What does the glow factor of spark plugs mean?

Hot and cold spark plugs

Manufacturers produce hot and cold spark plugs with different ignition index , for different engines, operating conditions and fuels. Incorrect selection of parts can cause inconsistent starting and engine performance, impaired performance and increased fuel consumption. In this article you will learn what the glow factor is, which plugs are hot and which are cold and how to choose the right spark plugs for the car.

What is the difference between cold and hot spark plugs

Determine whether a spark plug is cold or hot can be determined by its caliber number, which is usually encoded in the marking. This characteristic indicates the rate at which the working part heats up. Spark plugs with the lowest wick number waste less heat, so they heat up faster and are considered hot, and more heat-resistant plugs with a high wick number are cold.

The difference between hot and cold spark plugs: video

For proper ignition operation, the temperature of the hottest part of the plug should be between 450-850°C. Below 450°C, fouling builds up on the electrodes, impairing conductivity and spark formation. Heating the electrodes above 850°C increases the risk of glow ignition, in which the mixture is ignited by the red-hot electrodes instead of the spark discharge. In both cases, the engine works worse, ignition skips or untimely ignition of the mixture is possible, as well as its afterburning in the exhaust manifold, which is fraught with a number of problems, including increased wear of the cylinder head.

The main difference between a cold spark plug and a hot spark plug by design is the area and shape of the center electrode insulator, which determines the rate of heat transfer. The hotter the plug, the greater the height of the taper of the insulator and its surface area. This element is usually hidden inside the threaded part, but you can see it if you want. See the table for the difference between hot and cold plugs.

Cold spark plug – the insulator is short and recessed into the housing

Hot spark plug has a long insulator skirt

Type of plug Center electrode insulator taper Operating temperature, °C
Height Location
Cold Significantly less than thread height Recessed in thread housing 600-850
Hot Larger than height of thread Protrudes above thread 400-500
Medium Approximately equal to thread height Flush with threads or protrudes 2-4 mm 500-600

There are no other design differences between hot and cold spark plugs. The number of electrodes, their thickness, shape and the material from which they are made have no influence on this. Platinum, iridium, three-electrode and classic one-electrode nickel spark plugs come in hot, cold and moderate. The caliper number is not determined by the gap between the electrodes either. The use of the plugs will be determined by the temperature of the engine.

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When to put cold and hot plugs

The only thing that affects the caliper number of the plug is the speed of heating of its working part . Therefore, this parameter is selected depending on the thermal loads in the engine combustion chamber:

The use of cold and hot plugs

  • Hot plugs – are installed on low-speed engines of relatively large volume and low power (up to 50 hp per 1 liter of volume), not subject to overheating.
  • Cold plugs are used in high-speed engines, supercharged engines, internal combustion engines with high compression ratio (over 12), with a return of about 100 liters and more, in sports and tuned cars.
  • Medium plugs – are used in most “civil” atmospheric engines up to 2.5 liters.

In addition to the design features of the engine, laid down by the manufacturer, the choice of caliber number is also influenced by the mode of operation of the car. Ideally, the temperature of the spark plug should be such that it reaches 450°C as quickly as possible, but does not exceed 850°C during the whole time of operation.

A hotter spark plug than recommended by the manufacturer can be used for regular slow driving for short distances and frequent idling in traffic jams. It is also acceptable to use seasonally in winter for short trips, when the engine and plugs do not reach the operating temperature, which contributes to the deposition of plaque on the electrodes. A hot spark plug in such conditions is self-cleaning better.

The logic of selecting spark plugs based on need – click to enlarge

It is worth choosing cooler plugs when using a car under load: long trips on highways, off-road driving, participating in races and drifting. Also change the spark plugs for colder in the case of engine tuning, when increasing the working volume or compression ratio, installing a turbine or a gas turbine.

For the majority of small cars operating in the regular mixed mode, the plugs are designed with a medium glow plug number. There is no sense to change them for cold or hot ones in the usual operation mode. Moreover, in most cases, it will have a negative effect on the engine operation.

