Plug failure
01-21-2003, 10:52 PM
#1
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Plug failure
What causes a spark plugs tip to break? Is it heat? Lean conditions? Age? Wrong heat range for aplication?
I've seen this happen frequently and wanted to prevent this myself.
Also, what are the pros or cons of getting too much of a colder plug?
I've seen this happen frequently and wanted to prevent this myself.
Also, what are the pros or cons of getting too much of a colder plug?
01-22-2003, 04:35 AM
#2
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Plugs fail for a number of reasons
Plugs break due to pre-ignition, or detonation pressures. Normally, the ceramic insulator will crack or break first, however, that insulator can fall into your combustion chamber and then get pounded into fine ceramic sand by the action of the pistons and valves. Not good. The majority of the ceramic insulator will get "blown" out the exhaust ports, but not until it is pounded down to a small enough size to sneak out the small exhaust valve opening. This can do damage to valve seats, valve faces, piston tops. Also ceramic sand/small particles can damage cylinder walls/rings. It can also damage the spark plug itself by hitting the earth or ground electrode.
Detonation and pre-ignition can also cause extremely high combustion chamber temperatures. This can melt center electrodes of spark plugs, or melt the earth (side) electrode, or both. The most "extreme" cases, damage to either the positive/earth electrode can be so bad, that either the two fuse together, short circuiting the spark plug, or damaging it in such a way that plug firing is not possible. This can cause plug wires to arc, or coil packs to arc possibly damaging both.
Spark plug temperature
Spark plug temperature is a fine balance. Too hot of a plug will "pre-ignite" the incoming air/fuel charge. Too cold of a plug will not "self clean", and it will foul with carbon deposits and incomplete combustion byproducts to the point that the plugs will not fire properly, or with proper duration and intensity.
Burning more air and fuel in the same combustion chamber volume means one thing; increased heat and increased pressure. Decreasing the combustion chamber volume (raising the Compression ratio) also increases temperature by increasing pressure.
With Nitrous Oxide use, or Forced Induction, we've got got MORE fuel and MORE oxygen...yet we are burning it within the same combustion chamber space. This means increased pressure. Increased pressure means increased temperature. The denser air/fuel charge also burns HOTTER, than a less dense charge. That increase in temperature is going to heat up the spark plug.
If the plug is "too hot" it will pre-ignite the incoming air/fuel mixture. Too cold of a plug will foul with carbon and unburned hydrocarbon deposits and combustion byproducts. In extreme cases, too cold of a plug can foul up with carbon, then when Nitrous is used, the carbon build up can stay hot and cause pre-ignition, even though the temp of the plug is cold, the carbon buildup stayed hot. (one reason why you might want to consider swapping plugs for a normal heat range when not using nitrous).
When the temp of the spark plug is "proper" the spark plug cleans itself. It stays hot enough to burn off deposits, but not hot enough to self ignite the air/fuel mixture until a spark is provided.
You want to use the hottest spark plug you can use, and still avoid pre-ignition. There is no advantage in going with a "colder" spark plug. It will just foul and cause you problems later.
For those of you that use nitrous or Forced Induction...compromises have to be made. Ignition conditions and combustion chamber conditions change drastically from Normally Aspirated (N/A) to Nitrous or Forced Induction (FI) use.
Nitrous
Using Nitrous raises the combustion chamber temperature and pressure. This heat is transmitted to the spark plug. If the plug cannot “shed” the excess heat, it will be “too hot” and will ignite the air/fuel mixture before the spark plug actually fires. Going with a plug that is too cold will foul the plug (see above description of cold plugs). Nitrous comes out damn cold, as such, it cools the intake air/fuel charge, which lowers it's susceptibility to pre-ignition. It also "cools" the combustion chamber, and absorbs the heat of the combustion chamber. Nitrous air/fuel charges are usually COLDER than ambient, which means they must absorb more “heat” before self igniting. Localized hot spots can STILL cause pre-ignition with nitrous, they may NOT be cause by the spark plug.
