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Spark Plug Technical Info

Old 12-01-2006, 03:39 PM
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As the source for spark plugs, we pride ourselves in the technical spark plug information contained on our website. We enjoy providing that technical information on the forums, so here are some

of the common topics and questions in regards to spark plugs. If you have any additional questions, please feel free to ask!

Subject covered by this post:

Basic Spark Plug Construction
How do I cross reference from one brand to another?
What are resistor plugs?
What are V-cut or U-grooved plugs?
Multi-Ground Plugs
What are Fine Wire Plugs?
What Is Platinum?
What Is Iridium?
How long will my iridium plugs last?
Can I use Iridium plugs with nitrous injection?

Basic Spark Plug Construction

Let's start out with the basic construction of a spark plug.

Starting at the top, the TERMINAL can come 3 ways:
stud - some wires are made to fit over plugs that don't have a terminal nut on top, the plug is produced with the terminal nut left off.
solid - the terminal nut is permanent and can not be removed. Used particularly in the motorsport and marine industry when there is a lot of movement and vibration and a removable terminal nut

could come loose.
removable - the plug comes with a terminal nut, but it can be removed.

HEX - This is the area your socket grabs when removing or installing plug. For automotive applications, plugs usually come with a 5/8 or 13/16 hex. Vehicles prior to about 1980 allow for a 13/16

hex, most after 1980 only allow 5/8.

SEAT - Plugs are available in a tapered seat or with a gasket. The two are not interchangable - in order to use a plug with a tapered seat, your cylinder heads must have been made specifically for

the use of a plug with a tapered seat.

REACH - The plug reach is measured from the seat to the end of the threaded are (do not include ground strap in measurement).

THREAD DIAMETER - Accurate Measurement of the cylinder head or removed plug is necessary to determine the plug diameter, which may range form 8mm to 18mm.

GROUND ELECTRODE - Ground electrodes come in a variety of shapes and sizes. They are also called by a variety of names depending on manufacturer, IE: trapezoid cut ground, tapered cut

ground, fine wire ground, angled ground, trimmed side electrode, wedge shaped ground, inverted V-tip ground, cut back ground, etc. All have the same purpose, to reduce quenching and

shadowing. Ground straps will be discussed in more detail in future technical threads.

CENTER ELECTRODE - A traditional center electrode is 2.5mm. Manufacturers have improved spark plug performance by creating fine-wire, taper cut, necked down and v-power center

electrodes. Each of these will be discussed in further detail in future technical threads.

GAP - A spark plugs’ tip temperature and the voltage necessary to fire the plug are directly affected by the gap setting. Most manufacturers set the gap from the factory for that plugs most popular

application. Unfortunately, that plug may have hundreds of applications from automobiles to golf carts. Setting the gap for your particular engine is important as insufficient spark plug gap can

cause pre-ignition, detonation and even engine damage. Whereas too much gap can result in a higher rate of misfires, loss of power, plug fouling and poor fuel economy. Even if the preset gap is

supposed to match your motor, it is always best to physically check that the gap is adjusted properly for your motor prior to installation as the gap may have been changed during shipping.

How do I cross reference from one brand to another?

From the home page, type the part number you wish to cross-reference from into the "PART NUMBER/CROSS-REFERENCE" box located in the center of the page.
DO NOT include the manufacturers name in the part #, In other words if you are crossing over an AC MR43T, you would put in MR43T, NOT ACMR43T.

If the plug you want to cross is available, that part will be displayed in the results, then click on "Cross-reference to other brands"You will be supplied with the equivalent part number, or if the part

number you selected is common with more than one manufacturer, you will be asked what brand you are crossing from.

What are resistor plugs?

At the moment the spark jumps the gap it causes a high frequency burst of energy, this is known as RFI (radio frequency interference). This is why resistor spark plugs were introduced in the mid

1960's. Placing a resistor within the spark plug suppresses the RFI. Without resistor plugs in your car you can experience static on your radio as well as interfere with other sensitive electronic

equipment. Some later model vehicles as well as newer Powersport engines must use resistor plugs for a proper "talkback" to the electronic ignition. Outboard marine Capacitive Discharge

Ignition (CDI) such as used on some Johnson and Evinrude marine engines require a special inductive type resistor (such as a Champion Q-type or NGK Z-type). Use of non-inductive resistor type

plugs on these motors can cause misfire and poor performance.

What are V-cut or U-grooved plugs?

V-cut ground electrodes are called a variety of names depending on manufacturer. Although they look similar, they may be called a tapered cut ground, trimmed side electrode, wedge shaped

ground, v-trimmed electrode, inverted v-tip ground, v-power (NGK) or U-groove (Denso). So what's the point? The v-cut give the flame a groove to grow in and forces the spark to the outer edge of

the ground electrode - placing it close to the air/fuel mixture and creating a larger flame that ignites quicker for a more complete combustion, even in the case of lean air/fuel mixture. A

conventional, flat ground electrode design of regular plugs crushes the flame (sometimes called quenching), thereby preventing full spark potential.

Standard Plug Causes Quenching

NGK V-Power Reduces Quenching

Denso U-Groove Reduces Quenching

Multi-ground Plugs

Some combustion chamber designs, such as rotary motors, require that the spark plugs have the ground electrode placed to the side of the center electrode rather than below as on a traditional

plug. This firing tip design tends to erode the tip of the ground electrode faster than a traditional plug, and as erosion at these points creates a larger gap between the center and ground

electrodes, plug misfire will occur. Thus, by having more ground electrodes, you extend plug life. It is important to note that multi-ground does not mean multi-spark - electricity follows the path of

least resistance, there will still only be one spark at a time. Therefore a multi-ground plug will not perform any better and may actually perform worse than a traditional plug, unless the engine is

designed for a multiple ground plug.