The table below lists the main situations in which you can deviate from the manufacturer’s recommendations, indicating the reasons for this deviation.

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When hot plugs are installed, and when cold plugs are installed

Purpose of installation Why the candle should be cold or hot What happens when the wrong wick number is installed
For sports cars High RPM loads make the spark plugs hotter, thus allowing the fuel to ignite spontaneously. Cold spark plugs avoid this Fuel ignites prematurely, and this leads to a glow plug ignition. The resulting loss of power and increased wear on the engine
For off-roading
For vehicles that frequently stand in traffic jams The crankshaft speed does not exceed 1,000 rpm when idling. As a result the thermal loads on the combustion chambers are lower and the spark plugs are less warmed up. That is why it is better to use hot spark plugs in the city with heavy traffic. A cold spark plug will work with a constant underheating, due to which its electrodes will be quickly covered with soot. Spark formation worsens because of these deposits, the engine starts bouncing and starts worse
For CNG equipment During driving on gas the combustion chamber does not receive additional cooling due to evaporation of fuel (which comes already in the form of vapors), which slightly increases the heat load on the spark plug, and cold plugs will compensate for this. Besides in itself the propane-butane or methane burns cleaner than petrol and forms less cinder on the electrodes. Hot spark plugs run out sooner because they operate at higher thermal loads.
For a high mileage engine On a worn engine with high oil consumption, installing hotter spark plugs slows fouling and improves stability. Cold plugs quickly accumulate deposits from oil combustion products, which do not burn out due to underheating. The engine starts to run worse, it starts to stall.

The following section will help you understand if there is a difference in spark plugs from different manufacturers and how to determine which spark plugs are hot and which are cold by marking.

How to choose a spark plug with the correct caliper number

You can quickly distinguish hot spark plugs from cold ones by the marking and shape of the center electrode insulator. The easiest way to determine the caliper number of a spark plug is to examine the taper of the insulator: cold plugs have a low taper that is recessed into the housing, and hot plugs have a high taper that protrudes over the threaded part. This method will allow you to understand which plugs are hotter by holding the two parts in your hands.

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The caliber designation in the marking of Russian-made spark plugs

The difference between a hot spark plug and a cold one is easiest to find by the inscription on the insulator. It is easy to identify Russian-made products, marked according to GOST R 53842-2010, because they have only 7 standard caliber numbers:

  • 8, 11 and 14 – hot;
  • 17 and 20 – medium;
  • 23 and 26 – cold.

Other values are also allowed, so manufacturers can produce plugs with a caliber number, such as 13, 18 or 27, which, respectively, will be hot, medium or cold. But in general, the smaller the number, the hotter the candle, and the more – the cooler. If you consider candles from other manufacturers, the relationship could be reversed or not as noticeable.

In Europe, there is no universally accepted standard for the designation of glow plug numbers and companies use their own markings. To determine the correspondence between them will help table of glow plugs caliber of different manufacturers.

Standard Cold Medium Warm
GOST More than 20 15 to 20 8 to 14
Denso 24, 22 20 17, 16, 14
Bosch 5 6, 7 8, 9, 10
NGK 8, 7 6 5, 4
ISKRA 95 85, 65 55
BRISK 12, 14 15 17, 19
Champion 7 8, 9, 10 11, 12
AutoLite 3 4 5
Magneti Marelli. 3, 6 7 8, 9

Most popular manufacturers have a higher rating, the colder the spark plugs, and some companies, such as Bosch has a reverse marking, so the selection of this nuance should be taken into account.

The question “Which plugs are better: hot or cold?” is incorrect, because you should select them in accordance with the manufacturer’s tolerances. He may indicate the values of the caliber number according to GOST, and may only indicate the designation of the part of a particular manufacturer.

Knowing how different companies correlate cold or hot spark plugs, and what is recommended to install according to the instructions, you can easily pick them up for different operating conditions. For example, if the Denso website recommends Q20TT for the standard Lada Priora motor, then for the tuned power plant you can take a similar model with the index 22 or 24.

Answers to frequently asked questions

Which spark plugs are cold and which are hot?

