A spark plug exists to ignite the mixture of air and fuel in the combustion chamber.
How a spark plug works
Electricity comes from the ignition system, often by way of an individual coil sitting directly on the plug. The current flows along the center electrode to its tip and jumps the gap to the ground electrode; the resulting arc lights the mixture.
Why plugs foul
Age is not the only thing that damages a plug. Running with the wrong air-to-fuel balance and oil contamination are the other major causes. Too much air (a lean mixture) can make the plug run too hot and harm the electrodes. Too much fuel (a rich mixture) can leave unburnt fuel and carbon on the electrodes, which blocks the spark, and oil can foul the electrodes by coating them too. Either way, the plug is fouled.
Some causes of fouling are outside your control, but the type of plug can also have an influence.
Copper, platinum and iridium
Spark plugs have been around since 1860. In modern car engines, the type is defined mainly by the metal used on the center electrode and on the side electrode, since metals differ in toughness and in how well they conduct heat. The design of the ground electrode varies too, but it does not matter if you are replacing like with like: knowing the metal is enough.
Copper
After silver, copper is the best-conducting metal, but it is soft and melts at a low temperature. For that reason a copper plug has a tip of nickel alloy over a copper core. The alloy is nearly as conductive, wears well and is the cheapest material. Do not equate cost with quality. In high-performance use these plugs produce the coolest burn along with the greatest peak power, yet their durability is the poorest of the modern types.
On average they last 20,000 to 30,000 miles, and as few as 10,000 miles under hard use. Vintage cars with distributors are where copper plugs are most common. They also work in modern engines, but stick with the plug type the engine was designed to use. If you want to compare makes, NGK spark plugs are one place to look.
Platinum
A platinum plug swaps the nickel alloy for a platinum disc fused onto the copper. Platinum is far harder and carries heat better, so the plug lasts about three times as long as a copper one. It runs hotter, and that helps stop carbon building up on the electrodes. Plan on 60,000 to 100,000 miles.
Iridium
An iridium plug has a copper core with an iridium tip fused on. It is stronger and handles heat better than platinum, and you can recognize it by the fine-wire tip. Because iridium is so tough, makers can shrink the tip to less than 0.5 mm without hurting life. The smaller tip concentrates the spark and demands less of the ignition system. The drawback is simply money, since iridium is not cheap, although an iridium plug's minimum service life is equal to the maximum for platinum. Denso spark plugs are one range to look through.
Double-metal plugs and wasted-spark ignition
Double platinum and double iridium plugs carry the same metal on the ground electrode as well. Distributor and individual-coil ignitions do not need them. Wasted-spark ignition systems do, because in those systems the current has to travel from the center electrode out to the ground electrode, then return.
In a wasted-spark layout one coil serves a pair of cylinders, so there are two sparks per cycle, one on the compression stroke and one on the exhaust stroke. A common reason to fit double-metal plugs is an owner replacing a distributor ignition who does not want to pay for individual coils.
Heat range
A plug has to operate inside a particular temperature window. If it runs too cold, deposits of carbon, fuel and oil accumulate on the electrodes and interfere with the spark. If it runs too hot, the electrodes melt. Because combustion chamber temperatures differ so much from one engine to the next, no single design fits all of them, and heat range is how makers deal with that.
Heat range describes how quickly a plug sheds absorbed heat to the cylinder head. Makers control it by altering the ceramic insulator surrounding the core, along with the metal threads around it.
A hotter plug has a smaller amount of ceramic insulation around its core, so more heat is retained in the core and moves out more slowly. A colder plug is built with a lot more insulation, which means the cylinder has to run hotter on average for the plug to stay within its ideal range.
Modifying an engine for performance nearly always results in more intense combustion, so tuners tend to recommend a colder plug. If the engine is unmodified, you can ignore heat range when picking your next plug.
Gapping
Most new plugs do not need to be gapped, because modern plugs nearly always arrive set to a safe gap. The exceptions are vintage cars, race cars and engines that have been heavily modified. Putting a colder plug into an engine that has gained power falls into that last group: it will not necessarily need a new gap, but it happens much more often there.
If you want to depart from the plug the car came with, research the options and give serious thought to working with a tuner who is familiar with what you have changed. Engines can look similar on paper, but individual characteristics strongly shape what ends up working best.
What drives plug cost
Price follows engine type more than anything. A high-powered track-day car needs specialized plugs that have short service intervals, whereas a common engine takes a common plug. Specialization, production volume and metal all feed into what a plug costs, so find out what your car actually calls for.
Three examples: the Porsche 991.2 GT3 uses copper plugs in a unique Bosch design, serviced at 10,000-mile intervals; the Mercedes M272 V6 uses double platinum plugs that are very common; and a vintage car needs only basic copper, such as the Bosch WR5DC used in a Porsche 914. You can browse Bosch spark plugs if you want to look.
For the full figure, take the cost of one plug and multiply it by the cylinder count; an engine with two plugs per cylinder needs the plug count used instead. The age of the coil or coils deserves a thought too: they are every bit as vital to producing the spark and could be due for replacement.
Key takeaways
- Start with the electrode metal: copper, platinum or iridium.
- Cost is not the same as quality, and copper still has a place in engines designed for it.
- On an unmodified engine, heat range is not a concern.
- New plugs usually come pre-gapped; gap them yourself only for vintage cars, race cars and heavily modified engines.
- Before switching from the plug type the car came with, research the options and consider a tuner who is familiar with your changes.
- Genuine BMW spark plugs are another range you can browse.
General information only. Always check the specification for your own vehicle.