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Spark Plug Heat Range

Bulletin 1002

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All NGK spark plug application


The optimum operating temperature for a spark plug is 500-850 C. The spark plug acts as a heat exchanger by removing the thermal energy away from the combustion chamber and into the cylinder head. The rate of transfer is determined by the insulator nose length, the material used to construct the center and ground electrode and the ceramic insulator. NGK spark plugs use a heat range index ranging from 2 to 12 with 2 being the hottest spark plug and 12 being the coldest. Spark plugs may physically appear to be the same but have a different heat range based on the insulator design or shape. Selecting an incorrect heat range for your application may cause fouling of the spark plug if too cold, or possibly severe catastrophic engine failure if too hot for the application.


Surface area between the ceramic insulator and metal shell is what designates a spark plugs heat range. The difference from one heat range to the next is the ability to remove approximately 70-100 C from the combustion chamber. In the following case the fourth digit is the heat range identifier. A BKR7E-11 dissipates more heat over a larger surface area than the BKR5E-11. The “5” heat range, or, the BKR5E-11 cannot dissipate as much heat making it a hotter spark plug. See the examples below.



Choosing the correct spark plug for your application is critical. An approved NGK retailer or will help with your selection for the common stock applications. For modified applications select the correct heat range spark plug by applying your modifications to an application chart. Following the installation of a spark plug into a modified application, test the spark plug heat range by reading the spark plug insulator after a short run to understand how the spark plug will react to the modified application. Never assume a spark plug can operate effectively in these conditions, read the spark plug to understand its effectiveness in the motor.


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