Since the dawn of the automobile, spark plugs have been an integral part of gasoline engines because they conduct the electrical energy from a vehicle’s ignition system needed to finalize the combustion process. After the gas/air mixture has been fully compressed inside the cylinder head, spark plugs serve a miniature bolt of lightning to create an explosion which pushes a piston downward.
Spark plugs aren’t needed on diesel engines, which run at higher compression ratios sufficient to create combustion without the help of a spark. While 99% of engines feature one spark plug per cylinder, some high-performance engines such as modern Chrysler Hemi V8s have used two plugs per cylinder since their introduction.
Spark plugs are designed with an inner central electrode that’s covered by a visibly white porcelain insulating shell. That central electrode is connected by a heavily insulated wire to the output terminal of the vehicle’s ignition coil or magneto. When more expensive spark plugs advertise a "fine wire" core, they are referring to an inner central wire with added metals to conduct electricity better and/or increase longevity.
A threaded shell at the bottom of the spark plug allows it to be screwed into a vehicle’s cylinder head, and the very bottom tip of the spark plug actually extends inside the engine. The metal used to coat that bottom tip is how different types of spark plugs are known and defined.
Types of spark plugs available today are known as "copper", "platinum", "double platinum", and "iridium".
Copper Spark Plugs
Sometimes referred to as "standard" or "normal", copper-tipped spark plugs are lower in cost and have the shortest lifespan due to copper’s naturally tendency to erode away over time. However, their value lies in the ability of copper to conduct electricity better than any other type of material used on spark plug tips. As a result, copper-tipped plugs run cooler, and deliver more power in performance driving applications without reaching overheated temperatures which reduce power and shorten spark plug life.
Because copper produces the best spark under the adverse conditions generated by turbochargers or higher compression ratios, manufacturers of such engines typically use copper plugs as original equipment. Depending on a number of external factors, copper spark plugs typically last 20,000 to 40,000 miles. Additionally, natural gas engines tend to run best on copper plugs.
Platinum Spark Plugs
Platinum-tipped spark plugs are more expensive because platinum is a more rare element in nature than copper is. Because platinum is a less conductive material, it doesn’t deliver the effective power transfer of copper and may become overheated easier under performance driving conditions. Where platinum spark plugs shine is their great longevity under normal driving conditions. Platinum does not erode the way copper does, so the gap at the tip of the spark plug does not widen as the metal wears away – a factor that causes a drop in power, lower mileage, and intermittent misfires on startup that can trigger check engine lights on modern vehicles.
The lifespan of a set of platinum spark plugs is typically double that of copper ones, although some vehicle manufacturers using them specify long plug change intervals of up to 100,000 miles thanks to precise computer control of air fuel mixture at all running temperatures.
Tip: Due to a natural chemical reaction, spark plugs that will stay in your car for a long duration have a tendency to fuse themselves to cylinder heads made out of aluminum. If you’re installing a set, coat each spark plug with anti-sieze lubricant before installing. Because most spark plugs feature a two-piece design, the removal process can leave the bottom part stuck in the aluminum cylinder head easier. Look for one-piece spark plugs to avoid this headache.
Double Platinum Spark Plugs
Spark plugs simply known as "platinum" feature a platinum tip only, compared to "double platinum" spark plugs which feature platinum on the tips and other areas such as the ground electrodes. Some double platinum plugs may advertise a "fine wire" center core featuring one or more platinum discs inside of it. Although these represent a step up in price, double platinum spark plugs typically yield slightly higher performance with the traditional long life platinum plugs are known for.
Iridium Spark Plugs
Iridium-tipped spark plugs offer better power, more complete combustion that leads to smooth-running engines, and a longer lifespan than copper plugs. Depending on application, iridium plugs can even approach the lifespan of some platinum plugs. Iridium plugs are the highest in cost, and typically feature "fine wire" centers designed to conduct electrical energy better.
Tip: Iridium spark plugs typically do not require gapping adjustment because the iridium material itself can become easily damaged during the process.
Gapping Spark Plugs
"Gapping" refers to adjusting the distance between the curved conducting electrode and the plug tip where the electricity eminates from. Gapping is done by bending the electrode itself via a special "gap tool", and achieving proper gap distance is important to maximizing power, fuel consumption, smoothness, and overall life span your spark plugs will deliver.
Too narrow a gap can yield a spark that’s too small and weak to fully ignite the gas/air mixture while too wide a gap can cause misfires during normal driving, especially at higher revs. Misfires may be noticeable or they may be so intermittent that they’re noticeable only to the car’s engine computer – resulting in a pesky check engine light that won’t stop coming on and off. If the gap’s incorrect in either direction, misfires may not occur but power and economy can still suffer. (Helpful youtube video showing how to gap a spark plug https://www.youtube.com/watch?v=lk70oyUEftY)
Spark Plug Heat Range
No matter what type of spark plug you choose, its heat rating is important. Heat range is the measure of how fast the spark plug tip dissipates combustion heat. While spark voltage output and operating temperature of the spark plug itself are not connected, plugs with a higher heat rating better serve the needs of driving styles where more engine torque is needed – such as performance driving, drag racing, or towing.
Too "hot" a spark plug can cause premature ignition of the air/fuel mixture (known as engine-damaging knock), and too "cold" a spark plug can cause incomplete combustion and formation of carbon buildup on the tip. Either will cause the engine to run disappointingly and wear out spark plugs before their time.
Tip: If you’ve modified your car for more power by upping the engine compression ratio, increasing a turbocharger’s boost, or even installing an add-on turbocharger or supercharger, you’ll benefit from hotter temperature spark plugs. Otherwise, it’s best to use factory specifications as a guideline for normal driving applications.
(helpful youtube video detailing more about spark plug heat ranges https://www.youtube.com/watch?v=ST_qXTwlDI4)