What Is the Spark Plug Heat Range? (All You Need to Know!)

Spark plug image with text - heat range

The optimum heat range is the range in which a component works well from a thermal standpoint. The same applies to spark plugs as well.

So what is the spark plug heat range? The optimum spark plug heat range is between 5000 C to 8000 C (or between 9000 F to 15000 F). The heat range of a spark plug varies depending on the electrode material, insulator, and whether the spark plug is of the hot or cold type.

The heat range of a spark plug is important because operating outside of the heat range can cause problems in the engine.

Too low a temperature, there will be carbon deposits. Too high, there will be pre-detonation of the fuel.

Let’s dig deeper into this aspect here.

Optimum Spark Plug Heat Range

The optimum spark plug temperature range is between 500 to 800 degree celsius (or between 900 to 1500 degrees Fahrenheit).

By spark plug temperature, we are referring here to the center electrode temperature. This is where the spark originates.

Again, the operating temperature can vary based on the electrode material, engine conditions, insulator, and the spark plug type (hot vs cold).

Spark plug temperatures are important because the optimum range is where the engine will work smoothly. Spark plug temperatures beyond the desirable range will create problems in the long term.

If the spark plug is operating at a temperature below the optimum range, then you will start noticing carbon deposits. The low electrode temperature allows the unburnt fuel to start depositing.

If, on the other hand, the spark plug temperature is too high, there will be pre-detonation. The air-fuel mixture will start burning even before the spark ignition.

As you can notice, both these instances are bad for the engine. Too low or too high a spark plug temperature is not at all desirable.

Factors determining heat range

Spark plug stock image on a black bg

Optimum range aside, different types of spark plugs will have different operating heat ranges. These heat ratings are determined by several factors, major of which are discussed here.

#1: Electrode material

The type of spark plug material used in the center electrode plays a major role in determining the heat range.

Usually, spark plug electrodes can be of 3 material types:

  • Copper
  • Platinum (single or double)
  • Iridium

Copper spark plugs are low-cost and economical. They are easy to source as well. But, they wear out fast and have a short lifespan. More importantly, they have low melting temperatures (1085 celsius).

Both iridium and platinum (single as well as double platinum) spark plugs have higher life spans. They don’t wear out fast, perform better, and can withstand high temperatures.

Of course, all these advantages come at a cost. These spark plugs are costly.

When it comes to heat range, copper spark plugs usually have lower heat ranges. Copper cannot withstand extremely high temperatures. As a result, the heat range of copper spark plugs is usually low.

Spark plug optimum heat range - infographic

On the other side, platinum and iridium have higher heat ranges. They can sustain high temperatures and are preferred in hot spark plugs.

Tldr. Copper spark plugs have lower heat ranges. Platinum and iridium spark plugs have higher heat ranges.

#2: Thermal conductivity and length of Insulator

The insulator is a key component in a spark plug. They restrict thermal and electrical conductivity mainly between the two electrodes. But, also the thermal flow away from the electrodes.

Put simply, more insulation translates to less heat transfer from the electrodes to outside the spark plug (and engine as well).

In addition, the insulating properties of the material used affect the heat range as well.

Typically, sintered alumina or aluminum oxide is used as the insulating material for spark plugs.

Apart from the material, the position of the insulator nose affects the heat range as well. The longer the insulator nose, the more it will restrict heat transfer. Hence, the hotter will be the spark plug with a high heat range.

Let’s discuss more on this aspect in the next section.

#3: Hotter or colder spark plug

Hot or cold spark plugs are differentiated by the design of the insulator nose.

The longer the insulator nose, the less heat is transferred away, so the hotter the spark plug.

Similarly, the shorter the insulator nose, the more heat is transferred away, so the colder (less hot to be precise) the spark plug.

Not surprisingly, hotter spark plugs will have higher heat ranges. And colder spark plugs will have colder heat ranges.

Spark plug heat range for hot vs cold spark plugs

For hot and cold spark plugs, you can check out our infographic to look at various spark plug manufacturers’ heat ratings here.

There is some caution to be exercised here though. Cold spark plugs that operate at a lower heat range can get carbon buildup if the temperature gets too low (less than 4000 C).

Similarly, hot spark plugs operate at a much higher heat range. The risk here is pre-detonation. If the temperature goes beyond 14000 C or 15000 C, the fuel mixture in the engine will burn before the spark ignition itself.

Hotter or Colder Spark Plug? Which is Better

Hot vs cold spark plug - illustrative diagram

Both the hot and the cold spark plugs have their pros and cons.

Hot spark plugs are useful to prevent any carbon buildup on the spark plug tip.

On the other hand, cold spark plugs are great if you are facing engine knocking, pre-ignition, and power loss. They prevent these problems by reducing the engine temperature significantly.

For low compression engines and engines with problems in complete combustion, hot spark plugs are better.

On the other hand, for high compression engines like turbo and superchargers, and nitrous and high-performance engines, cold spark plugs are preferred.

What heat range spark plug should you use?

4 Spark plugs on a blue background

Typically, you should go for a hotter spark plug.

For your vehicle engine – be it a motorcycle or car, choosing the spark plug will be a bit of trial and error. Generally, a hotter spark plug is preferred.

So, start with a hot spark plug. And read the spark plug from there.

To identify the hotter spark plug by the manufacturer, you can refer to our heat rating chart.

Once you are using the hotter spark plug, you can adjust based on your engine requirements the next few times until you hit the sweet spot.

For the next trial, if you notice carbon buildup in your used spark plug, go for an even hotter spark plug next time.

But if you are seeing engine knocking and pre-detonation, choose a colder spark plug.

Conclusion

The optimum spark plug heat range is between 5000 C to 8000 C (or between 9000 F to 15000 F)

If the spark plug is operating at a temperature below the optimum range, then you will start noticing carbon deposits.

If, on the other hand, the spark plug temperature is too high, there will be pre-detonation. The air-fuel mixture will start burning even before the spark ignition.

The heat range of a spark plug varies depending on several factors, but mainly:

  • the electrode material
  • insulator
  • hot or cold spark plug

For your vehicle engine, generally, a hotter spark plug is preferred.