How to Choose Proper Spark Plugs for Your Engine

The spark plug is a simple engine part, seems like there is nothing to worry about when choosing it. Yet there is a lot to discuss asking “how to choose spark plugs for my vehicle?”. Even though spark plugs themselves are basic pieces of equipment, they are involved in a complex process – combustion. Actually, they are the exact reason why air-fuel mixture is ignited in a gas engine.

Effective combustion is the foundation for power and fuel efficiency – you need to burn all the fuel to get the most out of it.

Also, different engines work in their own way: conditions inside a cylinder of turbo inline 4 engine are way different than in naturally aspired V8. Those conditions require specific spark plug characteristics to ensure maximum performance.

How to choose correct spark plugs

The fast answer is to check the manufacturer’s recommendation – it’s the best way to ensure the engine will be working as it was designed.

The full answer is a bit longer. At the first sight, all spark plugs are the same, but there are 3 characteristics that differ, which drastically impact the engine behavior.

Spark plugs characteristics

Three main features of a spark plug include:

Electrodes material

Materials have a different lifespan, but all of them create the same spark.

Gap

The spark occurs between two electrodes, and the distance between them is called “gap”.

Heat range

Spark plugs need to stay at a specific temperature range to work fine, but different engines produce different amounts of heat.

What is the best spark plug material?

The material doesn’t directly impact the performance, but the spark plug longevity. The materials that we are talking about are center and ground electrodes because the spark is created between those two.

Copper (Nickel)

Those are the basic spark plugs. Their electrodes are made of copper with a nickel coating. Even though they create the same spark as other materials, their longevity is the worst, since both copper and nickel are soft metals and don’t last long in rough conditions.

Longevity: up to 30,000 miles

Platinum and Double Platinum

Those spark plugs aren’t fully made of platinum, since they’d be way too expensive to produce. Instead, they have a platinum disk on the center electrode (regular platinum) or both center and ground electrodes (double platinum). That way spark connects with platinum instead of softer metals, which drastically increases spark plug working cycles.

Longevity: up to 60,000 miles for single platinum; up to 100,000 miles for double platinum

Iridium

Fundamentally iridium spark plugs don’t differ from platinum. This type uses iridium discs, which tend to be stronger than platinum. As a result, iridium spark plugs last even more.

Longevity: up to 150,000 miles

Summary on materials

Obviously, the best spark plug type is iridium. The only downside is their price – they cost 10-20% higher than platinum. Yet spark plugs are not so expensive in general, so it’s better to overpay a little to get more mileage.

What is correct spark plug gap?

The gap is the distance between the ground and center electrode. Generally speaking, a bigger gap means a bigger spark. And the bigger spark can ignite more A/F mixture, making combustion more efficient.

On the other hand, with too much gap you are risking to get misfires: because of increased distance, it’s harder for the spark to occur.

The optimal gap is a maximum distance between electrodes while saving the stable spark plug work. All the manufacturers have gap recommendations for each engine. You won’t make a mistake sticking to them.

💡 Spark plug gapping

You can increase the gap on your own. The process of changing the gap calls “gapping”. Simply bend a ground electrode a little to increase the gap. Measure it with a spark plug gap tool. Try increasing the gap by 0.002 inches and test it. If your car accelerates smoothly after the change, it means you’ve got a slightly better fuel efficiency and performance. If not – reverse the gap to what you had before it. You can repeat the process to find the gap when the car starts jerking and downgrade a gap once or twice – it’ll be a sweet spot for your engine.

🚨 Be careful

Gapping platinum or iridium is not recomended, because you can damage the disc welded to the electorde.

What is “heat range”?

The spark plug requires a special temperature condition to work the way it was designed. The optimal operating temperature is above 500°C and below 950°C.

  • Below 500°C spark won’t burn leftover fuel, causing a carbon outgrowth. Carbon is a bad current conductor, so it can prevent spark from occurring, causing misfires.
  • Above 950°C hot spark plugs can lead to pre-ignition (detonation, knocking), which not only make your car jerking but also harms your engine.

Hot and cold spark plugs

There are “hot” spark plugs and “cold” spark plugs. The coding is different with each manufacturer: for example, NGK’s spark plug heat range varies from 2 to 11, where 2 is hottest and 11 is coldest.

Hot spark plugs

Hot spark plugs have lesser ceramic insulation. Instead, there is extra room for fuel to burning. It causes 2 things:

  1. This space is filled with hot gases from combustion, warming up the spark plug.
  2. Gas pocket prevent heat to be transmitted to the engine’s head via spark plug since gases are worse heat conductors than solids.

Cold spark plugs

Cold spark plugs do the exact opposite: have lesser room for hot combustion products and more effectively transfer heat from plug to engine.

How to chose right heat range for your engine

Each engine has a recommendation on spark plugs. It has a coding, from which you can determine the standard heat range. For example, Dodge recommends NGK LZFR5C-11 for 2012 5.7 HEMI with a heat range of 5. Knowing that NGK’s gradation is 2-11, we can replace them with spark plugs from another manufacturer with a heat range somewhere in the middle of their heat range.

Sometimes, you need to swap to different heat ranges:

  • If you stressing your engine a lot (i.e. racing), it’s recommended to pick spark plugs with a heat range of 1-3 units colder than the manufacturer recommends.
  • If you’ve increased the performance of an engine (i.e. increased boost), it’s also better to pick more cold spark plugs over the standard.
  • If you idling your engine on the highway most of the time, you should use a bit hotter spark plugs.

Ground electrode design

One more thing worth mentioning is ground electrode design. Most of the spark plugs came with traditional construction, but some engines require a specific spark plug design (i.e. rotary). NGK has a great article on this topic, we’ll jump straight to the conclusion: check the recommended spark plugs by the manufacturer and stick to it. There is no sense to change them since there is no fundamental difference in how a spark occurs.

Conclusion

Spark plugs are simple, but important part of your engine, you should carefully choose them. Most of the time is the best option is to use the manufacturer’s recommended plugs or find analog with the same characteristics.

Regarding material, it’s better to pick iridium spark plugs, since they create the same spark while being the most durable.

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Illia Korchahin
I'm in love with cars since childhood - this love brought me to an engineering degree. My second passion is web development and blogging - here I am, writing automotive articles on my website. Seems like dreams come true. Driving a 1996 BMW 328i Coupe.
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