Based on the number of ads that you see for them in various car magazines, I think the three most common modifications to any car are air intakes, exhaust systems, and spark plugs/wires. This makes sense because they are some of the easiest changes to make, and you’re less likely to break your car by attempting any of them.
As mentioned in our previous article on horsepower, air tends to be the limiting factor for engines. Stock fuel systems often have plenty of extra capacity that can be added if airflow is increased, which leads to the popularity of intakes and exhausts. So why are spark plugs in the mix? Unless you are driving a diesel, spark plugs are a fairly important, but do you need to upgrade them? Will it give any extra power? Some of the specialty plugs available are nearly four times as much as traditional copper plugs. What gives? More importantly, are they worth the extra cost?
To answer this, we first need to understand what a spark plug is and how it works. A spark plug, contrary to the name, does not actually cause the spark. A “spark” in an automotive engine is actually an electrical arc that jumps across a gap, much like a bolt of lightning. The plug plays no part in generating the energy needed to create this arc; it simply provides the air gap and the electrodes. The energy is created by the ignition coil or coil pack, which builds voltage until the spark occurs between the two electrodes.
If you zoom in on a spark plug’s tip, as we have done in the image above, you can see that there is a central electrode that sticks up and a side electrode that curves around it. The gap between the two of these determines how hot the spark will be because it affects the amount of electrical potential that is required to overcome the resistance of the air. This spark ignites the air-fuel mixture in the cylinder, and the lifeblood of the car is created.
Due to the corona effect, the air around the central electrode becomes ionized and creates a region of plasma between the electrodes. This effect is amplified around sharp points and edges, such as the corner of the flat tip of the central electrode. This concentrates electrical discharge to the corner, which means the corner will start to wear with time. Manufacturers have started producing plugs with multiple side electrodes to help extend the life of the center electrode by encouraging sparking from multiple points. Wearing of the central electrode reduces the strength of the ionization and increases the amount of voltage required to create the spark. For this reason, plugs need to be replaced to ensure that a spark is reliably created.
Early spark plugs were made with nickel electrodes and cores. As engines became more powerful and reliable, the conditions in the combustion chamber became more stressful. Nickel was found to be unsuitable as a material for the core of the spark plug because it failed to conduct heat away from the central electrode. A new design was introduced that paired a copper core with a nickel-chromium coating on the central electrode. This combined the thermal conductivity of the copper with the durability of the nickel-chromium.
Now that we have a basic understanding of how spark plugs work, let’s take a look at some of the high performance options on the market and determine if they are worth the cost.
Many spark plug manufacturers are producing plugs that use “noble” metals for their electrode coatings rather than nickel-chromium, primarily platinum and iridium. Some even use the noble metals for the core of the plug. The major benefits that they claim are increased durability, consistent performance, high-melting point, and in some cases, claims of higher performance. Do any of these hold water?
Increased durability and high melting point
Nickel-chromium alloys have a melting point of 2,618 degrees Fahrenheit compared to 3,220 for platinum and 4,440 for iridium, so it is likely that the durability of noble metal plugs would be higher. Furthermore, noble metals are very non-reactive, so they are less prone to oxidization than nickel-chromium. Oxidation serves to increase resistance, making the plug less effective.
Consistent performance over a long life
Given the durability of these metals, the gap between the two electrodes should remain fairly constant, which will make the spark consistent. This means that the engine will continue to perform as it is engineered for a longer life. Many nickel-chromium plugs need to be replaced every 20,000 to 30,000 miles, depending on wear and the specific plug, but noble metal plugs can last for more than 100,000 miles.
This would be more accurately described as “less performance loss.” Spark plugs do not increase the performance of an engine — even spark plug manufacturers are fairly forthright about that. Noble metal plugs can perform consistently and well, which helps to maintain the engineered performance of the engine. They won’t make the engine perform better than it did when it was new, but they will help it stay at that level longer.
Are noble metal plugs worth it? That’s up to you. If you want long-lasting plugs that will reduce the work you need to do replacing plugs and don’t mind paying extra for that luxury, yes. If you are designing an extremely high performance car where the plugs will be subjected to extreme conditions, they are probably a good idea. But if you are hoping to improve the performance of your car, your money will be better spent elsewhere.
Bonus: In our research for this article, we came across a link to an issue of Popular Science from 1942 in which spark plug technology was explained with illustrations from Walt Disney. Not only is this fascinating in itself, but the whole magazine is fun to read to see how much the world has changed.
Like classics? It’s always Throwback Thursday somewhere.