The ignition systems on modern cars is highly complex. Even on classic cars that technology has moved on but you probably don’t realise it. The humble spark plug is a highly complex part and the plug leads that supply the power are no exception.
Here I will try to explain some of the basics, how they are made, how they work and how you can make your own custom plug leads!
HT ignition cable sets can vary from £15 to £150 (or more depending requirements).
Spark Plugs can very from £2 each to easily over £20 each
Crimping tool £5 to £25
Spark plug Gap tool / feeler gauges £2 to £30
What is an Ignition, Spark Plug or HT lead?
It’s a simple but critical cable which transfers the power from the distributor cap to the spark plug, where it provides the energy to create a spark at the plug, which in turn ignites the air/fuel mixture.
The ignition lead is comprised of a conductive material surrounded by a silicone jacket. The primary purpose of the silicone surround is protection. Ignition leads sit in close proximity to the engine itself and so are subject to dramatic changes in temperature, caustic chemicals and frequent abrasion. A silicone outer layer helps to protect the conductive core from external damage, extending its lifespan.
The core itself, which carries the current, will either be made of copper or carbon, depending on the age of the vehicle. Older vehicles used copper, modern ones carbon. Now days older cars can use the modern carbon style for better reliability and consistency and still look stock.
It’s a common myth that bigger and more elaborate HT leads will give you more horse power. What they will do is provide the optimum efficiency and stability that may have been lacking on older worn leads. They should last longer and keep performance at a constant level.
How do you know if the HT leads have gone bad?
1. Decrease in power, acceleration, and fuel efficiency
One of the most common symptoms of an issue with the ignition cables is engine performance problems. The ignition cables carry the spark from the coil and distributor to the spark plugs so that engine combustion can occur. If there is any issue with the spark plug wires the engine spark can be disturbed, which can result in engine performance issues such as misfires, a reduction in power and acceleration, as well as a reduction in fuel efficiency. In severe cases bad cables may even result in engine stalling.
2. Visible wear or damage to cables
Visible wear or damage are symptoms of issues with the ignition cables. Old cables may dry out or perish, which may lead to cracks in the insulation. There are also instances where the cables may rub up against a hot manifold or engine components, which may cause them to melt and burn up. Both of these issues may compromise the cable’s ability to transport the spark to the spark. This can lead to misfires and other performance issues, and in more serious cases may even lead to the cables shorting against the engine.
3. Check Engine Light comes on (modern cars)
Faulty cables can lead to engine misfires as well as excessively rich air fuel ratios, both of which can set off the Check Engine Light if detected by the computer. Many modern vehicles are now being manufactured without the good old fashioned ignition cables, however they are still found on a very large majority of older cars.
Process To Make Your Own:
Regardless of the vehicle, making the wire is the same in principle. Most of the time the ‘generic’ HT leads tend to have the spark plug boot already connected. Most kits will give you an option of metal crimp ends and rubber boots depending on application.
These can be straight, 90deg or 115deg angles (as below) etc. depending on the manufacturer, engine fitting, style, age and type of engine etc.
These HT leads can be single colours, multi coloured, cloth wrapped, silicon covered or custom variations of anything in between.
The spark plug boot fittings can be pretty much anything on top of the spark plug terminal, to nothing with just the thread or a simple screw down for the clamped earlier designs.
Or any of the following styles; stud, clip, screw, solder or crimp etc.
My Mustang leads are set to the 115deg from the plug with an open end which allows you to cut the wire and route where you want it. Or you could buy prefabricated leads without the hassle or making them of course. But you may be limited to the routing of the wires that you wanted.
Process to attach the fittings.
Making your cable connection will require a plug crimping tool, usually around £10 to £25 for a ratchet set. They have the main crimping section at the bottom of the jaws, outer layer cutting jaw and then the inner cable crimp at the top, if needed.
Measure the length of the cable you need and trim approx 1″ longer than you need, this will allow for trimming and crimping the core. Better to be a fraction to long than too short and put stress on the cable.
Depending on the connection you have you may need to trim to the inner core and crimp twice.
