How to Select and Service Spark Plugs

Spark plugs: the forgotten Corvette service item


Today, spark plugs are pretty easy to forget. In new Corvettes, they seem to last forever. That wasn't the case years ago and, frankly, spark plugs are often critical when it comes to performance and economy. They shouldn't be neglected in your Corvette, and you should think of them as a window into the world of your engine. Certainly, proper selection is important, but once you understand what makes a spark plug tick and what to look for, you might be able to pick up performance, economy, and drivability--in some cases, lots of it.

Spark Plug Basics

Before looking at spark plugs as a Corvette tuning tool, let's discuss what makes a spark plug function. They aren't as simple as you think--in fact, they're precision pieces of equipment. Keep in mind, the vast majority of spark plugs offer similar (although not necessarily identical) construction to the ones shown here.

Ribs: Insulator ribs provide added protection against secondary voltage or spark flashover and also help improve the grip of the rubber spark-plug boot against the plug body.

Insulator: The insulator body is molded from aluminum oxide ceramic. In order to manufacture this part of the spark plug, a high-pressure dry molding system is utilized. After the insulator is molded, it's kiln-fired to a temperature that exceeds the melting point of steel. This results in a component featuring exceptional dielectric strength, high thermal conductivity, and excellent resistance to shock.

Hex: The hexagon provides the contact point for a socket wrench. The hex size is basically uniform in the industry and is related to the spark-plug thread size.

Shell: The steel shell is fabricated to exact tolerances using a special cold extrusion process. Certain types of spark plugs make use of a steel billet (bar stock) for shell construction.

Plating: The shell is almost always plated. This enhances durability and provides rust and corrosion resistance.

Gasket: Certain spark plugs use gaskets while others are "gasketless." The gasket used on spark plugs is a folded steel design that provides a smooth surface for sealing purposes. Gasketless spark plugs use a tapered seat shell that seals via a close tolerance incorporated into the spark plug.

Threads: Spark plug threads are normally rolled, not cut. This meets specifications set forward by the SAE along with the International Standards Association.

Ground Electrode: There are a number of different ground electrode shapes and configurations, but for the most part they're manufactured from nickel alloy steel. The ground electrode must be resistant to spark and chemical erosion, both under extreme temperatures.

Insulator Nose: There are many insulator nose shapes and sizes available, but all must be capable of shedding carbon, oil, and fuel deposits at low speeds. At higher engine speeds, the insulator nose is generally cooled so that temperatures and electrode corrosion are reduced.

Center Electrode: Center electrodes must be manufactured from a special alloy that's resistant to spark and chemical corrosion. Keep in mind that combustion-chamber temperatures vary (sometimes radically); the center electrode must live under these parameters. The area between the ground electrode and center electrode is called the gap.


Basic Spark-Plug Types

Fine-wire spark plugs are intended for high-performance applications and are particularly useful when the engine in your Corvette requires a wide heat-range latitude. This type of spark plug is well suited to conditions where the mixture is rich and there is a wide difference between cylinder-to-cylinder cooling.

There are at least a dozen different spark-plug configurations available in today's marketplace. Some are unconventional and extrinsic. The following is a rundown of some basic high-performance configurations you'll come across.

Regular Gap: The regular-gap spark plug is most often associated with production vehicles. The side electrode extends midway to the center electrode, requiring less voltage at high engine speeds and preventing particles of carbon (or other foreign material) from blocking the gap and shorting out the plug. This spark-plug design offers good protection against fouling and provides good service in street-strip automobiles.

Projected-Nose Gap: Projected-nose spark plugs can be used in any overhead-valve engine provided there is physical clearance in the combustion chamber. The heat-range characteristics of this spark plug differ from other spark plugs in that the incoming air/fuel charge cools the insulator tip at high engine speeds, preventing fouling that can be caused by excessively rich mixtures.

