A. FIRST CONCEPT TO CONSIDER WITH SPARKPLUG PURCHASING: ARC-OVER-VOLTAGE
This is the major concept behind your aftermarket spark plug choice.
quote from Cycle Magazine
http://www.fatbaq.com/mainpage.phtml?topic=plugs
Platinum and gold-palladium alloys can survive the combustion chamber environment as very small wires, and in that rests their great advantage.
Electrons leap away from the tip of a small-diameter, sharp-edged wire far more willingly than from one that's fatter and rounded. So the fine-wire plug requires less voltage to form a spark than one with conventional electrodes, and the difference becomes increasingly biased in the former's favor as hours in service accumulate and erosion blunts the iron-alloy electrodes.
There are, of course, drawbacks with precious-metal plugs: they are more expensive, and they are very sensitive to excessive ignition advance. The overheating you get with too much spark lead effects plugs' center electrodes before it can be detected elsewhere in an engine, and when subjected to this kind of mistreatment fine-wire electrodes simply melt. In one sense this is a disadvantage, as it means the ruination of expensive spark plugs. Seen in another way it's a bonus feature: it is better to melt a plug electrode than an engine.
Originally posted at
http://www.se-r.net/engine/specialty_spark_plugs.html
What makes good ignition?
Ignition occurs in a modern automobile when an arc is struck and current flows between the electrodes of a spark plug, or when current migrates across the conductive medium in a surface gap plug. While that may sound simple at first, the process becomes progressively more complicated as engineers try to optimize the type of spark plug with the ignition system generating the required voltage.
The amount of voltage necessary to arc the electrode gap is set by the following characteristics:
* The size of the gap... arc-over voltage is roughly proportional to the gap size
* The air/fuel ratio within the gap... the richer the air/fuel ratio (more gasoline vs. Air), the lower the required arc-over voltage
* The compression at the moment arc-over is to occur... the higher the compression, the higher the required arc-over voltage
* The composition of the electrode... certain metals for all the same conditions stated above will require less arc-over voltage than other metals. For example, platinum requires less arc-over voltage, all other things equal, than does steel
* The shape of the electrode... the sharper and more jagged the shape, the easier it is for voltage to jump
* The amount of fouling deposits trying to remove the electron flow from the arc... more fouling deposits and lower resistance to ground pulls more energy out of the spark gap.
While it may therefore seem desirable to lower the required arc-over voltage, since without arc-over there is a total misfire and no ignition, low arc-over voltage produces low spark power because spark power is directly proportional to arc-over voltage. That is, by doubling the required arc-over voltage, you double the instantaneous peak spark power, and the higher the spark power, the better the ignition.
All ignition is, therefore, a balance between the requirement to have sufficient arc-over voltage and increasing peak spark power for better, quicker ignition.
What benefits to specialty plugs bring to this mix?
One popular specialty type is platinum plugs. The primary advantage of these plugs, especially when used in an OEM ignition system (especially an older system, which may not be producing as much voltage as when it was new), is that platinum will require less arc-over voltage and therefore, particularly in a weak ignition, allows the gap to be jumped a higher percentage of the time.
For example, if at factory gap and with steel electrode plugs, it requires as much as 18,000 volts five percent of the time to jump the arc... due to the changing engine environment and running conditions... and if the OEM only produces 17,000 volts, then it follows that five percent of the time there would be a misfire.
Now, if one installs platinum plugs, which may only require, say, 13,000 volts to arc, the five percent misfiring with steel plugs would be eliminated. Since the ignition output on OEM ignition rolls off as rpm increases, platinum plugs in this case would allow the motor to reliably turn to higher rpm, thereby giving and increase in performance and possibly gas mileage.
The disadvantage of this method of reducing misfires is that the higher arc-over voltage, the better the spark when it does fire.
Therefore, platinum plugs will show a performance improvement with a weak ignition because the benefit from reducing the percentage of misfires more than outweighs the loss from reduced spark power.
Originally posted at
http://www.sdsefi.com/techcomb.htm
The average plug consists of steel shell which threads into the cylinder head, a ceramic insulator, an iron or copper core leading to a nickel or platinum center electrode and a ground electrode of similar material. The spark jumps between the center and ground electrode. Certain special application plugs may have multiple ground electrodes.
Different heat ranges are available depending on application. For constant high power applications, a colder than stock plug is usually selected to keep internal temperatures within limits.
Again, many "MAGIC" plugs come onto the market from time to time expounding the virtues of their incredible new design, usually offering more hp of course. Split electrode plugs are a waste of money because the spark will only jump to one of the electrodes at a time in any case.
