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| Carbon particle migration. |
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The wire design used to limit RFI in almost all OE applications is resistance wire which uses carbon particles within the core. Most every car manufacturer uses this wire type as OE, and every aftermarket supplier makes such wire available to its customers. This wire is not perfect in operation. In addition to the heat buildup mentioned above, there is a problem of carbon particle migration.
Electron flow depends upon physical contact between the carbon particles in the conductor. When mishandling occurs, such as bending, the carbon particles may move away from each other and lose contact. When this happens, the flow of electricity in the wire tends to polarize the carbon particles. When the particles polarize, they act like little magnets. When the "like ends" are next to each other, they tend to repel each other, meaning they try to move away from each other. When they succeed, a gap in the electron flow path occurs.
So long as some carbon particles are still in contact, some electrical flow can take place. However, the repelling and migration of the particles continues until a complete gap eventually occurs. When this happens, electricity flows only by jumping across one bunch of particles to another, thereby causing a spark inside the wire. (See Fig. 10)
Fig.10
This gap only gets worse, causing one of the following two problems to occur: 1) the wire burns through, giving the technician a visual clue to the problem; or 2) the gap becomes so large that it cannot be bridged, nor can the plug be fired. However, no external evidence of the gap can be found. The problem can only be detected in one of three ways: 1) analysis on an engine scope; 2) measurement for resistance with an ohmmeter (which will show up as "infinity" or an "open wire"); or 3) step-by-step replacement of the individual wire as a single unit, or replacement of the complete wire set.
| Wire damage. |
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Take a look at the diagram which shows the construction of resistance wire. The makers of the wire go to great lengths to make the wire as strong as possible. The core is made of glass fiber or some other strong "pull strength" material. The carbon particles are in a matrix of latex or other film to help maintain proper particle distribution. (See Fig. 11)
However, mishandling of the wire can still cause problems. In the old days when linen fibers were used as the core material, pulling the wire off the plug could actually break the wire internally. While a complete break is rare today, a "neckdown" condition in the core may occur when the technician pulls the wire off the plug, thereby causing excessive resistance. Neckdown problems can accelerate the polarizing condition mentioned above due to changes in the structure of the carbon matrix. (See Fig. 11).
Fig.11
Carbon core wire is simply not very strong, no matter how strong you try to make it. While it may last for many thousands of miles with careful handling, it is no match for real metal wire then it comes to maximum service life.
Another source of the problems is the spark plug boot and distributor nipple. If boots are well made and tight enough (and left on their terminal for any length of time), they may have enough "stick" to pose some real opposition to removal. The inclination of the person doing the job is to give the plus wire a good yank to get the terminal off. However, that is a big mistake! Instead, he should give the boot a small twist to break the seal between the boot and its sealing surface. Then the terminal can usually be removed with reasonable force or by using a spark plug boot removal tool.
| Pull strength enhancers. |
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To avoid damage to the wire, its conductor, insulation or jacket, manufacturers use a variety of methods to enhance its strength. We have already mentioned the use of high strength core materials such as glass fiber. However, for applications where continued rough handling might occur-or to provide a premium quality product of superior strength and reliability-- a braided covering may be used to enhance the wire's ability to withstand pulling. This braided layer is usually directly over the thick insulation and under the outer jacket.
| Other damage concerns. |
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A variety of elemental attacks take their toll on wires. Most car repair people worry about highly active materials like gasoline and cleaning solvents attacking plug wires and other rubberlike materials under the hood. However, these attacks are only part of the problem.
A list of chemicals causing problems to wires such as loss of strength, cracking, softening, etc., includes antifreeze, oil various vapor byproducts, as well as water, itself. Loss of strength due to water absorption is a major concern of materials specifiers. Ozone, from electrical spark production or local industrial atmosphere is another substance which can attack the wire cover and termination.
However, the most damaging problem other than mishandling is burning. The underhood temperature can be well over 300°F, particularly near the hot spot areas where spark plug wires wind their way past the exhaust manifold and head. In some cases, the wires pass within an inch or less of hot exhaust parts, experiencing temperatures well over 350°F.
