How to Read Spark Plug Heat Range?
4 Answers
Spark plug heat range is indicated by numbers, which are typically located in the middle of the model number. For example, in the model BP5ES11, the number 5 represents the heat range of this spark plug. Different brands have varying heat ranges, and each brand's heat range corresponds to specific vehicle models. More details about spark plug heat range are as follows: 1. The heat range of a spark plug essentially refers to an indicator of its heat absorption and dissipation capabilities. Higher-revving engines require spark plugs with higher cooling heat ranges. 2. Heat ranges are categorized into numbers 1 through 9, where 1-3 indicate low heat range, 4-6 medium heat range, and 7-9 high heat range. Most passenger car spark plugs fall within the 5-7 range.
The heat range of a spark plug actually reflects its heat dissipation capability. When I usually inspect them, I first look for the middle string of numbers in the model code. Take NGK's BKR6E-11 for example - the number 6 in the middle indicates the heat range. A higher number means faster heat dissipation, which is suitable for modified cars that frequently run at high RPMs. Lower numbers indicate slower heat dissipation, making them more appropriate for daily commuting. Once when my car had sluggish acceleration, I took it apart and found the previous owner had incorrectly installed spark plugs with a higher heat range - replacing the factory-specified 7 with a 9. Always check the factory manual for reference when selecting, as the coding conventions vary by brand. Denso typically marks the heat range at the fifth position, while Bosch often hides it at the end of the model number. When changing spark plugs, don't just consider the size - an incorrect heat range can either cause carbon buildup in the engine or melt the electrodes.
The heat range of spark plugs depends on the model number. That's how I teach customers when repairing cars. Remember the common brand coding rules: NGK uses the middle number, like the '1' in ZK16RU's '16' indicates the heat range; Bosch's FR7DC uses the middle '7' as the heat value. Lower numbers mean hot plugs with slower heat dissipation, suitable for daily drivers; higher numbers indicate cold plugs with faster heat dissipation, ideal for performance cars. Never swap them just because they look similar. My old Fit once had the wrong heat range installed by an apprentice, causing idle vibration like a massage chair. Always bring the old part to the auto shop when replacing, or send a photo of the model number to the store for verification—it's the safest way.
Over the years of modifying cars, understanding spark plug heat range is absolutely essential. Simply put, look at the number in a specific position of the model code. For NGK spark plugs, the number in the middle of the model represents the heat range. A difference of 1 in the heat range number means roughly an 80°C difference in the temperature at the ignition tip. If you're into track racing, opt for higher-numbered cold-type plugs—heat ranges like 9-11 can withstand high RPM heat dissipation. For daily commuting, medium heat ranges of 6-8 are the safest bet. Last week, while helping a friend inspect a used sports car, we found the spark plugs had the wrong heat range—turns out the previous owner had cheaped out and installed low-grade parts. Before modifying your car, always check the heat range range specified in the factory service manual.