You will find that most reputable engine builders in the higher forms of racing use pretty standard NGK, Bosch or Champion plugs with pretty standard electrode setups. A properly selected, standard plug will easily last 25,000 miles of hard use in most engines. A platinum tipped plug will easily last twice as long on most engines. There just isn't any rocket science here. Modern spark plugs coupled to modern ignition systems in a modern engine are extremely cheap and reliable. In most cases, on street performance and even race engines, a $2, off the shelf, NGK plug will work just fine.
B. SECOND CONCEPT TO CONSIDER: HIGH TEMPERATURE IS NOT YOUR SPARKPLUG ELECTRODE'S FRIEND.
quote:
Spark plugs have two jobs.
1) To produce a very high temperature spark to ignite the air/fuel mixture as quickly as possible.
2) To fine tune the temperature of the combustion chamber. Number 2 is the "heat range" of the plug, an explanation better than my own (and other good stuff) can be found on this site:
www.centuryperformance.com/spark.htm
Most plugs do this just fine, with each manufacturer being a little different in their specifications (i.e. a heat range of "5" in an NGK is not the same as a "5" in a Bosch). There are equivalency tables, but don't expect them to be too accurate.
So I'll just concern myself with topic Number 1).
To create a spark, we must produce so much voltage across the air gap that the air becomes ionized (the molecules split apart and the electrons fly willy-nilly). This creates very intense heat, hopefully plenty to ignite the compressed air/fuel mixture, and the plug has done it's job (#1).
The shape of the plug electrode is important. The sharper the electrodes (both of them!) become, the easier it is to ionize the air between them. If I take two smooth brass balls of 3" diameter and put 10,000 Volts across them, I can get a spark to jump when I push them to within about 0.125" of each other. If I take two long, sharp needles pointed at each other, that same 10,000 Volts will still spark when they are 1.5" apart! (For the technically minded: this can easily be seen from Gauss' law if you compare E between the extremes of either two point charges or two equally spaced infinite parallel planes).
There is also another good reason for sharp points:
"unshrouding" the spark.
Imagine that we put a big flat plate over the tip of the ground electrode, thus shrouding the spark from the combustion chamber. The spark would still happen, and the mixture would probably still ignite, but the burn would have to go out, around the plate, and back in to the center of the combustion chamber, resulting in piston rock and detonation. We'd much prefer that the burn happen in a very smooth, ideally hemispherical manner to produce a smooth pressure curve inside that chamber. By keeping the electrodes sharp, we unshroud the spark as much as possible, allowing the maximum contact between the spark and the air/fuel mixture, making it ignite more easily and the burn spread more smoothly.
So it would seem that the sharper the electrodes, the better.
This would be true except for two caveats having to do with heat.:
Caveat 1: Too sharp a tip will melt the electrode.
If the temperature of the (spark plug electrode) tip reaches the melting point of the metal that it's made of - you can kiss it goodbye.
Here are the melting points of some commonly used metals (Celsius):
Zinc == 420
Aluminum == 660
Copper == 1083
Steel == 1400-1500
Platinum == 1772
Iridium == 2410
This problem is mainly concerned with the volume of metal at the spark tip - if the temperature even instantaneously reaches the melting point, some of that metal will disappear. You can see that Platinum and Iridium coated plugs can withstand significantly higher temperatures, and thus can have sharper tips than their steel or copper counterparts. To add insult to injury, if some of the metal does disappear from a very sharp tip, then you've actually opened up the spark gap some. To prevent that from happening, we have to start with a wider tip, such that any small amount that is eroded will not change the size or geometry of the tip by too much.
Caveat 2: Too sharp an electrode tip will create a "hot spot" in the combustion chamber.
Even if you don't reach the melting point of the metal, you can still get it glowing hot. If that tip is still glowing red hot when the next compression stroke comes about (two full engine revolutions since the last spark) that residual heat can actually ignite the air/fuel mixture before the spark is supposed to occur. This is pre-ignition. It generally creates even more heat - leaving the spark plug even hotter than the last time thus repeating the cycle until you melt a piston. Ouch!
To avoid this, we want a wide area near the tip to conduct as much heat away from the tip as possible.
Here are some of the thermal conductivities of some commonly used metals (Watts / centimeter*Kelvin) :
Zinc == 1.16
Aluminum == 2.37
Copper == 4.01
Steel == 0.70 - 0.82
Platinum == 0.716
Iridium == 1.47
You can easily see why Copper is the metal of choice for the core of the spark plug. It's just about the best thermal conductor on earth. Occasionally, you still find plugs with an aluminum core - stay away!
