If your last plugs read a little cold or wet go hotter. If they stick in too far they could hit or "glow plug" and never come out. threads ending to self centering taper helps with that.
I think it also has something to do with how the insulator is designed. Some have short stubby insulators which probably run cooler, and some have long slender insulators that reach way back into the plug with an air gap around them, which probably makes them hotter at the tip. The heat of the spark plug tip has to make its way through the insulator and into the metal barrel and then into the cylinder head. The longer that path, the hotter the tip. How far it sticks out into the combustion chamber is important too.
Here you go. more info than you probably wanted to know. SPARK PLUG BASICS: The spark plug has two primary functions: Ignite air/fuel mixture Transfer heat from the combustion chamber Spark plugs carry electrical energy and turn fuel into working energy. A sufficient amount of voltage must be supplied by the ignition system to spark across the spark plug's gap. This is called "Electrical Performance." The temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called "Thermal Performance", and is determined by the heat range selected. It's important to remember spark plugs do not create heat, they only remove heat. The spark plug works as a heat exchanger by pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine's cooling system. The heat range is defined as a plug's ability to dissipate heat. The rate of heat transfer is determined by: The insulator nose length Gas volume around the insulator nose The materials/construction of the center electrode and porcelain insulator A spark plug's heat range has no relationship to 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 measurement is determined by several factors; the length of the ceramic center insulator nose and its' ability to absorb and transfer combustion heat, the material composition of the insulator and center electrode material. The insulator nose length is the distance from the firing tip of the insulator to the point where insulator meets the metal shell. Since the insulator tip is the hottest part of the spark plug, the tip temperature is a primary factor in pre-ignition and fouling. Whether the spark plugs are fitted in a lawnmower, boat, or a race car, the spark plug tip temperature must remain between 500C-850°C. If the tip temperature is lower than 500°C, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits. These accumulated deposits can result in spark plug fouling leading to misfire. If the tip temperature is higher than 850°C the spark plug will overheat which may cause the ceramic around the center electrode to blister and the electrodes to melt. This may lead to pre-ignition/detonation and expensive engine damage. In identical spark plug types, the difference from one heat range to the next is the ability to remove approximately 70°C to 100°C from the combustion chamber. A projected style spark plug firing tip temperature is increased by 10°C to 20°C. Tip Temperature and Firing End Appearance The firing end appearance also depends on the spark plugs tip temperature. There are three basic diagnostic criteria for spark plugs: good, fouled and overheated. The borderline between the fouling and optimum operating regions (500&def;C) is called the spark plug self-cleaning temperature. The temperature at this point is where the accumulated carbon and combustion deposits are burned off. Keep in mind the insulator nose length is a determining factor in the heat range of a spark plug, the longer the insulator nose, the less heat is absorbed, and the further the heat must travel into the cylinder head water jackets. This means the plug has a higher internal temperature, and is said to be a hot plug. A hot spark plug maintains a higher internal operating temperature to burn off oil and carbon deposits, and has no relationship to spark quality or intensity. Conversely, a cold spark plug has a shorter insulator nose and absorbs more combustion chamber heat. This heat travels a shorter distance, and allows the plug to operate at a lower internal temperature. A colder heat range is necessary when the engine is modified for performance, subjected to heavy loads, or is run at a high rpm for a significant period of time. Colder spark plugs remove heat quicker, reducing the chance of pre-ignition/detonation. Failure to use a cooler heat range in a modified application can lead to spark plug failure and severe engine damage. Below is a list of external influences on a spark plug's operating temperature. The following symptoms or conditions may have an effect