If there are no boot(s) on the cable you will need to put these on first. Make sure they are the correct way round. This is so often forgotten and will lead to a wasted crimp and an even shorter cable when you have to redo them. To make life easier to slide the cable through the boot is apply a little dielectric grease. this protects the rubber and also allows electrical current through it just in case it gets on the centre core.
My cables needed to expose the inner core only. Trim of the outer silicon and the inner layer to leave an exposed amount of core.
Fold the core cable over back on itself. making sure that the core is on the back of the fitting.
Place the fitting and cable into the crimps with the open side on the fold part of the jaws.
Slip the boot over the fitting and make sure it seats correctly. With both ends crimped and ready to fit test the integrity of the lead via a multimeter ‘ohms’ setting with a tester prong at each end of the cable. With everything in place you can fit the cable.
Note; the distributer cap boot fittings (male fit) tend to extend past the rubber as it has to sit deeper into the distributor cap as in this pictures here to make a contact.
Repeat for each lead you need.
This is probably the least thought about part of the engine and least understood. Yet these relatively cheap disposable parts can make a huge difference to the running of the engine, and the designs behind them can get highly technical. i will try to break it down into smaller sections.
What is the Spark Plug?
This is the part of the engine that detonates the air fuel mixture in the engine’s cylinders to make the explosion to force the piston down to rotate a crank, which in turn drives a shaft to the gearbox.
Design of the plug will dictated by the engine manufacturer, such as where it will be located how far from the chamber etc. things like centre mounted plugs on an over head cam tend to be thinner and longer to reach the top of the cylinder. Traditional side mounted mounted plugs can be shorter or fatter depending on where it needs to sit in the cylinder head.
A good rule of thumb for the correct depth is something like this, all the information will be in the manufacturers guides.
To shallow a plug and the spark will not ignite the fuel/air in the cylinders correctly. To long a plug and there is a danger that the top of the piston could hit the plugs. The incorrect depths could also play a part in heat dissipation, fuel economy, power and even the overall drivability of the car.
A selection of various designs from short to long, to thin to fat and marine application.
The firing end has a number of different ways to create the spark, single, double and even quadruple toe electrodes, angled etc. Each design will give a specific spark for a particular engine. Here is a description of the more common ones.
Standard spark plugs typically feature a traditional ground electrode.
Double fine wire electrode (DFE) spark plugs apply a fine wire pin to the ground electrode in addition to a fine wire center electrode. A smaller electrode requires less voltage to jump the gap, resulting in fewer misfires, which translates to increased fuel economy and horsepower. A smaller electrode also reduces flame quenching. Reducing the electrode size on a standard nickel plug would result in a drastically shortened life span, so smaller electrodes require exotic metals such as platinum or iridium to maintain (and at times surpass), the longevity of a traditional spark plug.
A flat ground electrode is shorter and closer to the metal shell and center electrode, providing a faster path to transfer heat away from the ground electrode. Its low profile design is resistant to vibration.
Developed for engines that tend towards increased carbon depositing as a result of their design, hybrid spark plugs have a standard ground electrode as well as two smaller ground electrodes on each side. When the insulator becomes clogged with carbon, the ignition voltage jumps over to the side electrodes, enabling the plug to operate even under severe conditions. Once the plug has reach operation temperature and the deposits are removed, it returns to “normal” operation, with the spark jumping between the center and main ground electrode.
A low angled ground electrode is shorter and closer to the metal shell and center electrode, providing a faster path to transfer heat away from the ground electrode. Its low profile design is resistant to vibration. A smaller electrode requires less voltage to jump the gap, resulting in fewer misfires, which translates to increased fuel economy and horsepower. A smaller electrode also reduces flame quenching. Reducing the electrode size on a standard nickel plug would result in a drastically shortened life span, so smaller electrodes require exotic metals such as platinum or iridium to maintain (and at times surpass), the longevity of a traditional spark plug.
