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 Spark Plug FAQ's

FAQ's  Handbook Selection Tips Indexing

 Q: What is the function of a spark plug?

     The spark plug has two primary functions:

 1)     To ignite the air/fuel mixture.   Electrical energy is transmitted through the spark plug, jumping the gap in the plugs firing end if the voltage supplied to the plug is high enough. This electrical spark ignites the gasoline/air mixture in the combustion chamber.

2)      To remove heat from the combustion chamber.   Spark plugs cannot create heat, they can only remove heat. The temperature of the end of the plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. The spark plug works as a heat exchanger by pulling unwanted thermal energy from the combustion chamber and transferring heat to the engines cooling system. The heat range of a spark plug is defined as its ability dissipate heat from the tip.

 Q: What is a resistor plug?

   In a resistive plug, the center electrode is split inside the insulator body and a resistive element is inserted between the two pieces of the center electrode. Ignition voltage and current are conducted through this resistor to reduce radio interference from the electromagnetic field that is generated each time the spark jumps the electrode gap inside the cylinder.

 Q: What is the difference between a truck plug and a regular plug?

   The truck plug is designed specifically for pickup truck and SUV engines. A special ceramic insulator is used for additional strength and the electrodes are larger.

 Q: What exactly is a spark plug "heat range"?

   The heat range has nothing to do with the actual voltage transferred through the spark plug. Rather, the heat range is a measure of the spark plug's ability to remove heat from the combustion chamber. The heat range is determined by the insulator nose length and its ability to absorb and transfer combustion heat, the gas volume around the insulator nose, and the materials/construction of the center electrode and porcelain insulator.
   In identical spark plug types, the difference from one heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber. A longer the nose on a spark plug forces the heat from the tip to travel farther before it is absorbed by the cylinder head, which retains more of the heat in the plug tip - making the plug "hotter" than a similar plug with a shorter nose. Engine temperature will affect a spark plug's operating temperature, but not the plug's heat range.

 Q: What is the operating temperature of a spark plug tip?

   Whether a spark plug is installed in a lawn mower, boat, truck, or race car, the plug tip temperature must remain between 450° C and 850° C. If the tip temperature is below 450°C, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits. These deposits will accumulate and foul the plug gap, leading to a misfire. When the tip temperature exceeds 850°C, the ceramic around the tip will blister and the electrodes will melt. Preignition and detonation caused by the overheated tip are likely to cause engine damage.
   The borderline between the fouling and operating regions (450°C) is called the spark plug self-cleaning region. This is the temperature at which the accumulated deposits burn off of the center insulator. A function of temperature only, the self-cleaning region has no relationship to spark quality or intensity.

 Q: What engine parameters affect the operating temperature of a spark plug?

   Air/Fuel mixture:  Rich mixtures reduce the tip temperature, causing fouling and poor drivability. Lean mixtures increase both tip temperature and combustion chamber pressure, resulting in pre-ignition. It is important to read the spark plugs frequently while tuning after engine modifications, the spark plugs will help you achieve an optimum air/fuel ratio.
   Compression ratio/forced induction:  As effective compression ratio increases through engine modifications or forced induction, a colder plug using a narrower gap and higher ignition voltage is required.
   Ignition timing:   Advancing the ignition timing by 10° increases the plug tip temperature by approx. 70°-100°C.
   Engine speed and load:  Increases in firing-end temperature are proportional to engine speed and load. When consistently traveling at high engine speeds or hauling, towing, or pushing loads - install a cooler plug.
   Weather: Temperature, humidity, and barometric pressure all affect air density and intake volume. Higher intake volumes result in leaner mixtures and hotter plug tips.

   Unless you are racing and need maximum performance under today's precise conditions, try to select a plug heat range that runs the plugs near the center of the normal operating range so that the plugs will be able to handle a variety of loads and weather conditions.

 Q: Is the correct spark plug gap important?

    Always use factory recommended gaps unless the engine has been modified. The manufacturer has designed the complete ignition system as a package; the voltage and timing of the spark from the ignition system is tailored for a specific plug gap. Changing the gap can lead to intermittent misfiring, which will foul the plug.
   When altering the gap size for high performance ignition modifications, never go more than +/- 0.008" from the stock setting because the gap surfaces between the ground and the center electrode may not be parallel for larger changes. Higher compression or boosted engines generally require a smaller plug gap and a higher ignition voltage.

 Q: How do you set the spark plug gap?

    Use a wire type feeler gauge to measure the gap, do nut use flat or tapered feeler gauges. If adjustment is needed, use a gapping tool to bend the ground electrode; never pry against the center insulator to open the gap.

 Q: How important is proper spark plug installation torque?

   Torque is the most critical part of installation. A spark plug can only transfer heat effectively when it is fully seated on the cylinder head. Under-torqued spark plugs will not seal and transfer heat effectively. Over-torquing can distort a spark plugs internal seals and cause poor heat transfer. A plug with poor heat transfer will allow cylinder temperatures to rise dramatically - eventually leading to detonation and plug or engine damage.
   To ensure proper torque when installing spark plugs, clean the threads in the head and on the plug, use anti-seize sparingly, and don't guess - use a torque wrench.

 Q: What is pre-ignition?

Defined as: ignition of the fuel/air mixture before the pre-set ignition timing mark. This is caused by hot spots in the combustion chamber, which results from advanced ignition timing, a spark plug that is too hot, high compression, low octane fuel, lean air/fuel mix, insufficient engine cooling, or carbon in the combustion chamber. Pre-ignition and detonation are separate events, but pre-ignition frequently leads to detonation.

 Q: What is detonation?

Detonation involves rapid, uncontrolled burning of the air/fuel mixture while the piston is still rising. During normal combustion, the flame front propagates across the cylinder at a controlled subsonic speed with the piston near TDC. During detonation, the flame front goes supersonic and/or collides from different directions. This rapid detonation strikes the top of the piston as it is still being pushed upward in the cylinder by the crankshaft. The shock wave resulting when the detonation flame front strikes the top of the piston causes the piston to rattle in the cylinder. The sound of this shock wave and the sound of the rattling piston is what people commonly refer to as ping. Detonation radically increases cylinder pressure and temperature, and can quickly burn the end of the spark plug. Excessive heat is usually what causes engine damage when detonation is ignored.

 Q: What is misfire?

    A misfire occurs when the spark is not strong enough to ignite the cylinder charge at the proper moment of the power stroke. A weak spark can be due to a fouled plug, defective coil, to much gap for the compression ratio, incorrect timing, leaking plug wires, crossfiring plug wires, weak ignition module, etc.

This page would not have been possible without the help of Hank LaViers,
who graciously loaned me his extensive Mopar engine library

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This page was edited on: May 3, 2004