Originally I was going to write this article on reading spark plugs. I will touch on spark plug reading, but in reality it can be time consuming on modern engines to remove spark plugs. Often the spark plugs will only be inspected in the event of an issue or during maintenance. With the use of wide band O2 sensors or pyrometers and knock sensors, reading the spark plugs for tuning is possibly not necessary for many applications.
To read spark plugs accurately you will need to replace them every time a reading is to be taken.
Spark plug reading with get coverage along with the differnet types and their constraction.
Types of spark plugs
Standard or copper core
The name comes from the centre core construction of copper. These are cost effective and more conductive than most other types. In comparison they do have a higher wear rate.
BP5ES Standard Copper

V grove
Still having a copper core V grove have a V shaped centre electrode and concave ground electrode, this guides spark to occur between the sharper outside edges improving ignitability and performance.

V Groove


A fine wire platinum centre electrode, adds ignitability, easier starting and fuel efficiency. The reasoning behind a fine wire electrode or electrodes is to open up more area for flame expansion. Laser or double platinum has a platinum centre and ground coated electrode proving the above with a better life via reduced wear rate. Cost wise they can be over twice the price of copper core plugs.

Iridium plugs simply replace the platinum with iridium. Fine wire and laser or double is still used. Iridium is more durable than platinum hence offering a slightly longer service life at slightly increased expense over the equivalent Platinum plug.
Both Platinum and Iridium are very poor conductors of heat. Trying to get heat out of spark plugs tips doesn’t happen easily. This makes them not well suited to race applications.


Double Platinum Spark Plug


Multi ground electrodes
Multi ground electrode plugs have exactly that, more than one ground electrode. This will reduce the number of imperfectly ignited cycles. They are often extremely highly durable and reliable. Good combustion is possible in high pressure, high output engines. Often multi ground spark plugs will have a sliver centre electrode.

Yttrium spark plugs are designed with additional ground electrodes. They name comes from the addition of Yttrium metal to the spark plug electrodes. They are a cost effective alternative to other precious metal spark plugs and offer lower emissions and fuel usage.

Silver fine wire
Silver is the best electrical and heat conductor. It has superior ignition ability and has lower required supplied voltage. This provides exceptional spark for hard to ignite fuels. A silver fine wire centre electrode allow for a thicker dielectric (porcelain) for greater strength. Cost is a greater than other options.

Racing spark plugs can and do have many different variations of the features listed above. Testing for your application is the only option in racing environments. Silver electrodes and colder heat ranges are very common.




Spark plug gap.
The spark plug gap is where the spark discharge is designed to occur. It is considered to be the point between the centre electrode and the ground electrode, this is the path of least resistance. Larger spark plug gaps require a higher voltage in the ignition system and are generally used in lower compression naturally aspirated engines. This larger gap promotes a smoother idle.
Running a large gap in a high cylinder pressure application (high compression and forced induction) increases the voltage demand for the spark to jump the gap. It is possible for the spark to find an easier path to ground and result in an engine misfire. More cylinder pressure means smaller gap and or stronger ignition system requirements.

Indexing Spark Plugs.
Indexing is setting the spark plug ground electrode open end towards the largest section of the combustion chamber. Each spark plug is indexed this way. This has been proven to increase horse power in certain style of combustion chambers. Indexing has more relevance for older wedge style combustion chambers than hemi or pentroof style.

Reading timing
Ignition timing has an effect on the combustion chamber’s temperature. Therefor it has an effect on the colour of the spark plugs ground electrode. It is possible to get an indication of the ignition timing by looking at the position of the colour change on the ground electrode. Ideally the colour change will be on the curve on the ground electrode and often a slight blue colour. If your timing is to far advanced the blue will become a green tinge, the next change after that is the melting of the electrode.


Timing indicator


Heat range
If you add timing and torque drops off and you have no colour on the ground electrode but colour on the porcelain is good you have to cold a spark plug, if you have lots of colour on the ground electrode and minimal on the porcelain you have to hot a plug.

What is heat range?
Heat range is how hot or cold the spark plug runs. A hotter heat range means that the spark plug stays hotter. This heat burns off any deposits. With increased throttle opening and larger engine loads the spark plugs run naturally hotter. Using a colder heat range spark plug is perfect in these situations. Racing uses plugs even colder again. Using to cold a spark plug for your situation will allow fouling of the plugs. To hot a plug can cause detonation and engine damage.

Heat of Spark Plug Materials

Spark plug heat


Porcelain spotting
Two types of spotting on the porcelain occur. Black spots occur when the spark plugs is running to hot and causing the plug and not the spark to start combustion. This creates two flame fronts and uncontrolled combustion. If your tune is correct, you need a colder plug. Shiny specs on the porcelain are a solid indicator of detonation. The specs are piston material that has been blasted off the piston. The tune is incorrect and timing needs to be removed and or fuel added and retested.


Salt and Pepper


Part numbers
All spark plugs have manufacturer numbers. These numbers indicate all the information about the plug. Information relating to the thread size, reach, heat range, firing end construction, firing end material and gap size to name a few. The manufacture’s websites explain their coding.

Different fuel types.
All of the available fuels have different requirements for tuning and therefore will have different spark plug requirements. As with any modification we make to a vehicle we need to consider every aspect to get the best results.

Related Tag: Vehicle Engineering Brisbane


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