Some engine designs require the spark plugs have the ground electrode placed to the side of the center electrode rather than below as on a traditional plug. This may be for combustion chamber design as in the case of a rotary engine, or a surface gap design as used in leaner air/fuel ratio’s on industrial engines. The side electrode design tends to wear faster than a traditional plug. Erosion at these points creates a larger gap between the center and ground electrodes, causing plug misfire. Thus, if the engine design requires a side discharge plug, more ground electrodes extend plug life.
Multi-Ground plugs are offered in 2, 3 and 4 ground electrode designs. It is important to note that multi-ground does not mean multi-spark, there will still only be one spark at a time. Caution should be made in selecting a “high performance” plug.
If your car came OE with a multi-ground plug, your engine will likely wear through single electrode plugs, especially fine wire plugs, at a rapid rate.
Projected square platinum electrode (PSPE) spark plugs apply a square shaped tip of platinum to the end of a shortened ground electrode.This ground electrode shape allows the spark to be focused between the fine-wire center electrode and projecting platinum ground electrode. A smaller electrode requires less voltage to jump the gap, resulting in fewer misfires, which translates to increased fuel economy and horsepower. A smaller electrode also reduces flame quenching.
In a semi-surface discharge design, the voltage path skims across the surface of the insulator. When the spark discharges, it burns off any carbon build-up. The wide gap improves ignition capability and is less sensitive to gap growth. Additionally the concave cut in the ground electrode promotes even gap growth.
A slant ground electrode is shorter and closer to the metal shell and center electrode, providing a faster path to transfer heat away from the ground electrode. Its low profile design is resistant to vibration.
A thin square pad of platinum is welded to some ground electrodes to ensure durability.
True surface discharge or surface gap spark plugs have no side electrode, instead utilizing the entire face of the plug shell as a ground to ignite. Thus the gap remains constant through the plugs entire life. They have no given heat range as the electrode design prevents the firing tip from overheating, and the insulator is flush with the metal shell to dissipate heat quickly. Therefore, these plugs are susceptible to fouling in cold applications.
Surface discharge plugs may be required in high compression applications or with high energy ignition systems. They are also used in rotary engines as they present a flush face to the combustion chamber, eliminating interference with an electrode tip and exposing the spark to the entire air/fuel mixture for improved combustion.
Many variations of the surface discharge plug exist, including the semi-surface discharge, intermittent gap, supplementary gap, and surface air gap plug. All designs create a spark along the insulator nose to remove carbon build-up.
A taper cut ground electrode serves a similar function to a cut back ground, fine wire ground and angled ground electrode. Also known as an inverted v-tip, tapered v-profile, trimmed side, v-trimmed or wedge shaped ground electrode, all trimmed designs have the same purpose: to reduce quenching and shadowing by reducing the surface area between the electrodes which could hinder the growth of the flame nucleus.
A trapezoid cut ground electrode is a variation of a taper cut ground, which serves a similar function to a cut back ground, fine wire ground and angled ground electrode. All trimmed designs have the same purpose: to reduce quenching and shadowing by reducing the surface area between the electrodes which could hinder the growth of the flame nucleus.
When the air temperature is high, as in the summer, the inlet air temperature becomes higher, increasing the load on the engine. In times like this, it’s better to choose a plug with a higher heat range.
Various circumstances such as engine conditions and driving conditions are rough measures for choosing a plug. If you are doing lots of strenuous driving for a considerable time using normal plugs, the plug will overheat. This is why the idea of a Standard plug fits all for all car’s doesn’t work.
The heat is controlled by the design of the inside of the spark plug head More material that can be in contact with the cylinder the greater the cooling. (The dark grey shading represents a hollowed out area between the insulator and the thread.)
Each manufacturer has their own coding for heat ranges, this is a comparison of some popular plugs.
If the horsepower has been increased through tuning, the increase in explosive power leads to an increase in combustion chamber temperature, making pre-ignition more likely; in such cases it is necessary to choose a plug with a higher heat range and a higher level of heat resistance provided by Denso. The same with other plugs of course just different numbering approach.