For Nitrous use, in general, you should go 1 plug range colder for smaller nitrous amounts, once you start to get into the 75+ range, you should consider going 2 heat ranges colder. Things vary from Nitrous kit to kit, and from engine to engine. What works great for one guy, may NOT work great for you. The above is just a guideline. Go with what works in your engine. If 2 heat ranges colder works...go with it.
Consider running with 'normal' plugs for daily driving, then change out to the colder plugs when you go to the drag strip. (Changing plugs is not that hard!)
Forced Induction
For Forced Induction, you also need to go with colder plugs. The denser air/fuel combustion chamber mix means hotter combustion chamber temps, and that means you need a colder plug.
Forced induction is similar to nitrous, in that it crams more oxygen and more fuel into the combustion chambers, and this increases temp and pressure.
FI usually has HOT intake air charge (hotter than ambient). This temperature difference between FI and Nitrous means than FI needs colder plugs than nitrous does. Even with the BEST intercoolers, the air/fuel charge will be hotter than ambient. The hotter the air/fuel charge is, the less temperature increase it takes to ignite it.
Similar to Nitrous, in "off boost" running, (which is most of the time) the plug may be too cold, allowing deposits to form that might impair the function of the spark plug that will harm performance.
Normally Aspriated
Very high compression N/A engines, or engines running "alternative" fuels also require colder sparkplugs. Anytime you increase the combustion chamber temperature (increasing compression does this), or run an alternative fuel (Nitro Methane, or alcohol fuels) you need a colder plug.
In closing
Keep in mind, that "colder" plugs are NOT affected by octane rating and vice versa. Octane is a measure of how "susceptible" a fuel is to ignite at a given temperature and pressure. The Colder plug is just there to "lower" the temperature of the plug. If there are other "hot spots" within the combustion chamber, the fuel will still ignite, if the hot spot is "hot" enough, regardless of octane value.
[edit...this is a preliminary document, it is not in it's final form...plz do not take everything in it as "gospel". Please instead consider it a "guideline".]
[ January 22, 2003, 01:35 PM: Message edited by: Random ]
Plugs break due to pre-ignition, or detonation pressures. Normally, the ceramic insulator will crack or break first, however, that insulator can fall into your combustion chamber and then get pounded into fine ceramic sand by the action of the pistons and valves. Not good. The majority of the ceramic insulator will get "blown" out the exhaust ports, but not until it is pounded down to a small enough size to sneak out the small exhaust valve opening. This can do damage to valve seats, valve faces, piston tops. Also ceramic sand/small particles can damage cylinder walls/rings. It can also damage the spark plug itself by hitting the earth or ground electrode.
Detonation and pre-ignition can also cause extremely high combustion chamber temperatures. This can melt center electrodes of spark plugs, or melt the earth (side) electrode, or both. The most "extreme" cases, damage to either the positive/earth electrode can be so bad, that either the two fuse together, short circuiting the spark plug, or damaging it in such a way that plug firing is not possible. This can cause plug wires to arc, or coil packs to arc possibly damaging both.
Spark plug temperature
Spark plug temperature is a fine balance. Too hot of a plug will "pre-ignite" the incoming air/fuel charge. Too cold of a plug will not "self clean", and it will foul with carbon deposits and incomplete combustion byproducts to the point that the plugs will not fire properly, or with proper duration and intensity.
Burning more air and fuel in the same combustion chamber volume means one thing; increased heat and increased pressure. Decreasing the combustion chamber volume (raising the Compression ratio) also increases temperature by increasing pressure.
With Nitrous Oxide use, or Forced Induction, we've got got MORE fuel and MORE oxygen...yet we are burning it within the same combustion chamber space. This means increased pressure. Increased pressure means increased temperature. The denser air/fuel charge also burns HOTTER, than a less dense charge. That increase in temperature is going to heat up the spark plug.