Hot spark plugs are plugs that have a large insulator area of the center electrode, so they heat up quickly. Candles that have a small insulator taper area take a long time to warm up, so they are called cold.

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Can I use lower gauge plugs?

It makes sense to lower the rating and to put plugs with a lower index than the manufacturer provides only when you drive regularly at low speeds, often stand in traffic, and when there are no heavy loads. Under such conditions, plugs will be less covered with deposits. If your car is frequently exposed to loads, traveling at high speeds, hotter plugs are not desirable.

How do you determine the glow plug ratio?

The easiest way to determine the wattage is by external characteristics: the lower it is, the bigger is the height and bigger is the insulator area of the central electrode. You can also use the marking, but you need to have the correlation table handy. In the universal GOST marking, cold candles are with a number up to 14, and hot – 21 or more, but many manufacturers use their own system.

What happens if you install a candle with an unsuitable thermal range?

Installing a cold spark plug in an engine designed for hot plugs entails its constant operation at a lower temperature. Under these conditions, fouling of the electrodes is accelerated, causing the spark plug to perform worse. If, on the contrary, a hot spark plug is installed in a heat-loaded engine, it will work with overheating, which leads to glow ignition. These phenomena adversely affect the engine life and performance.

Can I turn a cold spark plug into a hot one?

The caliche of the spark plug is determined by its design, so it is impossible to change it. The only parameter that can be adjusted manually is the value of the spark gap between the electrodes.

Cold and hot spark plugs. Caliche number.

Modern spark plugs are individually selected for different engine designs and driving conditions. Therefore, it is impossible to specify a spark plug that will function in all engines without problems. In this article, we will look at what cold and hot spark plugs are and the associated wick number.

What is the wick number?

The wick number is a value that indicates the time it takes for the spark plug to reach the wick condition. The higher the heat rating, the lower the heat of the spark plug. Accordingly, a low wick number is “hot”, and a high wick number is “cold”.

Low loads work fine for a “hot” spark plug, but it can become “glow plug hot” with sustained high-intensity sparking. The result is a loss of engine power. The spark plug must always be replaced after checking the thermal characteristics and rectifying any faults.

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The temperature rises differently in the combustion chamber of different engines, requiring spark plugs with different thermal equivalents. This thermal equivalent is expressed in terms of the so-called glow number.

The heat equivalents, expressed as caliche, are the average temperatures measured on the electrodes and insulator that correspond to the engine load. At the insulator skirt, the operating temperature should be in the range of 400°C to 850°C. Temperatures above 400°C are required because at these temperatures soot and oil deposits are removed and the spark plug is thus self-cleaning.

However, the temperature at the insulator should not rise above 850°C either, as glow ignition may occur at temperatures above 900°C. In addition, at very high temperatures the electrodes are additionally exposed to chemically aggressive compounds or are destroyed. Avoiding glow plug ignition is possible, you only need to follow a few simple rules: Firstly – avoid early ignition setting; secondly – pour the fuel that corresponds to the engine; and thirdly – watch the appearance of the spark plugs.

When are cold and warm spark plugs used?

You have to keep in mind that the conditions of the spark plugs are different in summer and winter, it follows that it is best to have two sets of spark plugs: summer with “cold” and winter with “hot”. If you drive in winter and often stand in traffic jams, it is best to put hotter plugs, but if in summer you race at high speeds, and even for long distances, of course, put cooler ones.

So: For long distances and high speeds – “cold” plugs, and for short distances and at low speeds – “hot” plugs.

Also the size of the engine affects the choice of plug, the bigger the engine, the “cooler” the plug. The same spark plug may be “cold” for one engine and “hot” for another. How much the spark plug will be heated in the process, and how it will give off heat, also depends on the material of the insulator and the length of the thermal cone.

Marking of spark plugs.

Examples of spark plug markings: – A low wick number (e.g. BP4ES) – “hot spark plug”, high heat absorption due to the long insulator skirt – A high wick number (e.g. BP8ES) – “cold spark plug”, low heat absorption due to the short insulator skirt.

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Thank you for reading the article to the end! Good luck on the road!

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