Types of Spark Plug Cores:
Sparks like to jump from the sharpest point on the center electrode to the sharpest point on the side electrode. Ideally you want a plug that retains its sharp edge for the most amount of miles covered. Precious metals like platinum and iridium are harder metals and have higher melting temperatures than the nickel alloy electrodes found in traditional copper spark plugs. The smaller the diameter of the center electrode, the lower the voltage needed to start the spark.
These plugs have a solid copper core, but the business end of the center electrode is actually a 2.5mm-diameter nickel alloy. That’s the largest diameter electrode of all the spark plug types. Remember, the smaller the diameter, the less voltage required to initiate the spark. Nickel alloy is also softer than either platinum or iridium, so the sharp firing edge you get right out of the box tends to wear out quickly. Despite those shortcomings, copper spark plugs are still a good choice for certain applications. Copper spark plugs are best for older (pre-‘80s) vehicles with low voltage distributor-based ignition systems. Don’t use copper spark plugs in high-energy distributor-less ignition systems (DIS) or coil-on-plug (COP) ignition systems. They’ll wear out too quickly.
There’s one exception to that advice. Some late-model high-performance engines were designed specifically for copper spark plugs. In those cases, copper spark plugs are considered to be high performance spark plugs. If your owner’s manual calls for copper spark plugs, don’t upgrade to platinum spark plugs or iridium spark plugs.
A single platinum spark plug is basically styled after a copper spark plug with a platinum disc welded to the tip of the center electrode. Since platinum is harder than nickel alloy, it holds its sharp edge for as long as 100,000 miles. Platinum spark plugs also run a bit hotter, preventing spark plug deposit build up and fouling.
Platinum spark plugs are usually the best spark plugs for newer vehicles with electronic ‘distributor-based ignition systems’ (DIS). If your owner’s manual recommends platinum spark plugs, don’t downgrade to copper spark plugs to save money. However, you can upgrade to either double platinum spark plugs or iridium spark plugs.
Double platinum spark plugs were designed for “waste spark” DIS. In a waste spark system, the spark jumps from the center electrode to the side electrode for the cylinder that’s on the compression stroke. To return the electrical pulse back to the ignition coil pack, the spark jumps backwards (side-to-center) on the partner cylinder. Since the partner cylinder is on its exhaust stroke, nothing ignites and the spark is “wasted.”
You can’t use single platinum spark plugs or traditional copper spark plugs in these systems because the side electrodes aren’t designed to handle the reverse spark. But double platinum spark plugs, with a platinum disc welded to their side electrode, work exceptionally well. Both the center and side platinum discs remain sharp, allowing sparks to fly easily in both directions without causing rapid electrode wear.
If your owner’s manual recommends double platinum spark plugs, then those are the best spark plugs to use. You should never downgrade to single platinum spark plugs or copper spark plugs. However, you can upgrade to an iridium/platinum combination plug (an iridium center electrode with a platinum-tipped side electrode).
Iridium is harder than platinum, and in most cases, iridium spark plugs last approx 25 percent longer than comparable platinum spark plugs. Because iridium is costly, iridium spark plug manufacturers reduce the diameter of the center electrode to as little as .4mm. In addition to saving money, the “fine wire” center electrode on iridium spark plugs increases firing efficiency.
Many carmakers require iridium spark plugs or iridium/platinum combination spark plugs for coil-on-plug (COP) ignition systems. If your owner’s manual specifies iridium spark plugs, don’t downgrade to platinum spark plugs, or double platinum spark plugs, or even copper spark plugs. They won’t perform as well as the iridium spark plugs and cause some potential problems with running.
As the spark ignites the air/fuel mixture, the colder side electrode tends to “quench” the flame. To combat quenching, some spark plug manufacturers cut a “U” or “V” shaped channel into the “spark receiving” surface of the side electrode. The larger channeled area reduces quenching and allows the flame to grow more quickly. Other manufacturers split the end of the side electrode to reduce the flame’s contact with the side electrode and allow the flame to shoot straight down into the cylinder.
Are these type of plugs better? There’s no general consensus or conclusive evidence on whether these designs work better or not. However they will perform at least as well as a non-channelled plug, if not better.
