Slapped Together By Paradox Productions & Two Other Vagines

 

We have never made a secret of it and we are not going to start now! Squared Arches exists and continues to improve thanks to the support we recieve from people like you who have ideas on how we can make the best VN / VP resource available. If you have any suggestions of what you would like to see added to the site or any information you could contribute to help make what we have even more comprehensive we need to hear from you. Perhaps you have spotted a mistake that needs correction- let us know and we'll climb all over it! Full credit will be given for all corrections and information given.

And then of course we have our galleries of events, submitted cars where you can show off your ride and the forum for wasting some time and hopefully learning something new.
 

SquaredArches.com Spark Plug Guide

Written By SquaredArches.com

  • Applies To: VN / VP V6 and V8s
  • Example Vehicle:
  • Estimated Time In Beers: However many it takes you to read this.
  • Tools Required:

 

This article is a result of internet research. I have done my best to make it as accurate and informative as possible. If you spot any errors or have anything to add, please contact me.

Often it is the little things that make all the difference to how your car performs. This is especially true of the spark plug which when not properly suited to your engine will restrict how much power you make, as well as having the potential to cause damage.

Through this guide, we will explain what it is your spark plug does, how to select the right plug and to give you an idea about what your spark plug is telling you about how your car is running.

Part One - What is a spark plug and what does it do?

  

Essentially the spark plug exists to transmit an electric charge in the form of a spark to the combustion chamber and ignite the mixture of fuel and air.

However, the spark plug itself does not generate electricty and relies on the coil to create the charge. Because of this, it's important to remember that no design of plug can create more voltage or a hotter spark. Sure, some will be more efficient than others in transmitting voltage, but the coil is the main player- beware of snake oil salesmen!

As the image on the right shows, a spark plug is quite simple.

Externally visible is the connector or terminal which transmits the electrical charge from the lead to the electrode.

The ceramic part of the plug is actually an insulator to ensure the charge moves down the electrode without sparking elsewhere along the plug. Because ceramic material is typically a poor heat conductor, it also keeps the electrode hot enough to burn off any deposits that may build up. The ribbed design is to create more resistance and improved the insulating properties.

The metal jacket of the plug is what allows us to screw it into the head and also plays a part in how the spark plug disipates heat generated within the combustion chamber. The spark plug itself does not generate heat. The ground electrode that creates the gap if found at the lowest point of the thread.

The centre electrode itself runs the length of the plug with a small portion exposed at the end of the plug which will allow the spark to

jump the gap to the earthed electrode. Often the material used in this electrode is from where the plug will get it's name- for instance, Iridium plugs have an Iridium electrode.

Most of today's plugs are 'resistor' type plugs, where the electrode is split to include a resistor to prevent radio interference caused by the electromagnetic field generated each time the spark jumps the gap.

Part Two - Selecting a plug.

OEM plugs were typically choosen to give a good mix of reliability, efficiency and longevity. Working in conjunction with the whole ignition system they are a good option to get the job done at a relatively low cost and without needing to be changed every few months.

Over the last few years a whole range of different plug designs and materials have emerged. Platnium and Iridium plugs are worth much more than the copper equivilents, however will last for much longer- some claim up to 100,000km. For some people the extra convenience of having a longer lasting plug will justify the cost.

As well as the different materials, there are also different electrode designs. Some will have multiple ground electrodes and the myth exists that they will create multiple spark. This is not true, as the spark will only jump from the centre electrode to the nearest ground electrode. The main benefit is as one electrode wears away, the spark will jump to another, extending the life of the plug.

In many cases, the multi ground electrode designs are actually out performed by single designs. Theoretically, the gases around the plug are disturbed, but also the extra metal used means more heat is retained by the plug within the combustion chamber and effectively the heat range of the plug is increased which can lead to preignition.

