Yes, Spark Plug Tuning Is As Critical As Carb And Cam Selection

While usually an afterthought, consideration of your engine’s spark plugs are as equally important in a performance engine as your cam specs or carburetor size. The wrong choice can knock out all your efforts to put together a well-executed performance engine.

A hot rod or street machine engine that sees little or no all-out-competition use typically prefers a spark plug close to an OEM heat-range to keep the center electrode at the optimum temperature. Step into the world of high-compression and high-RPM engines, and you need a plug that will thoroughly transfer the increased heat away from the center electrode.

The heat-range criteria is to get heat to transfer from the electrode and inner-insulator area to the plug case, and thus, the cylinder head’s water jackets for cooling.

Knowing The Parameters Of Operation

Fundamentally, spark plug tuning is a matter of matching a plug to the fuel, air, and compression characteristics that generate heat within the combustion chamber. Your choice in plug selection features different “heat range” spark plugs. These different plug characteristics alter how heat is transferred from the spark plug case, through the cylinder head, and to the cooling system.

You need to use the standard practice of making a couple of full passes on the dragstrip with your street/strip or race car. Once you have made that pass, immediately shut down the engine. Idling the engine back to the pits will alter the plug markings you are trying to read. – Don Ward, E3 Spark Plugs

Different heat-range designs change how rapidly heat is transferring from the spark plug tip to the cooling system chambers within the head. A “colder” plug has a short distance of depth between the center-electrode insulator and the spark plug case. A cold plug allows for heat to transfer more quickly to the cylinder head. A “hotter” plug uses a ceramic insulator that extends to a deeper depth before it meets the case. The point where the insulator meets the outer plug case is the point where heat is transferred from the spark plug to the cylinder head. A hotter plug, with its deeper insulator depth, keeps more heat in the spark plug.

Possible Reasons To Use Colder Plugs:

  • High piston compression-ratios in normally aspirated engines, supercharged, or turbocharged applications, and/or the use of Nitrous Oxide.
  • High-octane racing gas, methanol, and oxygenated fuels deliver a greater completion of combustion.
  • Performance air/fuel mixture or advanced spark timing raises the spark plug-tip temperature. That can cause knock or pre-ignition.
  • Extreme acceleration and high-RPM driving/racing will raise plug temps. Prolonged circle track and road race applications require an even colder heat range.

This tuning between hot and cold spark plugs is crucial when your plugs are working in a high-cylinder pressure, high-RPM environment. But, throwing hotter or colder plugs at a performance engine without understanding the effects is not a simple cure for most ailments. A spark plug must operate at an optimum temperature to alleviate problems that will become evident on both sides of the preferred 1,500-degree plug temperature.

We cleaned the current plugs in the 482ci engine in our bracket Camaro for a benchmark reading. You can see the electrode tip edge is rounded, and this will hamper the plug's ability to jump a spark to the ground strap. We could see that the overall plug indicates a rich condition. The heat line that is visible on the ground strap (arrow) indicates we needed to add a couple of degrees of timing advance. We leaned the jetting a little, and bumped the timing 2 degrees before installing our new plugs.

Possible Reasons To Use Hotter Plugs:

  • An engine with a lower compression ratio or one used at low engine RPM will not generate enough heat in a plug that has a colder design.
  • When the heat range of a performance spark plug is too cold, a minimum temperature that burns carbon from the plug tip is not met.
  • Carbon deposits can accumulate (carbon fouling). A carbon-fouled plug will not jump a spark from the tip to the ground strap, but rather to the plug’s metal case, causing misfire or a dead cylinder spark.

With the new and slightly colder E3 spark plugs, we immediately turned off the car after the second pass. We swapped the number-three-cylinder plug with another and brought this one (left) back for comparison photos to an unused plug (right). Our heat line on the ground strap (arrow) is much closer to the base, and the electrode insulator is colored evenly down to where it meets at the spark plug shell.

A spark plug also requires a minimum amount of heat to keep the center electrode at an optimum temperature. This temperature keeps carbon from collecting on the plug, thus preventing detonation or pre-ignition. Use a hotter heat range for rich air/fuel mixtures. Allowing a plug to operate at temperatures under approximately 1,000 degrees will allow carbon to collect, which will foul your plugs.

Edelbrock has adapted a fresh approach by engineering the same popular performance spark plug into all of their cylinder heads. From Chevy, to Mopar, and even their 1950's-era Ford flathead, they all use the commonplace 14mm spark plugs with a 3/4-inch-reach and flat gasket seats.

Keeping An Eye On Things

Taking a visual reading of a spark plug is the most direct way to learn what your engine wants in relationship to a spark plug heat range and air/fuel mixture. We spoke to Don Ward from E3 Spark Plugs about the art of examining the combustion markings on a plug for tuning and plug choices.