on the actual temperature of the spark plug. The spark plug cannot create these conditions, but it must be able to cope with the levels of heat...if not, the performance will suffer and engine damage can occur. Air/Fuel Mixtures seriously affect engine performance and spark plug operating temperatures. Rich air/fuel mixtures cause tip temperature to drop, causing fouling and poor driveability Lean air/fuel mixtures cause plug tip and cylinder temperature to increase, resulting in pre-ignition, detonation, and possibly serious spark plug and engine damage It is important to read spark plugs many times during the tuning process to achieve the optimum air/ fuel mixture Higher Compression Ratios/Forced Induction will elevate spark plug tip and in-cylinder temperatures Compression can be increased by performing any one of the following modifications: a) reducing combustion chamber volume (i.e.: domed pistons, smaller chamber heads, mill ing heads, etc.) b) adding forced induction (Nitrous, Turbocharging or Supercharging) c) camshaft change As compression increases, a colder heat range plug, higher fuel octane, and careful attention to ignition timing and air/fuel ratios are necessary. Failure to select a colder spark plug can lead to spark plug/engine damage Advancing Ignition Timing Advancing ignition timing by 10° causes tip temperature to increase by approx. 70°-100° C Engine Speed and Load Increases in firing-end temperature are proportional to engine speed and load. When traveling at a consistent high rate of speed, or carrying/pushing very heavy loads, a colder heat range spark plug should be installed Ambient Air Temperature As air temperature falls, air density/air volume becomes greater, resulting in leaner air/fuel mixtures. This creates higher cylinder pressures/temperatures and causes an increase in the spark plug's tip temperature. So, fuel delivery should be increased. As temperature increases, air density decreases, as does intake volume, fuel delivery should be decreased Humidity As humidity increases, air intake volume decreases Result is lower combustion pressures and temperatures, causing a decrease in the spark plug's temperature and a reduction in available power. Air/fuel mixture should be leaner, depending upon ambient temperature. Barometric Pressure/Altitude Also affects the spark plug's tip temperature The higher the altitude, the lower cylinder pressure becomes. As the cylinder temperature decreases, so does the plugs tip temperature Many mechanics attempt to "chase" tuning by changing spark plug heat ranges The real answer is to adjust air/fuel mixtures by rejetting in an effort to put more air back into the engine Types of Abnormal Combustion Pre-ignition Defined as: ignition of the air/fuel mixture before the pre-set ignition timing mark Caused by hot spots in the combustion chamber...can be caused (or amplified) by over advanced timing, too hot a spark plug, low octane fuel, lean air/fuel mixture, too high compression, or insufficient engine cooling A change to a higher octane fuel, a colder plug, richer fuel mixture, or lower compression may be in order You may also need to retard ignition timing, and check vehicle's cooling system Pre-ignition usually leads to detonation; pre-ignition an detonation are two separate events Detonation The spark plug's worst enemy! (Besides fouling) Can break insulators or break off ground electrodes Pre-ignition most often leads to detonation Plug tip temperatures can spike to over 3000°F during the combustion process (in a racing engine) Most frequently caused by hot spots in the combustion chamber. Hot spots will allow the air/fuel mixture to pre-ignite. As the piston is being forced upward by mechanical action of the connecting rod, the pre-ignited explosion will try to force the piston downward. If the piston can't go up (because of the force of the premature explosion) and it can't go down (because of the upward mo-tion of the connecting rod), the piston will rattle from side to side. The resulting shock wave causes an audible pinging sound. This is detonation. Most of the damage than an engine sustains when "detonating" is from excessive heat The spark plug is damaged by both the elevated temperatures and the accompanying shock wave, or concussion Misfires A spark plug is said to have misfired when enough voltage has not been delivered to light off all fuel present in the combustion chamber at the proper moment of the power stroke (a few degrees before top dead center) A spark plug can deliver a weak spark (or no spark at all) for a variety of reasons...defective coil, too much compression with incorrect plug gap, dry fouled or wet fouled spark plugs, insufficient ignition timing, etc. Slight misfires can cause a loss of performance for obvious reasons (if fuel is not lit, no energy is be-ing created) Severe misfires will cause poor fuel economy, poor driveability, and can lead to engine damage Fouling Will occur when spark plug tip temperature is insufficient to burn off carbon, fuel, oil or other deposits Will cause spark to leach to metal shell...no spark across plug gap will cause a misfire Wet-fouled spark plugs must be changed...spark plugs will not fire Dry-fouled spark plugs can sometimes be cleaned by bringing engine up to operating temperature Before changing fouled spark plugs, be sure to eliminate root cause of fouling