Standard Plug

Multi-Ground Plug

What are Fine Wire Plugs?

Fine wire center electrodes come under a variety of names depending on manufacturer, IE - Tapered point, Ultra-Fine electrode, Taper cut electrode, Necked down electrode). Originally designed

to improve starting and reduce fouling in two-stroke engines, this design was found to improve performance in four-stroke engines as well.

All operate on primarily the same principle, a spark plug with a fine wire electrode will perform better than a traditional plug. There are two reasons for this, first is because a smaller center

electrode requires less voltage to jump the gap. This means fewer misfires, which should be seen in higher mileage and more horsepower. The second reason is smaller center electrodes reduce

quenching. The smaller center electrodes have required exotic metals such as platinum or iridium so that they can still maintain (and usually surpass) the longevity of a traditional spark plug.

Currently the finest wire performance plugs available are made by Denso at 0.4mm diameter, by NGK at 0.7mm diameter, and by Champion at 1.1mm, a traditional center electrode is typically 2.0

to 2.5mm.

Standard Center Electrode

Fine Wire Center Electrode

What is Platinum?

Platinum is a precious metal used by nearly all spark plug manufacturers on their long life and/or performance spark plugs. This is because of platinums high melting point which makes it useful in

two ways.

#1 - On long life spark plugs, a thin wafer of platinum is bonded at the firing point to the center electrode (and possibly ground electrode) solely so they dont wear as fast as a traditional plug.

#2 - On a fine wire performance plug, the very tip of the center electrode is made of platinum so that the fine wire tip will last longer.

Do not be fooled, all platinum plugs are not created equal, Platinum is a very expensive precious metal, a $2 platinum spark plug will not have much platinum in it, and therefore will not last as

long as a $12 platinum spark plug. Some platinum plugs have only the center electrode tipped with platinum, while others have both the center and ground electrodes platinum tipped.

(By the way, it is still not suggested that platinum plugs be used on vehicles with nitrous injection. Thus far, there has been no problems reported regarding using iridium plugs with nitrous.)

What Is Iridium?

Iridium is a precious metal that is 6 times harder and 8 times stronger than platinum, it has a 1,200 degree higher melting point than platinum and conducts electricity better. This makes it

possible to create the finest wire center electrode ever. (See "What is a fine wire plug" for information on the benefits of fine wire plugs)

Prior till now, spark plug manufacturers have favored platinum for their long life or performance spark plugs due to its high melting point. Also, the technology did not exist to machine and bond

iridium on a spark plug electrode (at least in a cost effective manner).

Champion spark plugs have produced iridium industrial and aviation spark plugs since the 1960's, but they still sell for over a hundred dollars per plug. Just now is the technology cost effective to

use iridium in a spark plug for automotive applications.

The strength, hardness and high melting point of iridium make it very well suited for a fine wire plug. The primary iridium plug manufacturers at this time are Denso with a 0.4mm center electrode,

while Champion and NGK have 0.7mm center electrodes. These are the best performance plugs on the market for traditional automotive use and many racing applications.

SIDENOTE: Thus far the tech's we have spoken with report no problems using iridium plugs with Nitrous.

Metal Properties

How long will my iridium spark plugs last?

A traditional iridium plug such as a Denso ITL16 or NGK LZTR6AIX-13 both have iridium tipped center electrodes, however the ground electrodes are the traditional nickel construction(the same

ground electrode that single platinum and traditional 'copper' plugs have). The ground electrode will wear out first. The manufacturers are saying 40,000 to 60,000 miles on iridium plugs. But they

have to temper their projections as driving conditions and motor modifications differ. Typically we have found you can expect 60,000 to 80,000 miles on an unaltered motor. (the family mini-van has

had the Denso's for 76,000 miles, I examined 2 of the plugs, they had some minor spooning on the ground electrode, I put them back in and will likely change them in another 5,000 miles).

Both Denso iridium racing plug and some NGK Iridium racing plugs have iridium center and a platinum ground electrodes. If installed to a regular engine they would likely last longer than most

people keep their car (barring any motor problems that can cause premature plug death). But, these usually come in heat ranges too cold for an unaltered motor and are usually used in racing

applications where all bets are off.

For the traditional automotive market, the longest lasting plugs are the NGK "IFR" series and the Denso "SK" series. These have Iridium center and platinum tipped ground electrodes, however

these are extremely limited in application as they only manufactured these in a couple of heat range configurations.

Nitrous 411 & FAQs

Nitrous Oxide is an efficient means of getting more oxygen into the combustion chamber. Initially the
N2O injection will cool the incoming charge due to the energy expended on vaporization into the air fuel mixture. However, do to the dramatic increase in power output, the sustained use of N2O

increases cylinder temperatures to the point you may require a spark plug one or two heat ranges colder. If the nitrous is used only for very short bursts, (less than 10 seconds), then the standard

plug can likely be used with no spark plug heat change necessary.

Can I use platinum plugs with nitrous injection?

No, it is not suggested to use platinum plugs with nitrous oxide injection.
There have been instances where the platinum tip has lost its bond to either the center or ground electrode when they were used in a motor with nitrous.
Thus far the tech's say they have had no problems using Iridium plugs with nitrous.

Can I use Iridium plugs with nitrous injection?

Yes. We double checked with the tech’s on this one, they say, while they have been watching for problems, thus far, there has been no reports of any problems in using iridium plugs with a

nitrous system.
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