Examining the ceramic insulator is the first step in plug reading. With a lighted spark plug magnifier, you can look deep inside the plug casing for signs of incorrect heat range. If the white insulator shows signs of “sparkles” or has a glossy appearance as compared to a new spark plug, the plug being used is too hot. The altered insulator finish indicates that the plug has been super-heated, and the aluminum/ceramic composition has been altered. If you see dark deposits starting to form on the insulator, the plug is too cold.

“Another point to examine is the spark plug’s side electrode or ground strap, [ground straps in the case of E3 racing spark plugs],” Don describes. “On the ground strap, there should be a heat line. Examining this heat line will indicate a rich or lean mixture as related to the heat generated in the head chamber.”

For a naturally aspirated engine, you want that heat line to be evident from the bend, almost to the point where the ground wire is welded to the shell. For a boosted engine, you want the heat to come midway up the ground strap in the bend area.

"Indexing" your spark plugs is the practice of making sure your ground strap is opposite of the dome of high compression pistons. The process of clocking the electrode gap towards the piston helps ignition under high pressure. Once we find a plug/cylinder index match, we log it with this Allstar Performance tool. You will never need to index new plugs in your heads again. You simply thread new plugs into the tool and use your log book to match the plug with a cylinder.

If ignition within the combustion chamber is too cold, it is indicated by a heat line that is closer to the tip than recommended. This indicates the air/fuel mixture is possibly too rich, and/or there’s not enough timing advance in the ignition. A heat line too close to the base of the ground strap signifies too much chamber temperature. This lean condition needs to be rectified with more fuel and/or less timing advance.

The final inspection point is the electrode tip protruding from the ceramic insulator. Look for any color changes on the tip as compared to a new version of the same spark plug. Any “blue” colorizing of the electrode indicates the plug itself has been overheated. If the very edge of the electrode is rounded as compared to new, again from overheating, the ability to create a spark jump between the electrode and ground strap requires more energy from the ignition source.

A spark plug magnifier is an inexpensive tool to have at the track. They are lighted and have a typical 10x magnification that allows you to look closely for the slightest bit of coloring on the insulator and to easily inspect heat patterns.

Spark plug selection has become easier in many cases. “All Edelbrock aluminum heads feature use of a 14mm spark plug with a 3/4-inch reach and a flat-gasket seat. Common spark plugs are E3 spark plug number E3.48 or Champion’s RC12YC. This is our starting point for engines under 9.0:1 compression ratios. The assortment of colder or hotter spark plugs for this plug designation are numerous,” says  Smitty Smith of Edelbrock.

In spark plug-eze, many manufacturers’ part numbers incorporate the heat range within the spark plug. But, heat range specifications are not universal, as varied brands have their own numbering system for different heat-range applications. For example, Champion spark plugs indicate the heat range in the middle of the plug number. The Edelbrock-recommended RC12YC, has a heat range of 12. Champion plugs get hotter, as the number gets higher, and colder as the number gets lower. Most popular spark plug brands used in performance and motorsports applications offer a multitude of heat range choices.

“In a boost application, like with our Edelbrock E-Force Supercharger, we suggest new spark plugs in an OEM-stock heat range for our Stage 1 system,” Smitty says. “When we sell a Stage 2 or Stage 3 Professional Tuner System, they generate more boost, which increases cylinder head pressure. For those, we recommend spark plugs that are one or two heat ranges colder.”


Too much, or not enough, heat in your spark plugs can hurt your engine’s performance as much as any other performance fundamental such as fuel, or ignition timing.

With Edelbrock pushing out 300 different part numbers for aluminum cylinder heads, we asked Smitty about spark plug heat-range characteristics between steel and aluminum heads. “It is pretty much an across-the-board thing between cylinder head material,” he said.

Don Ward agrees with Smitty for most spark plug lines, but also recommends – specifically with E3 spark plugs – that you go a heat range cooler than “normal” when using a cast-iron head. The philosophy is, you can go a heat range cooler due to the slightly slower heat dissipation as compared to aluminum heads.

We asked Don if the advent of O2 and EGT sensors have spoiled racers when it comes to analyzing spark plugs. “Absolutely not,” he tells us. “I was at a drag race last weekend in Charlotte, North Carolina, and we were reading plugs for customers all weekend. Oxygen sensors are great. They will help you tune an engine, but they are made for unleaded fuels. Most race fuels today still contain lead. The lead will become a conductor on the sensor, and it will read that the engine is going lean. You really need to look at the spark plug and see if it says the same thing the sensor does.”

So the next time your buddy tells you that you need a colder spark plug, have him explain why. If he can’t, tell him how you know you have the correct ignition starters in your engine.

Article Sources

About the author

Todd Silvey

Todd has been a hardcore drag racing journalist since 1987. He is constantly on both sides of the guardwall from racing photography and editorship to drag racing cars of every shape and class.
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