A Quick Guide to Spark Plug Selection

We’ve all done it at some point in our lives: made the assumption that the hottest burning, most expensive spark plug is the best choice for our engine. In truth, spark plug selection really depends on the characteristics and specifications of your engine. Not running the correct plug type or temperature will result in poor performance, short lifespan of the plugs, and even engine failure.

Temperature

If a little is good, then more is better… right? Well, like most aspects of performance, this isn’t always the case. When it comes to spark plugs, it’s easy to think that a hotter spark is always better, since the primary goal of a plug is to ignite the fuel at just the right moment, creating the combustion needed to generate the most power. But, to achieve the best ignition, the temperature of the plug must meet the demands of the application.

“If the tip of the spark plug is too hot, it can cause pre-ignition. This can lead to detonation/knocking, and damage may occur,” explains The Green Spark Plug Co., the only company in the world that specializes in spark plugs and accessories for veteran, vintage, and classic engines. “If it is too cold, electrically-conductive deposits may form on the insulator causing a loss of spark energy or the actual shorting-out of the spark current.”

Fuel and air within the combustion chamber is susceptible to pre-detonation in high-compression engines. As the fuel mixture compresses, its temperature will increase. This means you want less heat introduced to burn all of the fuel in the chamber. You can see why a too-hot plug tip in this scenario can easily cause pre-detonation.

Conversely, if compression is low, the spark plug will need to provide higher temperatures to ignite the entire fuel and air mixture. If it doesn’t, then the unburnt fuel and air will form deposits on the plugs and throughout the engine. These deposits love to feed on the performance and economy traits of an engine.

Plug Type

So, what impacts plug temperatures? The first and most important thing to keep in mind when selecting plugs for an engine is the material of the electrode. There are three materials commonly used in spark plugs: copper, platinum, and iridium.

• Copper Plugs

  These plugs feature a copper core electrode covered in a nickel-alloy coating. Because copper plugs are the coolest-running option, they are best for performance applications with higher-compression. However, they also have the shortest lifespan of about 20,000 miles.

• Platinum Plugs

  Platinum plugs burn at a high temperature. This helps prevent the risk of deposits forming, making them highly desirable for lower combustion engines. As platinum plugs are harder than copper, their lifespan can easily reach 100,000 miles.

• Iridium Plugs

  Iridium plugs service the same engines as platinum plugs, as they are also a hot plug. However, because iridium is a harder, more resilient metal than platinum, these plugs can have a lifespan as long as 125,000 miles. It is not uncommon to find many late-model vehicles sporting these technologically-advanced plugs straight from the factory, as they deliver great acceleration and fuel efficiency.

Fuel and Boost

Compression plays a big role in spark plug selection, but so does boost. For the most part, when an application is boosted, the same principles apply as they do to high compression engines—meaning copper plugs are choice.

It’s also important to mention that as you install plugs in any vehicle, you will need to “gap” them. Spark plug gap refers to where the spark discharge is designed to take place. Proper gaps are important to the performance of the plug, and every manufacturer provides a specific gap to use.

Check out the video below for some guidance on how spark plug gap can affect the performance of your boosted engine, as well as what adjustments you should make.

Similarly, different fuel types affect plug selection since they each have different cooling capabilities. For example, alcohol will cool components more than gasoline will.

“It appears that the increased use of oxygenated fuels (up to 10%) does not directly affect the core nose temperature of the spark plug,” explains The Green Spark Plug Co. “However, the addition of the oxygenates will tend to lean out the air/fuel mixture and, since these alcohols require more energy to vaporize, may tend to actually cool down the spark plug and combustion chamber.”

While copper plugs are pretty much always the best option for performance applications, all of the factors above will still play a role in proper spark plug selection.

What if I’m running an ignition box?

Ignition boxes are highly desirable for classic car owners as they offer up multiple sparks as opposed to a single spark per ignition. As you likely guessed, this also has an effect on plug selection.

MSD was the first company to develop and offer the multiple sparking, capacitive discharge ignition for engines. These ignitions remain some of the most popular in the aftermarket, thanks to their proven performance and reliability. It might surprise you to learn that the company specifically warns against the use of “specialty” or “hype” plugs when using their ignitions. Instead, MSD recommends that users “stay with a normal copper plug.”