The biggest concern within a modified engine is going to be the increased heat within the combustion chamber which may make the original plugs unsuitable. If the plug is too hot, it can lead to preignition which can result in detonation or pinging and damage to the engine. If it is too cold the plug's self cleaning properties will be reduced and they will foul more rapidly and need to be replaced.

Whether a plug is hot or cold, refers to how well it dissipates heat from the combustion chamber. Typically, this depends on the ceramic tip around the centre electrode and how much of the threaded jacket is obscured. The less ceramic insulator, the more heat will be conducted through the thread, to the head. The diagram above shows the difference in design between a hot or cold plug.

If you are driving a stocker, you really can't go too far wrong by simply finding the appropriate part number in your NGK, Bosch or Champion catalogue and going from there. If you have carried out engine mods, sometimes the OEM recommended plug just won't cut the mustard.

Part Three - Reading your plugs.

It's often a good idea to pull your plugs each service to take a look at how your engine is running. You should also be able to tell if you have the right heat range plug installed.

NORMAL - This plug has been used in the correct temperature range in a healthy engine. In a petrol engine, the deposits will be light grey, tan or white however on LPG the colour on the plug will almost be brown.

Some fuel additives may leave red or purple deposits and are usually healthy, although some octane boosters leave conductive deposits which reduce the effectiveness of the plug.

OIL FOULED - Typically if the plug has substantial oily deposits it suggests the rings or cylinder walls are badly worn. However if the PCV is disconnected or otherwise not working it can result in built up crankcase pressure which can force oil or oil vapours into the combustion chamber.

The oil deposits will prevent the spark jumping the gap and can result in a misfire.

CARBON FOULED - If the deposits are dry, black appearence they are typically carbon deposits. This can be caused by running too rich, a weak ignition system or a spark plug that is too cold and not self cleaning correctly. If only one or two plugs are affected it could be a problem with the leads or a sticking fuel injector.

Carbon fouling can also lead to misfires in the same way as oil fouling.

WORN OUT - If you plug looks like this, you have left it far too long before replacing them. The car might misfire during accelerator, be difficult to start and have poor fuel economy.

Leaving a plug like this in the car, can do damage to other ignition components.
INSULATOR GLAZING - Shiny deposits on the insulator usually indicate that temperatures have suddenly increased under accelerator. Because of the sudden increase, the deposits have melted and created a conductive coating that can cause misfires at high revs.
MECHANICAL DAMAGE- Something has physically clouted the ground electrode. Usually this is some foreign object that has found it's way into the combustion chamber and due to valve overlap may have affected other cylinders.

OVERHEATED - Chalky white insulators and possibly blistered or pitted electrodes are signs the plug has overheated.

This can be caused by over advanced ignition timing, problems with the engine cooling system, running lean or using a plug that is too hot.

SPLASH FOULED - Most commonly this is caused by using fuel additives or throttle body cleaners before installing new plugs, especally if the car was long over due for a new set.

Old deposits are loosened when performance is restored and the collection of assorted crap is thrown from the piston, against the hot insulator surfaces.

ASH DEPOSITS - Are typically light brown, encrusted deposits often caused by fuel additives. The plug is rarely the cause of the problem but this can lead to misfires as the electrode is 'masked.'

 

GAP BRIDGING- This is very rare, however sometimes deposits can be caught between the electrode and ground electrode bridging the gap.

Without a gap, there is nothing for the spark to jump and no ignition and a misfire will result.

PREIGNITION - Initially, preignition may present as an overheated plug and many of the same symptoms apply. Over time and with sustained preigition the centre, ground electrodes and insulator may all melt.

Often damage to the piston and exhaust valves will also result if the problem is left unresolved.

DETONATION - In this case, the insulator has been fractured, however in more subtle cases black or grey spots may be visible on the insulator.

Typically this is caused by lean air fuel mixtures, advanced ignition timing or insufficient octane in the fuel.

The same causes of detonation can result in broken spark plugs, pistons or intake valves.

Thanks for checking out our guide and I hope you got something out of it!