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Old 12-01-2006, 04:15 PM
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Default Spark Plug Technical Info

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. Thi
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Old 12-01-2006, 07:27 PM
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Default RE: Spark Plug Technical Info

Good write up this is pinned
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Old 08-19-2010, 12:40 PM
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spark plugs come pregapped
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Old 05-30-2011, 12:30 AM
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I would like to appreciate your work, it's too informative.. Keep up the good work.
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Old 04-03-2012, 06:43 PM
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Posts: 13

i never saw any info on the:
What Is Iridium?
How long will my iridium plugs last?
Can I use Iridium plugs with nitrous injection?
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