Advance Essentials: Spark Plugs Basics


It’s time to tune up your car, so you head to the Advance Auto Parts website. Once there, you enter the year, make, model and engine size of your vehicle. You want to buy the best spark plugs so you click on “spark plugs” and up come more choices than you expected. In most cases, you’ll see at least four different types of spark plugs, from at least three different manufacturers. Will it be platinum spark plugs? Copper spark plugs? Iridium spark plugs? How do you know which are the best spark plugs to buy? Simple. Just pick the spark plug based on the type of ignition system in your vehicle. Here, we’ll further explain the differences between platinum spark plugs, copper spark plugs, iridium spark plugs and other spark plug types. Plus we’ll give you some solid advice on which are the best spark plugs for your vehicle.

Let’s start with the two most important spark plug facts. First, sparks like to jump from the sharpest point on the center electrode to the sharpest point on the side electrode. So you want a plug that retains its sharp edge for the most miles. Precious metals like platinum and iridium are harder and have lower melting temperatures than the nickel alloy electrodes found in traditional copper spark plugs. Second, the smaller the diameter of the center electrode, the lower the voltage needed to start the spark. Keep those shopping tips in mind as you review the best spark plugs for your vehicle.  

Copper spark plugs—where to use them

Copper spark plugs have a solid copper core, but the business end of the center electrode is actually a 2.5mm-diameter nickel alloy. That’s the largest diameter electrode of all the spark plug types. Remember, the smaller the diameter, the less voltage required to initiate the spark. Nickel alloy is also softer than either platinum or iridium, so the sharp firing edge you get right out of the box tends to wear out quickly. Despite those shortcomings, copper spark plugs are still a good choice for certain applications. Copper spark plugs are best for older (pre-‘80s) vehicles with low voltage distributor-based ignition systems. But don’t use copper spark plugs in high-energy distributor-less ignition systems (DIS) or coil-on-plug (COP) ignition systems. They’ll wear out too quickly.
There’s one exception to that advice. Some late-model high-performance engines were designed specifically for copper spark plugs. In those cases, copper spark plugs are considered to be high performance spark plugs. If your owner’s manual calls for copper spark plugs, don’t upgrade to platinum spark plugs or iridium spark plugs. Single platinum spark plugs—a step up from copper spark plugs

A single platinum spark plug is basically styled after a copper spark plug with a platinum disc welded to the tip of the center electrode (see photo). Since platinum is harder than nickel alloy, it holds its sharp edge for as long as 100,000 miles. Platinum spark plugs also run a bit hotter, preventing spark plug deposit build-up and fouling.
Platinum spark plugs are usually the best spark plugs for newer vehicles with electronic distributor-based ignition systems and some (DIS) systems. If your owner’s manual recommends platinum spark plugs, don’t downgrade to copper spark plugs to save money. However, you can upgrade to either double platinum spark plugs or iridium spark plugs.Double platinum spark plugs—twice as good?

Double platinum spark plugs were designed for “waste spark” DIS ignition systems. In a waste spark system, the spark jumps from the center electrode to the side electrode for the cylinder that’s on the compression stroke. To return the electrical pulse back to the ignition coil pack, the spark jumps backwards (side-to-center) on the partner cylinder. Since the partner cylinder is on its exhaust stroke, nothing ignites and the spark is “wasted.” You can’t use single platinum spark plugs or traditional copper spark plugs in these systems because the side electrodes aren’t designed to handle the reverse spark. But double platinum spark plugs, with a platinum disc welded to their side electrode, work exceptionally well. Both the center and side platinum discs remain sharp, allowing sparks to fly easily in both directions without causing rapid electrode wear.
If your owner’s manual recommends double platinum spark plugs, then those are the best spark plugs to use. Never downgrade to single platinum spark plugs or copper spark plugs. However, you can upgrade to an iridium/platinum combination plug (an iridium center electrode with a platinum-tipped side electrode).

Double platinum spark plugs

On the compression stroke
The spark shoots from the center electrode to the side electrode


Double platinum spark plugs

On the partner cylinder exhaust stroke
The spark shoots from the side electrode to the center electrode — why double platinum spark plugs are needed in a waste spark DIS system



Iridium spark plugs—the best kind around

You guessed it: iridium is harder than platinum. In most cases, iridium spark plugs last about 25 percent longer than comparable platinum spark plugs. Because iridium is costly, iridium spark plug manufacturers reduce the diameter of the center electrode to as little as .4mm. In addition to saving money, the “fine wire” center electrode of iridium spark plugs increases firing efficiency.
Many carmakers require iridium spark plugs or iridium/platinum combination spark plugs for (COP) ignition systems. If your owner’s manual specifies iridium spark plugs, don’t downgrade to platinum spark plugs, or double platinum spark plugs, or even copper spark plugs. They won’t perform as well as the iridium spark plugs in this case.Spark plugs with “U,” “split” and “V” groove side electrodes