If the plug is "too hot" it will pre-ignite the incoming air/fuel mixture. Too cold of a plug will foul with carbon and unburned hydrocarbon deposits and combustion byproducts. In extreme cases, too cold of a plug can foul up with carbon, then when Nitrous is used, the carbon build up can stay hot and cause pre-ignition, even though the temp of the plug is cold, the carbon buildup stayed hot. (one reason why you might want to consider swapping plugs for a normal heat range when not using nitrous).
When the temp of the spark plug is "proper" the spark plug cleans itself. It stays hot enough to burn off deposits, but not hot enough to self ignite the air/fuel mixture until a spark is provided.
You want to use the hottest spark plug you can use, and still avoid pre-ignition. There is no advantage in going with a "colder" spark plug. It will just foul and cause you problems later.
For those of you that use nitrous or Forced Induction...compromises have to be made. Ignition conditions and combustion chamber conditions change drastically from Normally Aspirated (N/A) to Nitrous or Forced Induction (FI) use.
Nitrous
Using Nitrous raises the combustion chamber temperature and pressure. This heat is transmitted to the spark plug. If the plug cannot “shed” the excess heat, it will be “too hot” and will ignite the air/fuel mixture before the spark plug actually fires. Going with a plug that is too cold will foul the plug (see above description of cold plugs). Nitrous comes out damn cold, as such, it cools the intake air/fuel charge, which lowers it's susceptibility to pre-ignition. It also "cools" the combustion chamber, and absorbs the heat of the combustion chamber. Nitrous air/fuel charges are usually COLDER than ambient, which means they must absorb more “heat” before self igniting. Localized hot spots can STILL cause pre-ignition with nitrous, they may NOT be cause by the spark plug.
For Nitrous use, in general, you should go 1 plug range colder for smaller nitrous amounts, once you start to get into the 75+ range, you should consider going 2 heat ranges colder. Things vary from Nitrous kit to kit, and from engine to engine. What works great for one guy, may NOT work great for you. The above is just a guideline. Go with what works in your engine. If 2 heat ranges colder works...go with it.
Consider running with 'normal' plugs for daily driving, then change out to the colder plugs when you go to the drag strip. (Changing plugs is not that hard!)
Forced Induction
For Forced Induction, you also need to go with colder plugs. The denser air/fuel combustion chamber mix means hotter combustion chamber temps, and that means you need a colder plug.
Forced induction is similar to nitrous, in that it crams more oxygen and more fuel into the combustion chambers, and this increases temp and pressure.
FI usually has HOT intake air charge (hotter than ambient). This temperature difference between FI and Nitrous means than FI needs colder plugs than nitrous does. Even with the BEST intercoolers, the air/fuel charge will be hotter than ambient. The hotter the air/fuel charge is, the less temperature increase it takes to ignite it.
Similar to Nitrous, in "off boost" running, (which is most of the time) the plug may be too cold, allowing deposits to form that might impair the function of the spark plug that will harm performance.
Normally Aspriated
Very high compression N/A engines, or engines running "alternative" fuels also require colder sparkplugs. Anytime you increase the combustion chamber temperature (increasing compression does this), or run an alternative fuel (Nitro Methane, or alcohol fuels) you need a colder plug.
In closing
Keep in mind, that "colder" plugs are NOT affected by octane rating and vice versa. Octane is a measure of how "susceptible" a fuel is to ignite at a given temperature and pressure. The Colder plug is just there to "lower" the temperature of the plug. If there are other "hot spots" within the combustion chamber, the fuel will still ignite, if the hot spot is "hot" enough, regardless of octane value.
[edit...this is a preliminary document, it is not in it's final form...plz do not take everything in it as "gospel". Please instead consider it a "guideline".]
[ January 22, 2003, 01:35 PM: Message edited by: Random ]