“The reason for this is that [specialty] plugs are hotter-burning,” explains an MSD technician. “Combined with the hotter ignition of the MSD CD, there is not enough time for the plug to cool off between firings. Eventually, the plug could start to glow like a ‘glow plug’ in a diesel engine. As your air/fuel mixture is compressed in the cylinder and getting ready to fire, the heat from the plug could cause the air/fuel to ignite prematurely causing detonation problems.”

“That’s why, we recommend using a plain copper plug,” he continues. “1-2 steps colder than factory recommendation and increase the gap on the plug in .005 increments until you have a nice smooth-running engine and there is a nice tan/caramel color on the porcelain. This could end up to be in the .050-.060 range.”

Plug Wires and Distributors

Having the right spark plugs is important to preserving or restoring an engine’s performance. But it’s not the only part of the ignition system worthy of your attention. A good set of plug wires, a distributor, and coil are just as important.

The coil or coil packs are the source of energy, so there’s no mistaking the value of having an excellent coil to set things alight. If you’re dealing with a carbureted application (carb swaps not included), the distributor and plug wires are responsible for transferring that power to its final destination.

Plug Wires

Keep in mind: once you start upping engine performance, your factory plug wires are likely to offer far too much resistance. Essentially, the more power you make, the more critical a complete burn becomes. You want the most energy possible making it to the plugs, so you should opt for plug wires with low internal resistance.

Don’t worry – you don’t need to spend an arm and a leg to get some decent plug wires. SpeedFX is in the game to provide vehicle owners with high-quality parts at competitive prices.

“The Speed FX wires have more ohms per foot over the stock wires and are priced-to-sell in black, red, and blue in 8.5 millimeters,” says Frank Hartman of A&A Auto. “Plus, they come in 180-degree and 90 degree-ends.”

SpeedFX plug wires are a universal fit for V8 engines and are rated at 500 ohms per foot. Not being preassembled makes them an ideal choice for any application, as you can cut them to the appropriate length and create a clean look under the hood.

That entire system is worthless without a brain, though, which is why you need a good distributor. (Unless you have a program to handle the thinking for you.) There are many things to discuss when it comes to distributor tuning, but we’re going to keep things short and sweet.

Distributors

Your distributor is responsible for determining which plug will fire and when. The where is important, but the when really determines how an engine runs. All the parts you throw at an engine aren’t worth a nickel if the timing isn’t right. You know this, because after you installed a new distributor, low-end RPM was excellent, but the thing falls on its face as engine speeds climb. So, what gives?

Well, most distributors work with some advance system. Some feature a mechanical advance, others a vacuum advance, and some feature a combination of both.

In any case, the advance is in place to overcome engine speed. When you install a distributor, the factory spec will typically require that ignition occurs somewhere around 10 degrees before TDC. The reason you set it up with an advance is to make sure there’s enough time for the spark to reach the chamber. This way, it can produce energy at the perfect moment. Think of it like putting a lead on a moving target. As the engine speeds up, the spark needs to be sent sooner. That’s why your distributor needs some form of an advance.

A change in timing along the lines of 10 degrees at idle and 34 degrees of total advance at 3,000 RPM is typical. However, there is no universal formula. Your driving habits, the build’s specs, and many other factors come into play. The good news is, researching comparable builds will get you in the ballpark. Then you need to decide if you want to work with the factory distributor or an alternative setup.

HEI Distributors

Many folks take the step to install an HEI distributor because they make a much cleaner look under the hood – and they are virtually maintenance-free. While you don’t need an HEI to get results, another significant advantage of this setup is that you can easily make adjustments to the timing advance. The counterweights, springs, and bushings are directly under the rotor, which is far easier to adjust than many other setups.

If you’re after an HEI, consider SpeedFX yet again. As you can expect, these are some of the best-bang-for-your-buck ignition components on the market, as they are subjected to the same standard of quality as all other products in the SpeedFX line. They are also backed with excellent warranties and customer service.

SpeedFX HEI distributors are readily available for plenty of applications, including LA Mopar engines. They rely on robust materials for construction, and spring and bushing kits are readily available. “At A&A, we have sold hundreds with very little comebacks,” says Hartman. “Plus, they have a generous 90-day warranty.”

Dialing in the timing curve will give you more performance gains than any other ignition modification you can throw at an engine. Even if the timing is already close, bringing it the rest of the way can make a significant difference in the way your ride behaves.

Like any other modification or repair, spark plug selection requires some informed research. Consult your vehicle’s service manual for the recommended plugs and be sure to pick the brain of the parts store employee, as they can provide more useful insight than Google.

This post was updated on March 11, 2021.