Most spark plugs aren’t made to last forever. Even the Iridium tipped longest-lasting spark plugs wear out over time (or more precisely, miles traveled). The good news is that you can swap out an old and worn set of spark plugs for a new set about as easily as you change a tire. Worn or damaged plugs are an easy problem to fix.
But what kind of spark plugs should you use when it's time to replace the originals? Should you go for OEM-quality plugs or a much less expensive set of plugs from the local automotive store? A spark plug is a spark plug, right? No. After-market plugs are cheaper for a few reasons, but they're not good reasons. It is absolutely worth it to get OEM spark plugs - here's why.
The Trouble With Cheap Spark Plugs
Most aftermarket spark plugs are made to be as inexpensive as possible. Aftermarket part companies assume that you don't know why you'd want one set of spark plugs over another, so they go with the lowest cost components and hope they can win your business by being the cheapest option. But here's the thing:
- Material Quality Matters: Aftermarket spark plugs usually cost less than OEM plugs, but that's because the materials in these plugs are often lower grade than OEM plugs. For example, most OEM plugs are tipped with a high purity platinum-iridium alloy and have nickel-coated threads (which resist corrosion and ensure easy removal in 100k miles). Many after-market plugs either use a lower (less pure) grade of iridium-platinum alloy, or they use only platinum, which means the plugs don't last as long. They often omit the nickel plated threads to save money. The center electrodes aren't always consistently pure, decreasing performance. The ceramic insulators are often more fragile than OEM ceramics. Etc.
- Precise resistance is essential: Every spark plug has what is called an "interference suppression resistor," which ensures that the electrical "noise" created by the rapid-firing spark plug doesn't produce signals that interfere with the proper function of the rest of the electrical system. If these interference suppression resistors are sub-standard, they will cause everything from audible static in the stereo system to functionality problems with complex systems like navigation, ABS, etc.
- Plug Fitment and Gap Aren't Always Consistent: If your aftermarket plugs are even just a tiny amount longer or shorter than the OEM plugs - or if the gap is off by a few hundredths - they can reduce power and fuel economy. If they're off more than a tiny amount, they could cause engine damage.
- Electrode Design and Shape is a Big Deal: Most OEM spark plugs have carefully designed electrodes which are ground into a very specific shape. This shaping is costly, as it adds complexity and time to the manufacturing process. Yet this shaping (also known as 'profiling') is important to maximizing the performance of your vehicle, as the shapes determine the efficiency of the spark and the combustion event. Auto manufacturers go to great lengths to pair the correct spark plug electrode to the engine. If you change the electrodes from a sophisticated 'pin to pin' design to a cheap blade and point design, you'll lose horsepower and fuel economy.
So why do OEMs use the best quality plugs? Because they're concerned about fuel economy and performance. They need to maximize fuel economy to meet federal regulations...even a small improvement in gas mileage can save an automaker millions in CAFE fines. Manufacturers also need to maximize the performance of their vehicles to sell you - the customer - their cars. They understand than an extra $5 per spark plug is money well spent when it's time to convince you to buy a brand new car. Finally, they spend money on plugs because they have to warranty the plugs, and it's cheaper to do it right the first time.
The Hidden Cost of Cheap Spark Plugs Is At The Gas Pump
It's easy to say "I don't care if my plugs last another 100k miles" or "I just want to save money," especially if the vehicle you're buying plugs for doesn't have another 100k miles of use in it. However, it doesn't take long for a cheap set of spark plugs to cost you.
If you buy an inferior set of spark plugs that decrease your engine's fuel economy just a couple of percentage points, you'll end up increasing your fuel costs the same amount. If you spend $40 a week on gasoline, for example, and your plugs decrease gas mileage 2%, you'll add about a dollar a week to your fuel bill (2% of $40 is 80 cents). In less than a year, you'll spend an extra $40 on gasoline because of your "cheap" spark plugs. Over the course of 5 years, a cheap set of spark plugs will be very expensive.
So even if you're trying to save money on spark plugs and don't care about longevity, OEM plugs are still the way to go. They'll ensure maximum performance and fuel economy, and that will save you on one of your largest vehicle expenses (fuel).