This spark plug is useless in almost any application except where nitro methane is the fuel of choice. The retracted-gap spark plug is an all-out race spark plug designed for applications where there is limited physical clearance between the pistons and/or valves. It's primarily intended for use on supercharged, highly modified and/or nitro-powered applications. The protection against fouling is substantially limited. Bottom line, don't use one in your Corvette. It won't help.

Fine-Wire Gap: The fine-wire spark plugs are intended for race applications and are particularly useful when the engine requires wide heat range latitude. Additionally, this type of spark plug is well suited to conditions where the mixture is rich and there is a wide difference between cylinder-to-cylinder cooling.

Retracted Gap: The retracted-gap spark plug is an all-out race spark plug designed for applications where there is limited physical clearance between the pistons and/or valves. It's primarily intended for use on supercharged, highly modified and/or nitro-powered applications. The protection against fouling is substantially limited and, as a result, it should not be used for street applications.

Selecting A Plug

Different types of spark plugs are designed to do different things, but they all have one common function: ignite the air/fuel mixture in the Corvette combustion chamber and, at the same time, ignite the air/fuel charge with some manner of efficiency. But there's a caveat: There is no perfect spark plug for a particular application. Some race engines run extremely hot (i.e., nitro-burning Top Fuel Dragsters). Because of the temperature extremes experienced in something like a Top Fuel Dragster engine, a cold spark plug is required. On the flipside, a daily driven Corvette might have a cold-running engine. The type of spark plug utilized is just the opposite: It must be relatively hot. Hot engines equal cold plugs; cold engines equal hot plugs. These are the basics of the spark-plug heat range.

Some Corvette engine combinations prefer more spark-plug gap than others. A gap of approximately 0.025 inch is commonly used in conjunction with early Corvette point-triggered ignitions. On the other hand, contemporary electronics can easily fire gaps in the range of 0.045 to 0.060 inch (or more).

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 perspective, the length of the insulator nose determines the heat range of the spark plug.

Cold spark plugs normally have a short heat flow path, which results in a quick rate of heat transfer. Additionally, the short insulator nose found on cold spark plugs 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, consequently, the water jacket).


So what's wrong with using spark plugs that are too cold for your engine? Simple. The engine loads up, carbon forms, and the spark plugs need constant attention. If the spark plugs are too hot, they begin to glow. The result is detonation that can easily ruin a cylinder (along with the piston). Of course, these are extreme cases, but even a spark plug that's slightly too hot inevitably fails over a period of time. In essence, the combustion-chamber temperature of your engine combination dictates the heat range of the spark plug.

But there's more to the heat range selection process than just fuel type and hot versus cold parameters. For example, in a performance application, a Corvette with an automatic transmission is generally operated in a wider rpm range than a stick-shift counterpart. It's also more difficult to clean out a car with an automatic transmission (cleaning out means blipping the throttle to obtain crisp throttle response). Because of this, the spark plug must be capable of burning off deposits that would otherwise cause fouling problems. At the same time, the automatic-transmission car is then accelerated to the higher reaches of the rpm range. Conversely, a stick-shift car is commonly run "on the cam" (typically in the higher reaches of the rpm band). By disengaging the clutch and working the throttle, a stick-shift car seldom, if ever, sees the lower speed ranges. As a result, the Corvette engine does not normally experience the fouling problems associated with automatic-equipped cars.

In order to select the right heat range for your engine, use a safe selection process. Start with a set of spark plugs that are too cold and work your way up until fouling stops. Use a spark plug that's several heat ranges colder than specified in a factory shop manual for your vehicle. According to the folks at Autotronic Controls Corporation (manufacturers of MSD ignition products), the tip temperature of the spark plug must stay lower than the normal pre-ignition temperature of approximately 1,400 degrees F. In addition, the tip temperature must remain hot enough to prevent oil or carbon fouling. This requires a temperature of approximately 800 degrees F. MSD feels you should try to use the hottest tip temperature that will "live" in the engine without creating any spark plug-induced problems (such as detonation). A projected-nose spark plug is preferred if it clears the piston dome (supercharged power plants will likely require the use of a side-gap plug).

