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Understanding Spark Plugs


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A cross reference to other plugs.
Harley Spark Plug Cross-Reference

[FONT=Verdana, Arial, Helvetica, sans-serif]This table shows the most common Champion/NGK equivalents.
Cross reference Champion to NGK page 1

Here's many tables to cross reference with. Click on the headers to change brand.
NGK Spark Plugs DENSO Spark Plugs Bosch Spark plugs Champion Spark Plugs at Wholesale Prices.

Listings for Denso plugs.
Find My Part: DENSO Iridium

Interesting read on plugs and telling their condition.
How you can read spark plugs and select them - by Gordon Jennings.

Spark plug frequently asked questions.
Spark Plug FAQ

[/FONT]Heat Range Reference

  • NGK Heat Ranges - The larger the number in the main body of the number, the colder the plug is. Example a R5671A-7 is a cold plug where a R5671A-10 is an even colder plug. The digits on the end of the plug code refer to the gap for that plug.
  • Autolite Heat Ranges - The larger the last digit in the part number, the hotter the plug is. Example a [FONT=Verdana, Arial, Helvetica, sans-serif]AR3935[/FONT] is hotter than a [FONT=Verdana, Arial, Helvetica, sans-serif]AR3932[/FONT] plug which is very cold.
  • Champion Heat Ranges - Typically Champion numbers in the same series the larger the number the hotter the plug.
  • Denso Heat Ranges - The heat range number located on the insulator indicates the temperature range of each DENSO spark plug. The lower the number, the hotter the plug, the higher the number, the colder the plug.

What Does It All Mean?

Hot plugs? Cold Plugs?
Definition: The heat range of a spark plug is its ability to dissipate heat from the combustion chamber to the cylinder head. The heat range, or dissipation capability, of a spark plug depends primarily on the length of the insulator nose. The heat from the combustion is transferred from the insulator nose through the center electrode to the plug housing, and from there it goes to the cylinder head.

A "hot plug" has a long insulator nose which exposes more surface area to the combustion gases. This keeps the plug temperature higher overall which is ideal for stop-start city driving conditions.

A "cold plug" has a shorter insulator nose, which minimizes the amount of surface area exposed to the combustion gases. Cold plugs are typically used in racing conditions because of its ability to transfer heat out of the cylinder chamber quickly. Most turbo, supercharged and nitrous oxide applications use this type of plug because of the tremendous heat they generate. A colder plug can also minimize the risk of pre-ignition and detonation, however if the plug is to cold there is risk of fouling during extended periods of idling and low-speed operation.

The challenge of spark plug design is finding the proper balance between the engine speed and the plug temperature to assure carbon fouling doesn't occur at low temperatures/low speed and pre-ignition at high temperature/high speed.

Heat Dissipation in a spark plug.
Of the 100% heat generated from the combustion, 20% is absorbed by fresh air from the intake of the following stroke. 58% of the heat is absorbed by the walls of the cylinder head which hold the spark plug in place. Twenty percent is absorbed by the insulator and side walls of the plug, the remaining 2% being absorbed by the spark plug wires.

A table to understand Auto lite plugs.


[FONT=Verdana, Arial, Helvetica, sans-serif] A table to understand Champion plug numbers.



A table to understand NGK plugs numbers.

A spark plug's heat range refers to how well the plug dissipates heat. A colder plug dissipates the heat quicker, and so runs cooler. Therefore, a hotter plug will help keep deposits from fouling a plug if used in an over rich or oil consuming engine. On the other side of the coin, a colder plug will be less prone to overheating and causing pre-ignition in a high compression engine or one that is run hard (Both high compression ratios and prolonged high speed will generate more heat).


Some of your older V twins (cast iron heads, points, ect) best results may be found using a colder plug in one cylinder than the other. In this case it is usually the front cylinder that takes the colder plug as this cylinder is not as likely to foul a plug at low speed."

(picked this info up somewhere...??)

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