One thing that I noticed is that if you run too hot a plug, you'll end up cooking the electrode. So, if you are concerned about the plug- take them back and get a colder plug so you don't risk any damage to the engine. However, you may find that if you run too cold a plug- they may foul easily, and these new plugs suck as they don't self clean like the old ones do. My car is supercharged, so I run a plug two steps colder than what a normally aspirated one would run, and I have never had any issues
First you need to know the recommended plug for that motor and go from there. A lower number than called for is colder and a higher number is hotter. Extended tip or longer porcelain is not an indicator of heat range. A prime example I can think of is a 292 Chev 6 I used to have. The correct plug for that motor had quite an extended tip
It's in a Worn out cammel hump 350, just trying to streach it through the winter 'till i get my new motor done So thanks, longer the plug the hotter, that's what I thought, No wonder why the plugs fouled up in a couple hundred miles, and i was running it as hard as i reasonably could too
some thing else to think about is, before you change your plugs is the best time to clean the carbon out by takeing your ride out on a good hard run, if you put your new plugs in and then try to burn the carbon out you run the risk of fowling your new plugs with the build up of carbon from the last plugs. there used to be a champion spark plug rep. on here.
heres a little more info, http://images.google.ca/imgres?imgurl=http://www.brisksparkplugs.com.au/images/technical/heat_range_insulator_tip_length.jpg&imgrefurl=http://www.brisksparkplugs.com.au/technical/heat-range.shtml&usg=__KMwMZ_FfkK9PL4GUFnxvVkIk0DI=&h=316&w=593&sz=24&hl=en&start=8&um=1&tbnid=Qm1Wm4Vg4d9xSM:&tbnh=72&tbnw=135&prev=/images%3Fq%3Dspark%2Bplug%2Binsulator%2Blength%26um%3D1%26hl%3Den%26sa%3DN
Whew, way much info. Let me summarize. The length (reach) has nothing to do with heat range; the heat range deals with engine heat, ie if you are running a bit warm in summer put a colder plug in - until foul at idle - these plugs will extract more heat out of the engine via the water (water passage very close to the plud); two tip styles are available, regular and protracted, the porcelin is obvious on the protracted ( advise to stay away from these in a hot rod, the act like little heating elements and can cause detonation); most plugs also come in either 5/8 or 13/16 socket size. The heat range is obviouse looking into the end of the plug, if you can see way down in there it has a small amount of mass and can't conduct heat; if you can't look down at all it has a great amout of mass and will conduct like crazy. I've seen a couple hundred degrees difference on the data recorder, easy.
the heat range deals with engine heat, ie if you are running a bit warm in summer put a colder plug in - until foul at idle - these plugs will extract more heat out of the engine via the water (water passage very close to the plud) Despite the text, the spark plug does not cool the engine. The heat range is combustion chamber heat, not water temperature. it has a small amount of mass and can't conduct heat; if you can't look down at all it has a great amout of mass and will conduct like crazy The length of the porcelain delays heat loss through the plug. More porcelain = hotter, not colder. U.S.-manufactured plugs (Champion, Autolite, AC) have higher part numbers for higher heat ranges: J12 is hotter than J10, &c. European and oriental plugs (NGK, Bosch, ND, Lodge, KLG) have lower part numbers for higher heat ranges: B6 is hotter than B7, &c.
I hope you are not confusing reach with the porcelain. If you are you could very well put in a plug to long and have some real issues. if the threads on the plugs you bought are longer than the threads on the one you took out you either have the wrong plug, or the wrong one was in there....
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