The Nose's Job

Projected-nose spark plugs are preferred for several reasons, but perhaps the most important is the plug's ability to act like a hot spark plug at low rpm levels, then cool off and act like a cold plug at high engine rpm. The projected-nose spark plug is cooled by the incoming air/fuel charge that effectively blows by the extended spark-plug tip. At low engine speeds, the increased insulator length improves the spark-plug temperature characteristics, allowing the plug to burn cleanly without fouling. Further, the projected nose helps place the spark plug in a well-suited position to initiate the ignition process.

The High-Power Ignition System Difference

The use of modern day high-powered ignition systems such as computer ignitions, multiple-spark ignitions, and many of today's digital systems also creates problems in spark-plug selection, but not in a way you might first imagine. 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 the spark plugs are "set and forget" components. Reading of the plugs with these ignition systems is certainly more difficult, but it isn't impossible.

Many Corvette enthusiasts make the switch from a conventional ignition system to a high-powered system of some sort, then claim the spark plugs are reading lean. Think about this: The air/fuel mixture hasn't been changed; how can the ignition system affect the air/fuel mixture? It can't, but because of the better burning properties of a modern ignition setup, changes may be required in the air/fuel mixture.


So why are the spark plugs so clean? Because the ignition system is efficient. In order to read spark plugs used in conjunction with a contemporary ignition system, MSD offers the following suggestions.

1. Pick up a copy of a spark-plug tuning guide (all of the major spark-plug manufacturers offer some form of a high-performance pocket tuning guide). Examine the correct color and appearance of your spark plugs and compare them to the guide. This should give you some insight into things like detonation, overheating, and coloration.

2. There are several methods to prepare for reading spark plugs, but in the case of a high-performance Corvette, there is only one right way to accomplish it. The engine must be shut off "clean." This means you should run through a quarter-mile and click off the engine, allowing the car to coast to a stop at a safe location. Don't drive the car any further. The idea is to stop the engine clean, then remove the plugs and inspect them. Obviously, this type of clean shutoff will be easier with manual-transmission Corvettes than it is with automatics. Nonetheless, it's necessary because the trip back down the return road can wipe out any valuable reading information provided by the spark plugs.

3. The point of effective heat transfer generates a ring on the spark-plug insulator nose. The ring almost looks like a shadow and is more easily observed with a spark-plug inspection light or loupe. The ring forms 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 applies only to gasoline-fueled race cars). The total range from rich to lean (ring near the tip to ring near the shell) may only take a change of 7-10 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 5-10 percent, because the ignition system is now burning more efficiently. In addition, the total spark timing might have to be reduced by approximately 3 degrees.

As you might have guessed, reading the spark plugs isn't a black art, but it isn't an exact science either. You have a number of variables open to you: Air/fuel mixture, spark-plug heat range, ignition timing, fuel type, transmission type, and a host of other things that aren't in your control (ambient temperature, weather conditions, altitude, and so on). The idea is to make the most of your situation and tune from there. Let your elapsed times be the final judge.

Bridging The Gap

Some Corvette engine combinations like a bunch of gap and others don't run well until the gap is tightened up. A gap of approximately 0.025 inch is commonly used, but some combinations (particularly those with high-powered contemporary ignition systems) can fire a gap ranging from 0.045 to 0.060 inch. In all situations, the gap, air/fuel mixture, and heat range must be coordinated to produce the best possible results from the engine. In simple terms, it's a case of trial and error coupled with the correct reading of the spark plugs.


Once you have the correct heat range dialed in, the gap sorted out, the spark plug type selected, the jetting squared away, and all of the other variables inside your Corvette handled, there is no need to continue messing with the plugs. The idea is to get your combination set up correctly, and to use the spark plugs as a tool in the setup. Once everything is right, no magic power will be found in the spark plugs. Remember, they're the window inside your Corvette engine. Don't forget them.