How to Select and Service Spark Plugs

Spark plugs: the forgotten Corvette service item

Wayne Scraba Mar 30, 2005 0 Comment(s)
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Today, spark plugs are pretty easy to forget. In new Corvettes, theyseem to last forever. That wasn't the case years ago and, frankly, sparkplugs are often critical when it comes to performance and economy. Theyshouldn't be neglected in your Corvette, and you should think of them asa window into the world of your engine. Certainly, proper selection isimportant, but once you understand what makes a spark plug tick and whatto look for, you might be able to pick up performance, economy, anddriveability--in some cases, lots of it.

Spark Plug Basics

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

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

Insulator: The insulator body is molded from aluminum oxideceramic. In order to manufacture this part of the spark plug, ahigh-pressure dry molding system is utilized. After the insulator ismolded, it's kiln-fired to a temperature that exceeds the melting pointof steel. This results in a component featuring exceptional dielectricstrength, 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 thespark-plug thread size.

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

Plating: The shell is almost always plated. This enhancesdurability 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 designthat provides a smooth surface for sealing purposes. Gasketless sparkplugs use a tapered seat shell that seals via a close toleranceincorporated into the spark plug.

Threads: Spark plug threads are normally rolled, not cut. Thismeets specifications set forward by the SAE along with the InternationalStandards Association.

Ground Electrode: There are a number of different groundelectrode shapes and configurations, but for the most part they'remanufactured from nickel alloy steel. The ground electrode must beresistant to spark and chemical erosion, both under extremetemperatures.

Insulator Nose: There are many insulator nose shapes and sizesavailable, but all must be capable of shedding carbon, oil, and fueldeposits at low speeds. At higher engine speeds, the insulator nose isgenerally cooled so that temperatures and electrode corrosion arereduced.

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

Basic Spark-Plug Types

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

There are at least a dozen different spark-plug configurations availablein today's marketplace. Some are unconventional and extrinsic. Thefollowing is a rundown of some basic high-performance configurationsyou'll come across.

Regular Gap: The regular-gap spark plug is most often associatedwith production vehicles. The side electrode extends midway to thecenter electrode, requiring less voltage at high engine speeds andpreventing particles of carbon (or other foreign material) from blockingthe gap and shorting out the plug. This spark-plug design offers goodprotection against fouling and provides good service in street-stripautomobiles.

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

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This spark plug is useless in almost any application except wherenitromethane is the fuel of choice. The retracted-gap spark plug is anall-out race spark plug designed for applications where there is limitedphysical clearance between the pistons and/or valves. It's primarilyintended for use on supercharged, highly modified and/or nitro-poweredapplications. 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 raceapplications and are particularly useful when the engine requires wideheat range latitude. Additionally, this type of spark plug is wellsuited to conditions where the mixture is rich and there is a widedifference between cylinder-to-cylinder cooling.

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

Selecting A Plug

Different types of spark plugs are designed to do different things, butthey all have one common function: ignite the air/fuel mixture in theCorvette combustion chamber and, at the same time, ignite the air/fuelcharge with some manner of efficiency. But there's a caveat: There is noperfect spark plug for a particular application. Some race engines runextremely hot (i.e., nitro-burning Top Fuel Dragsters). Because of thetemperature extremes experienced in something like a Top Fuel Dragsterengine, a cold spark plug is required. On the flipside, a daily drivenCorvette might have a cold-running engine. The type of spark plugutilized is just the opposite: It must be relatively hot. Hot enginesequal cold plugs; cold engines equal hot plugs. These are the basics ofthe spark-plug heat range.

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Some Corvette engine combinations prefer more spark-plug gap thanothers. A gap of approximately 0.025 inch is commonly used inconjunction with early Corvette point-triggered ignitions. On the otherhand, contemporary electronics can easily fire gaps in the range of0.045 to 0.060 inch (or more).

The thermal characteristics or "heat rating" of a given spark plugreflect the component's ability to transfer combustion-chamber heat fromthe firing end of the spark plug into the cylinder head. The range oftemperatures from idle to maximum rpm determines the heat range of thespark plug. From a design and construction perspective, the length ofthe insulator nose determines the heat range of the spark plug.

Cold spark plugs normally have a short heat flow path, which results ina quick rate of heat transfer. Additionally, the short insulator nosefound on cold spark plugs has a small surface area, which does not allowfor a massive amount of heat absorption. On the other hand, hot sparkplugs feature a longer insulator nose as well as a longer heat transferpath. This results in a much slower rate of heat transfer to thesurrounding cylinder head (and, consequently, the water jacket).

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

But there's more to the heat range selection process than just fuel typeand hot versus cold parameters. For example, in a performanceapplication, a Corvette with an automatic transmission is generallyoperated in a wider rpm range than a stick-shift counterpart. It's alsomore difficult to clean out a car with an automatic transmission(cleaning out means blipping the throttle to obtain crisp throttleresponse). Because of this, the spark plug must be capable of burningoff deposits that would otherwise cause fouling problems. At the sametime, the automatic-transmission car is then accelerated to the higherreaches of the rpm range. Conversely, a stick-shift car is commonly run"on the cam" (typically in the higher reaches of the rpm band). Bydisengaging the clutch and working the throttle, a stick-shift carseldom, if ever, sees the lower speed ranges. As a result, the Corvetteengine does not normally experience the fouling problems associated withautomatic-equipped cars.

In order to select the right heat range for your engine, use a safeselection process. Start with a set of spark plugs that are too cold andwork your way up until fouling stops. Use a spark plug that's severalheat ranges colder than specified in a factory shop manual for yourvehicle. According to the folks at Autotronic Controls Corporation(manufacturers of MSD ignition products), the tip temperature of thespark plug must stay lower than the normal pre-ignition temperature ofapproximately 1,400 degrees F. In addition, the tip temperature mustremain hot enough to prevent oil or carbon fouling. This requires atemperature of approximately 800 degrees F. MSD feels you should try touse the hottest tip temperature that will "live" in the engine withoutcreating any spark plug-induced problems (such as detonation). Aprojected-nose spark plug is preferred if it clears the piston dome(supercharged powerplants will likely require the use of a side-gapplug).

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