Condition of the Spark Plugs:
These little pieces of technology can tell you an awful lot about the condition of your engine, at each service you could take them out and check, but you don’t have to. Many years ago when spark plugs were a standard core, they used to be replaced with every car service along with points. With modern computer controlled injection, you should be running ‘normal’ most of the time.
On carburettor cars the tuning and air/fuel mixtures are much easier to spot.
A visual inspection of the spark plugs could like the following;
If the firing end of the spark plug is brown or light grey, the engine condition can be judged to be good and the spark plug is functioning correctly.
The gathering of different deposits on the firing end is influenced by oil leakage, fuel quality and engine operating period. The deposits can come from Carbon, Lead, Bromine, Calcium, Sulphur, Barium and Zinc.
Dry and Wet Fouling
Dry and Wet Fouling is another way the firing end can be damaged. If the insulation resistance between the centre electrode and the “shell” is over 10M ohms the engine can start up normally, however if the insulation resistance drops to 0 the firing end is fouled by either wet or dry carbon.
If your spark plug firing end looks yellowish brown on the insulator nose, this is found on spark plugs that have been damaged by lead. Also, this particular type of damage cannot be detected by a resistance tester at room temperature. Lead compounds combines at different temperatures; those formed at 370-420 degrees Celsius have the biggest influence on the resistance.
If your spark plug has overheated, the insulator tip is glazed or glossy. Deposits which have gathered on the insulator tip have melted, and there is a chance that the insulator will have blistered.
Breakage is normally caused by a thermal shock due to sudden heating or cooling – replace immediately!
A worn spark Plug not only wastes fuel but also strains the whole ignition, this is because is requires a higher voltage. A worn spark plug can reduce the engine efficiency by reducing the fuel economy and increases the exhaust emissions. For your reference, The normal rate of gap growth is about 0.01 0.02mm/1,000 Km for four stroke engines and about 0.02 0.04mm/1,000 Km for two stroke engines.
Erosion, Corrosion, Oxidation
The electrodes have oxidized, and when the oxidation is heavy there will be green on the surface. The surfaces of the electrodes are also fretted and rough.
An Abnormal Erosion is caused by corrosion, oxidation, or reaction with the lead. This results in abnormal gap Growth.
Lead Erosion is caused by the lead compounds in the gasoline which react chemically with the material of the electrodes (nickel alloy) at high temperatures. Crystals of nickel alloy fall off because of the lead compounds permeating and separating the grain boundary of the nickel alloy. Typical lead erosion causes the surface of the electrode to become thinner and the tip of the electrode looks like it has been chipped.
If the firing end is melted, this means it has over heated. Mostly, this will result in the electrode surface being rather lustrous and uneven. As note, the melting point of nickel alloy is 1,200 – 1,300 degrees Celsius.
What’s important is to inspect the condition of your current plugs, and then choose a plug accordingly.
Fitting the wrong plugs could damage your engine.
A chart for comparing different makes and designs of spark plugs.
Some cars run on better brands than others. In my past experience NGK were a great plug, but didn’t last long. Bosch platinum were great when warm on lots of motorway miles, but a little lumpy when cold. Trial and error depending on your driving style and engine will get the best results for economy and power.
Spark plugs are designed to last and give optimum performance for as long as possible. Racing plugs in racing engines tend to be designed to give the max for shorter periods of time. They wouldn’t necessarily make big performance gains in road cars, unless you intend to drive it like you stole it!
Copper cored plugs you can expect between 10,000 to 20,000 miles.
Platinum or Iridium tipped plugs can be expected to last 60,000 miles.
Some top quality branded plugs are expected to last 120,000 miles before changing.
You pay your money and take your chances. Buying cheap plugs where bits can break of in the cylinder could possibly destroy a good engine.
Setting the Spark Plug’s Gap:
The gap sizes are pre determined by the manufacturer to get the correct combustion in the engine’s cylinder or combustion chamber. These should be adhered to unless specialist tuning requires the changes.
