Replace spark plugs – TIPS

Tips for replacing spark plugs

I see lots of questions from DIYers asking about tips for replacing spark plugs. They know the plugs just unscrew and screw back in. But they wonder which is the best spark plug. And they see ads for e3 spark plugs, Bosch 4 spark plugs, iridium, double platinum, and copper. Let’s talk about those and other spark plug issues.

How spark plugs work

If we assume the ignition coil is working properly, generating the proper spark depends on thwo things: 1) The proper gap and 2) A sharp edge on the center and side electrodes. Here’s why.

The best spark always just from the sharpest edge of the center electrode and likes to land on the sharpest edge of the side electrode. As spark plugs wear, the center electrode loses metal and round off, making it much harder for the spark to make the leap. Plus, the same metal erosion affects the gap, enlarging it and making even harder for the spark to just the gap.

How long do spark plugs last?

There’s no single answer to that question. It depends on the type of spark plug used, the type of ignition system and whether the engine is equipped with a turbo. Some 4-cylinder engines with a turbo require new spark plugs in as little as 30M miles, while spark plugs in other engines with turbos can last upwards of 120K miles.

Which spark plugs are best?

It’s never a good idea to downgrade your spark plugs. If your engine calls for irridium, use that style plug. The same applies to double platinum. You won’t save money by downgrading because you’ll pay a higher cost in lost efficiency.

Using Copper spark plugs in place of platinum or irridum is a myth

Lots of performance guys recommend replacing platinum and iridium spark plugs with copper spark plugs. The theory is that copper is a better electrical conductor. That part is true as far as it goes. But copper electrodes erode quickly and therefore have a much shorter life. As the gap widens due to erosion, you get a cooler spark and more misfires. Generally speaking, copper spark plug have a maximum life of 25K miles. But for more than half of that life, they’re underperforming. Cooler ignition and misfires affect your gas mileage and the life of your catalytic converter. Unless you’re racing and plan to change your plugs after every race, it makes NO sense to use copper spark plugs in a modern engine, especially if it has a DIS ignition system.

A different spark plug won’t get you more MPG

Now let’s talk about advertising. You see lots of claims by spark plug manufacturers like Bosch, e3 spark plugs, PowerStar, etc. These companies claim they have found a new way to direct the flame kernel into the combustion chamber to increase horsepower and low end torque while increasing gas mileage, combustion efficiency, and lower emissions. If you believe all that, then I’m guessing you also believe in the tooth fairy, Santa Claus, and you faithfully buy lottery tickets. In other words, you must think the car makers engineers are stupid.

Keep in mind that increased efficiency, better performance, and higher MPG controls EVERY engineering decision in every car company these days. If an engineer can squeak ½ of 1% more efficiency simply by switching to a different spark plug design, they’d do it, because that small increase could translate into more market share and higher profits.

These specialty spark plug companies claim they’ve developed a better and faster way to move the flame kernel to air/fuel mixture. But here’s where the hype meets the brick wall. In older engines with traditional spark plugs the spark generates a doughnut shaped flame kernel. So the doughnut shape spread outwards toward the cylinder walls. The hype theory is this: If you can turn the flame kernel into more of a blow torch, you’d get a faster ignition. That’s the promise of these specialty plugs are designed to deliver a blow torch.

But modern leaner burning engines are different. To achieve higher MPG with lower emissions, engine designers completely redesigned the shape of the pistons and cylinder heads to create a swirl effect. In other words, they don’t wait for the doughnut flame front to spread on its own. They blend the flame into the swirl for rapid spread. In other designs, engine designers creates a pre-ignition chamber where they fire up a richer fuel mixture that shoots into the main chamber to ignite the lean mixture. So bragging about how a new spark plug directs the flame kernel really doesn’t apply to late model engines. Perhaps they work better in your 1955 Ford engine, but they really don’t improve performance in a late model engine. Again, IF THEY DID, the car makers would be using them to gain a market advantage.

Tips for installing spark plugs

Anti-seize on spark plug threads?

Car makers started using platinum spark plugs to reduce center electrode erosion and increase spark plug life while reducing emissions. So we saw spark plug change intervals increase from 25K miles to 60K miles. Then we noticed that the spark plugs were very difficult to remove. With aluminum heads and steel spark plug shells, we saw galvanic action literally weld the plugs to the head. So we were taught to use anti-seize on all replacement plugs to reduce the chance of stripping the spark plug threads on the next plug change.

But spark plug manufacturers no longer recommend applying anti-seize

Most new spark plugs have a nickel coating applied to the threads at the factory and no longer require anti-seize on most plugs. If in doubt, consult the spark plug manufacturer’s website to see if they want you to apply anti-seize. If you do apply anti-seize, you MUST reduce torque by at least 10%. AND, the nickel coating is considered a one-time use application. If you remove the spark plug at a later date, you MUST apply anti-seize when reinstalling it.

What’s is the correct torque for spark plugs?

Spark plug torque has been reduced substantially over the years due to the use of aluminum heads. This means you MUST use a torque wrench to properly seat spark plugs. Ba-humbug you say? Well, dude, read this!

• The porcelain portion of the spark plug is sealed to the metal shell with an adhesive. If you over torque the shell, you break the adhesive bond and create a leak that can lead to misfires.

• In addition, over torque can rip the threads right out of an aluminum cylinder head

• Under torquing, on the other hand, can allow combustion gasses to seep past the spark plug threads and cause a misfire. Worse yet, a loose plug can blow the spark plug right out of the cylinder head, and it’ll take the threads with it.

Lesson? You MUST use a torque wrench and follow the car maker’s or spark plug manufacturer’s recommendations for proper torque.

Want to learn more about spark plugs? Read these posts

©, 2015 Rick Muscoplat