plug insulator nose. This ring almost looks like a shadow and is more easily observed with a spark-plug inspection light. The ring will form very quickly and is an indication of burning in the combustion chamber. The closer to the tip the ring forms, the richer the fuel mixture. The closer to the spark-plug shell, the leaner the mixture. If there is no ring and the spark-plug insulator tip is bone-white, the fuel mixture is too lean. (This information only applies to gasoline-fueled applications.) The total range from rich to lean, or ring near the tip to ring near the shell, may only take a change of 7 to 10 percent in the air-fuel mixture.
When making the switch to a high-powered ignition system, you may find that the actual fuel curve will have to be fattened by 5 to 10 percent. The reason for this is that the ignition system is now burning more efficiently. In addition, the total spark timing might have to be reduced by approximately three degrees.
When reading your spark plugs, there are a number of variables to consider: air-fuel mixture, spark-plug heat range, ignition timing, fuel type, and a host of other things that are not in your control like ambient temperature, barometric readings, etc. Make the most of your situation, and tune from there.
The gap
Some engine combinations like a bunch of spark-plug gap, and others don't run well until the gap is tightened up. On the low end of the spectrum, a gap of approximately .025-inch is commonly used, but some combinations, particularly those with high-powered modern ignition systems, can fire a gap ranging from .045-inch to .060-inch. In all situations, the gap, air-fuel mixture, and heat range must be coordinated to produce the best possible results from the powerplant. It's a case of trial and error coupled with an accurate reading of the spark plugs.
A performance trick that is seldom discussed but has been accomplished on a matter-of-fact basis for years is spark-plug indexing. Indexing spark-plugs is a relatively simple task and can tighten the gap in engines with big domes and tight combustion-chamber clearances. To index the plug, mark the spark-plug insulator body with a felt-tip pen on the side where the ground electrode attaches to the spark-plug body. Position the plug so that the gap is facing the center of the cylinder toward the exhaust valve — this is the most common arrangement; some engines respond favorably to other gap locations.
Installation
Instead of rummaging through boxes and boxes of spark-plugs in an effort to locate the elusive combination of the perfect plug threads and respective cylinder-head threads, use aftermarket indexing washers. These soft-copper washers are available from Moroso (and other sources) under part number 71900 for 14mm taper-seat spark-plugs and 71910 for 14mm flat-seat spark-plugs. The washer kits are supplied in .060-inch, .080-inch, and .100-inch thicknesses. The softness and different washer thickness of the copper make it a simple task to thread the spark-plug into its respective cylinder and tighten it according to your index mark. Be certain that you do not double up on the washers! They are not meant to be used in tandem.
It is feasible to simply thread plugs into the respective cylinders and forget about them. But, if you take the time to select the right plug, gap it correctly, and index the gap in relation to the valves, you'll find a much quicker path to more horsepower.
Sources
MSD Ignition
1490 Henry Brennan Drive
El Paso, TX 79936-6805
(915) 857-5200
Moroso Performance Products
80 Carter Drive
Guilford, CT 06437-2125
(203) 453-6571