Jacket and insulating materials for these particularly difficult temperature conditions are chosen primarily for their temperature resistance -- and are selected very carefully. These materials which resist high temperatures tend to be expensive and can drive up the cost of individual wires and sets. However, since nothing else works, they are very necessary. Where heat resistance is required, other factors must be considered secondary!
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Insulation and jackets. |
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The size and type of the insulating material between the wire and outside of the cable determines how much electrical pressure the wire can sustain. The better the insulation and the greater the distance the core conductor is from the outside of the wire, the more electrical voltage pressure the wire can carry without failure.
The material used on the cover or jacket of the wire determines the ability of the wire to live within its service environment. Heat, oil, gasoline, solvents, moisture, etc., must be prevented from attaching the jacket, insulation or wire core. The better the jacket, the longer the wire will last.
Insulation. |
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Any non-conductor which covers the current carrying wire can be considered an insulation layer. In the past, rubber was the major insulation material. However, a variety of synthetic, rubberlike compounds are used today. As explained earlier, the total insulating quality of the wire is made up of both the barrier imposed by the synthetic rubber and the distance between the wire and the outside of the cable. This is why the latest types of high-energy ignition systems (capable of putting out 30,000 to 40,000 volts of energy as opposed to the 25,000 volts of older types of systems) have an 8mm wire diameter as opposed to the older 7mm diameter.
In general, all spark plug wires made by reputable and reliable manufacturers are made "to size" as required by OE requirements. They also have one of several types of suitable synthetic rubber as the major barrier between the wire and potential electrical paths outside the wire.
Jackets. |
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The jacket hides the insulation inside the wire and provides protection from the elements.
There are many types of jacket coverings used to protect wires. Each has an advantage such as price, strength, resistance to certain elements, resistance to heat, etc. As yet, there is no ultimate material which does all jobs equally well or better than the others -- given the requirements for meeting OE standards for size, etc. Clearly, some materials are better than others for various applications and, as a result, are used when the certain environmental attack conditions exist.
Jacket materials and qualities: |
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The following are some commonly used jacket materials and their advantages and disadvantages:
| Material | Continuous service temperature (in degrees F) |
Insulating (dielectric) strength |
Gas, antifreeze, etc. resistance |
| Hypalon | 300 | average | good |
| EVA | 325 | below avg. | fair/good |
| Neoprene | 250 | below avg. | fair |
| Silicone | 500 | excellent | fair/good |
| *PVC | 140-220 | highest | good |
| *XPLE | 275 | above avg. | good |
| *TPR | 300 | above avg. | poor |
| EPDM | 350 | very high | poor |
*These are common jacket/insulation materials used for ordinary 12-volt wire which connect various car components to battery voltage. They have less temperature resistance, but the best possible dielectric strength.
The plug wire jacket materials have some rather different qualities. If you are looking for heat resistance, silicone clearly has it over every other material. However, old, reliable hypalon has superior resistance to many of the chemicals found under the car's hood. As usual, the plug wire designer chooses the best of several alternatives based upon the application.
Conclusions: |
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There is no "best" material for all applications. When you are looking at a plug wire, you are looking at a highly engineered product. A lot of thought and testing goes into the design and manufacture of OE type wire BWD packages for your customers. We follow OE design requirements for the vast majority of applications. However, for certain applications where there is a known problem -- high heat conditions being most common -- we use silicone jacketing.
Jacket colors. |
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The color of the wire's jacket has nothing to do with its performance. Wherever possible, BWD tries to match the jacket color used on the OE vehicle. However, a substitute color in a wire of the right type, length and termination does not affect spark plug wire performance.
Plug wire terminals. |
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Terminals are designed to make a tight connection at the end of the plug wire. BWD uses 100% snap-lok spark plug terminals that are designed to snap onto a standard spark plug tip. (See Fig. 12)
Fig. 12
Distributor termination. |
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Older styles of distributor cap termination had a springlike tube which was designed to take a tight fit inside the distributor cap. This type of termination is going out of style as more powerful ignition systems put more demands on the terminals and their insulation.
Modern termination looks the same at both ends, with a positive snap-lok on the distributor terminals. In fact, on some GM cars a special retainer makes certain the wires make good contact and will not pull off accidentally.
In some cases, where extremely high potential electrical voltages are produced, manufacturers call for the use of a special silicone grease inside the distributor cap boot.
