For this reason, you may need to go with a colder heat range to prevent knock or pre-ignition. In fact, NGK estimates an increase of 70-100 degrees for every 10 degree advance in ignition timing. Use a hotter heat range for rich air/fuel mixtures.Īdvanced ignition timing: Advanced ignition timing will raise the spark plug temperature. Rich air/fuel mixtures can cause the plug temperature to dip, allowing carbon deposits to build up on the tip. Use a colder heat range for leaner air/fuel mixtures. Once again, you’ll need a colder heat range to rapidly transfer all that extra heat to the cooling system.Īir/fuel mixture modifications: Lean air/fuel mixtures raise the operating temperature, along with the plug tip temperature, possibly causing knock or pre-ignition. Increased compression ratio: Higher compression ratios mean higher cylinder pressure and temperature. This keeps the spark plug temperature high, which allows the plug to "self clean" and prevent fouling. Because they have a longer insulator nose length, heat is transferred from the firing tip to the head and subsequent cooling system, at slower pace. In broad terms, spark plugs are often referred to as “hot plugs” or “cold plugs.”Ī cold plug has a shorter insulator nose length-the distance from tip to spark plug shell-and transfers heat rapidly from its firing tip to the cylinder head.Ĭold plugs are ideal for high-rpm engines, forced induction applications, and other instances where the engine experiences higher operating temperatures.Īs well, hot plugs are good for applications that operate mainly at low rpms. Heat ranges are designated by each spark plug manufacturer with a number. Most spark plug manufacturers recommend that the tip temperature remain between 500-850 degrees Celsius.
If it the heat range is too hot, your engine could experience detonation, pre-ignition, power loss or damage. If the heat range is too cold, the spark plug will be unable to properly self-clean by burning off carbon deposits. The plug is simply the delivery tool, and does not increase spark intensity, despite manufacturer's claims to sell you their plug.īasically: Heat range is the speed at which a spark plug can transfer heat from the firing tip to the cylinder head and subsequent water jacket.Ĭhoosing the right heat range is crucial for high performance engines. Spark output is determined by the ignition source, and voltage generation of that unit.
Heat range is how well a spark plug dissipates heat. The heat range has nothing to do with the plug's spark output. There's a common misconception that the "hotter a plug is, the hotter spark it creates". The heat range is another physical aspect of a plug. Shovelheads and Big Twin Evos are all 14mm, while Evo Sporties and all twin cams are 12mm plugs. One of the most obvious characteristics is the thread diameter. Resistor type plugs have resistors built into them, to reduce and supress engine/electronic noise from filtering back into the radio or modern electrical circuitry of the vehicle. This is simply the difference between Resistor and Nonresistor type plugs.
Their Physical characteristics are differentiated by thread pitch/diameter, insulator design, heat range, total thread reach (into cylinder head) seat design, ground electrode shape/style and the type of material used for the center electrode. This information is a culmination of my mechanical experience as well as a very well written article from American Iron Magazine, by John Frank (July 2008), and research from both Champion spark plugs, NGK and various articles I've saved over the years.Īll spark plugs have physical and electrical characteristics. There seems to be a lot of controversy over spark plugs, so I decided to do a little write-up to clarify a few things, and to explain a few others.
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