Such an instance to increase the gaps could be changing from old school standard points ignition to a Pertronix electronic ignition. The sensor in the distributor cap does not have metal contacts to take the voltage. The sensor sends a signal to the ignition coil which can produce more voltage and power as a result. This in turn can allow a small increase in the spark plug gap to give a stronger more powerful spark. The theory is that the better spark gives better combustion of more fuel air mixture and could give you more horse power with the correct carb settings.
* Narrow Gap
When the spark plug gap is too narrow, or under specifications, the amount of room needed for the air-fuel mixture between the hot tip and the ground strap is decreased. The duration of the spark has less travel distance, thus not remaining hot enough with sufficient charge to ignite the air-fuel mixture. The symptoms of narrow gap include a noticeable (continuous) cylinder miss, hard starting if all the plugs have narrow gaps, rough idle and engine hesitation. A spark plug that does not fire, resulting from a narrow gap, will appear black or wet when inspected. The black or wet appearance indicates unburned fuel.
* Excessive Spark Plug Gap
Excessive spark plug gap results when the voltage has too far to travel. The increased length of the spark travel weakens it, robbing it of the hot, strong ignition charge it needs to fire the plug. Excessive plug gap also results in cylinder misfiring, a possible no-start condition, wet, black or fouled plugs, engine hesitation and rough idle. Excessive spark plug gap also happens as result of normal electrode wear and age.
Incorrect spark plug gaps as above can cause the following symptoms;
* Rough Engine Idle
An engine that has a rough, irregular engine idle is often due to spark plugs that are incorrectly gapped. Spark plugs that are gapped incorrectly it will upset the air/fuel mixture, which in turn affects engine combustion and idle.
* Engine Hesitation
If spark plugs are gapped too wide, the ignition spark loses some of its strength as it crosses the spark plug gaps. This can result in engine hesitation.
* Engine Missing
Spark plugs gapped incorrectly can cause an engine to miss, or run erratically, especially during idle. The incorrect spark plug gaps can cause uneven firing of individual spark plugs and delay engine combustion; both of which can cause an engine to miss or idle erratically.
* Poor Engine Performance
For an engine to run at optimal levels, its spark plugs have to be gapped according to factory specifications. Any spark plugs that are gapped incorrectly can result in poor engine performance by altering engine combustion, weakening the ignition spark, and delaying the arcing of the ignition spark as it crosses each spark plug diode.
* Engine Knocking
Engine knocking, or engine pinging, is a classic sign of an engine that has incorrectly gapped spark plugs. Engine knocking is the result of incomplete or delayed engine combustion, especially under acceleration.
To set the gaps.
Remove the spark plugs and inspect them for damage. Inspect the firing end for condition and to see how the engine is running.
Remove any loose fowling or oil on the plug first. Take a small brass wire brush and remove the deposits until the metal surfaces are clean. Brass is a softer metal than the spark plug and will not damage it, but should be strong enough to remove any build ups.
If your spark plug has a terminal cap, check that it is fully done up. Use a pair of pliers to pinch it tight. If the cap is loose this will cause issues for connection and will give you erratic sparks.
Use you spark plug gap tool of choice to set the gap if you need too. There are the disc styles, feeler gauges and open loop styles. I like feeler gauges or the hoop wire design. These can be metric or imperial measurements depending on your choice.
Once I know the current gap by placing the correct gap setting tool into the gap, I can either open or close the gap as required. To close you can gently tap the electrode on a solid metal surface, or use T-shaped part of a special plug gap tool to close it. In order to open use the tool again and lever it upwards. If you don’t have the tool, be careful not to damage the electrodes at the bottom when trying to open the gaps.
Two different ways to gap spark plugs. Remember that when tapping the electrode at the bottom, the weight of the plug itself is enough to close the gap.
DON’T over tap it when closing the gap, little and often. Check, repeat.
The gauge or setting tool that you use, should have a slight drag on it when you pull it through the gap.
When refitting the spark plugs back into the engine always tighten the first few turns by hand first to avoid cross threading. Don’t knock the spark plug when inserting it. This could alter the spark plug gap and give you problems as described above.
If in doubt – recheck it.
Replace the spark plug boot with a very thin smear of dielectric grease to stop the rubber boot sticking to the top of the plug.