So, what we want is the sharpest tip possible such that it does not melt the electrode nor does it stay so hot as to cause pre-ignition. Let's break it down:
Bare Copper
They have a low melting temperature and the tips will vaporize away - they have a very wide tip so each little bit that disappears will not change the gap size greatly, but they still must be inspected often to make sure there is sufficient electrode material left. They are great for very hot running engines which must avoid pre-ignition at all costs, since the wide tip will not stay hot(high boost forced induction and nitrous engines come to mind).
Platinum
Platinum plugs are usually constructed similar to copper plugs except that they have a thin coating of Platinum sputtered onto the electrode tips, about 0.010" thick (a human hair is about 0.005" thick). Because of the high melting point of Platinum, the tips can be made significantly sharper without fear of the gap changing shape. But the copper core is still sufficient to whisk the heat away fairly quickly. These are great all-around plugs, particularly for use on NA engines, and they should last a very long time. Very high heat engines should probably not use them because the sharper tips may not conduct enough heat away to prevent pre-ignition under adverse conditions.
Iridium
This is the new guy on the block. They are much like platinum plugs just with iridium in place of the platinum. Because of the extremely high melting point of iridium, they can have very sharp tips without risk of melting and they should last a very long time. These would be best for high-rpm NA engines where the sharpest tip is needed for the best spark, but there is little danger of pre-ignition....
** Disregard any BS about the electrical difference of the metals - the micro-ohm difference in 0.010" thickness of Copper vs. Iridium means exactly squat when there is a huge air gap equivalent to tens of mega-ohms of resistance right there in series with it.
So, platinum provides a smaller sharper electrode to allow a larger arc-over-voltage and will burn off before allowing an engine to detonate. Yes it is more expensive but it lasts longer.
Basically the smaller the electrode (has less Capacitance for those into electrical stuff) then less voltage it needs to fire. The bigger the electrode the more current it can flow (good in cars with heaps of power available, huge sparks).
Now what about "special racing plugs"? Well here is what the
www.ngksparkplugs.com site says:
quote:Originally posted from
http://www.ngksparkplugs.com
Be cautious! In reality, most "racing" spark plugs are just colder heat ranges of the street versions of the spark plug. They don't provide any more voltage to the spark plug tip! Their internal construction is no different (in NGK's case, as all of our spark plugs must conform to the same level of quality controls) than most standard spark plugs.
There are some exceptions, though:
Extremely high compression cars or those running exotic fuels will have different spark plug requirements and hence NGK makes spark plugs that are well-suited for these requirements. They are classified as "specialized spark plugs for racing applications".
Some are [B] built with precious metal alloy tips for greater durability and the ability to fire in denser or leaner air/fuel mixtures .
However, installing the same spark plugs Kenny Bernstein uses in his 300+ mph Top Fuel car (running Nitromethane at a 2:1 air/fuel ratio and over 20:1 dynamic compression) in your basically stock Honda Civic (running 15:1 a/f ratios with roughly 9.5:1 compression) will do nothing for you! In fact, since Kenny's plugs are fully 4 heat ranges colder, they'd foul out in your Honda in just a few minutes.
NGK as a company tries to stay clear of saying that a racing spark plug (or ANY spark plug) will give you large gains in horsepower. While certain spark plugs are better suited to certain applications (and we're happy to counsel you in the right direction) we try to tell people that are looking to "screw in" some cheap horsepower that, in most cases, spark plugs are not the answer.
[B]To be blunt, when experienced tuners build race motors, they select their spark plugs for different reasons:
* to remove heat more efficiently,
* provide sufficient spark to completely light all the air/fuel mixture,
* to survive the added stresses placed upon a high performance engine's spark plugs,
* to achieve optimum piston-to-plug clearance.
Some of these "specialized racing plugs" are made with precious metal alloy center/ground electrodes or fine wire tips or retracted-nose insulators. Again, these features do not necessarily mean that the spark plug will allow the engine to make more power, but these features are what allow the spark plug to survive in these tortuous conditions. Most racers know screwing in a new set of spark plugs will not magically "unlock" hidden horsepower.
Simply put, in high performance N/A engines there is no need for iridium (temp.) and platinum does afford some benefits over copper in a stressed engine. In FI engines, iridium may offer some advantages but using platinum does not hurt performance. You can use platinum coated copper electrode plugs which give you the heat advantage of copper and the arc-voltage advantage of platinum...just to throw more confusion into the mix.
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Originally Posted by mad70sx
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