As the spark ignites the air/fuel mixture, the colder side electrode tends to “quench” the flame. To combat quenching, some spark plug manufacturers cut a “U” or “V” shaped channel into the “spark receiving” surface of the side electrode. The larger channeled area reduces quenching and allows the flame to grow more quickly. Other manufacturers split the end of the side electrode to reduce the flame’s contact with the side electrode and allow the flame to shoot straight down into the cylinder. Are these channeled electrodes the best spark plugs you can buy? There’s no industry-wide consensus on whether these designs work better, but if channeled spark plugs make sense to you, buy them. They’ll perform at least as well as a non-channeled plug, if not better. 
Please note: Always consult your owner’s manual first. Follow the manufacturer’s recommendations to ensure warranties are not voided. Always replace the spark plugs before the recommended service interval. Waiting until the full service cycle could cause them to seize to the cylinder head, making removal difficult. In addition, the wear that occurs during the last 20% of a spark plug’s life will cost you far more in reduced gas mileage than the cost of a new set of plugs.

Recommended spark plug tools and accessories If you’re wondering how to replace copper spark plugs, iridium spark plugs, or any other kinds of spark plugs, check out the following recommended tools. They are what the pros use to remove spark plugs, gap new spark plugs, and insert new spark plugs properly.

Spark plug wobble sockets
If a standard spark plug socket isn’t enough to remove old iridium spark plugs or other kinds of spark plugs, use this special wobble socket. Its built-in wobble takes up less room so it can fit it into tight spaces. In fact, it takes less room than an ordinary spark plug socket with an add-on wobble extension.


Spark plug socket with swivel locking extension
The GearWrench spark plug socket with swivel locking extension provides maximum turning angle and the most clearance for tight spaces. The socket and extension are permanently locked together so it won’t disconnect from the socket as you remove the plug.


Flexible handle ratchet
You need plenty of angles when you’re trying to remove spark plugs from tight spaces. Combine this flex-head ratchet with a wobble spark plug socket or a swivel socket with locking extension to remove spark plugs in less time with less sweat.


Spark plug boot puller pliers
Spark plug boots tend to “weld” themselves to the plug. Pulling directly on the wire can damage the connection, separating it from the boot. Use spark plug boot puller pliers to grab the boot and break it loose. Then just pull the boot off the wire with the pliers.


Spark plug gap gauge
Always gap spark plugs according to manufacturer’s specifications before installing them. To use this gauge, find the gap specification and insert the gauge between the center electrode and the side electrode. Then slide the gauge towards the factory specification marked on the gauge. That will bend the side electrode outward, forming the proper gap.


Torque wrench
Spark plugs must be torqued properly during installation or you risk major engine damage. Over-tightening can destroy the threads in the aluminum cylinder head. If the spark plug is too loose, it can blow right out of the head, taking the threads with it. Instead, invest in a low-range ft-lb. torque wrench and retrieve the proper tightening specs from the plug manufacturer’s web site. 


Wire loom spacers
Spark plug wires are spaced in a certain order to prevent arcing-to-ground and cross firing. Wire loom spacers help prevent misfires that can kill gas mileage and possibly destroy the ignition coil, ignition module, or even the powertrain control module.


Apply just a drop of anti-seize to the spark plug threads (don’t get any on the electrodes or the porcelain). Then reduce the spark plug torque by about 10%. The anti-seize prevents the plugs from permanently bonding to the cylinder head threads.


Spark plug boot grease
Squirt a liberal amount of spark plug boot grease into the boot then install the boot onto the spark plug. This dielectric grease prevents high voltage “flashovers” (voltage leaking out of the boot to ground) and makes future boot removal easier.


Spark plug wires
Over time, the spark plug wire’s core conductor and the insulation surrounding it can break down. That leads to arcing and misfires that’ll destroy your gas mileage and possibly damage your ignition module or power train control module. Replace the ignition wires with a premium set to maintain proper ignition system operation.


Coil on plug boots
Some COP style ignition coils have replaceable boots. Extremely high firing voltages can damage the boot over time, causing misfires. If the boot on your COP coil is removable, replace it with a new boot and connecting spring to keep the ignition system in perfect running condition. 

Distributor cap
After years of distributing high energy sparks to the spark plug towers, the carbon tip at the base of the center tower can wear down and interrupt the electrical flow from the ignition coil to the individual spark plug towers. Centrifugal force then expels the ground-up carbon around the inside of the cap, causing “carbon tracking.” Carbon tracking causes spark plug misfires. Replacing the distributor cap every time you replace spark plugs is an inexpensive way to keep your ignition system working at maximum efficiency.



Replace the distributor cap and rotor as a pair. Doing so ensures the carbon button at the base of the ignition coil tower will mate with the new spring surface on the rotor. Plus, the “distributing” edge on the new rotor will be free of erosion and will transfer the spark with the least amount of electrical resistance.