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Spark Plug Tuning Considerations

Modifications and Spark plug considerations:
These are observations that have been made by us or passed on to us. This information is a generalisation only. Your engine tuner should be able to advise you on a suitable set of plugs for your vehicle. Please call if you require technical advice before purchasing. We cannot recommend a non standard part in any circumstance, but we can usually make a suggestion.


  1. Try to avoid platinum (particularly double platinum) plugs. There are suggestions that a chemical reaction may occur and some unusual deposits will form on the electrodes - these may impede ignition performance. Double platinum plugs (where a platinum 'chip' is welded into the ground electrode) may be at risk from the platinum chip becoming detached in extreme temperatures, the surrounding electrode material will doubtless have a lower melting point. A platinum chip dropping into the combustion chamber could be disastrous.
  2. Try to find a plug with a short ground electrode. This is generally thought to be more important than a colder heat range. Because temperature will increase so rapidly when using nitrous oxide, the temperature at the tip of the plug's ground electrode will increase very quickly - the longer the ground electrode, the longer it will take for the excess heat to be conducted away from the tip to the plug shell, and, ultimately, into the cooling system. An overheated ground electrode tip can cause detonation and may even fail completely.
  3. Try to find a 'non projected' plug. The design of a projected plug will usually aid ignition performance at low engine speeds by moving the spark position closer to the centre of the combustion chamber, the downside of this is that the centre electrode and central ceramic insulator become exposed to combustion gases and hence are at risk of damage under extreme conditions. By using a non projected or even a retracted spark position, the firing tip is more protected and plug failure is less likely. A non projected or retracted plug by design will have a shorter ground electrode which is beneficial for the reasons mentioned in 2. above.
  4. Use a non resistor or Iridium type plug if available. Please note if your vehicle manufacturer recommends a resistorised plug as standard we would not recommend using a non resistor plug, interference with engine and safety management systems may result! Some plug designs are only available with integral resistors. By using a non resistorised type plug, the amount of voltage available at the plug's firing end is slightly increased and therefore the risk of misfire under load is reduced. If combustion conditions are particularly extreme then an increased 'spark jump' voltage is required. Effectively, when more fuel and oxygen are compressed in the combustion chamber (as occurs in nitrous/turbo/supercharger use) the result will be similar to that of increasing the plug gap - if the gap becomes too large for the available voltage then the spark simply won't be able to jump the gap. Iridium spark plugs can help as they have a greatly reduced firing voltage (despite an integral resistor) - lower overall than most copper non resistor plugs. High performance ignition leads (such as Magnecor KV85 leads) can also help supply maximum available voltage to the plug
  5. Use a colder heat range plug. While colder plugs will be of limited worth during a large, short burst of nitrous, they will help to return the sparkplug tip to a safer operating temperature more quickly in between or after nitrous use. The increased heat of the nitrous burn will not have time to be dissipated more effectively by a colder plug in the usually short space of time for which nitrous is injected. A short ground electrode is more useful for maintaining 'safe' conditions during nitrous use (see 2. above)
  6. Learn by the mistakes of others. There are no official applications guides of suitable plugs for nitrous equipped engines. Web forums are a valuable resource when trying to identify a suitable part to use. It's unlikely that you will be the first to have a nitrous system fitted to your particular engine type. Do some research and try to find out what has worked for others and, perhaps more importantly, what hasn't worked.


  1. By increasing boost pressure you will be forcing more gas into the engine's combustion chamber, increased compression will result, an associated result of compressing a gas is an increase in temperature, increased temperature in the combustion chamber will increase the temperature of the sparkplug tip - possibly in excess of it's designed operating temperature range. Overheating of the sparkplug may be the result and associated detonation which will ultimately (and sometimes rapidly) lead to destruction of the sparkplug and possibly related damage to the engine internals - most often pistons. It is not advisable to increase boost pressure without also considering fuel requirements, if a suitable fuel map is run in conjunction with an increase in boost then an alternative plug may not even be necessary.
  2. To help maintain the plug's firing end at the correct temperature a colder plug grade is sometimes necessary. You should bear in mind that the majority of road going cars will rarely be used at 'full boost', by selecting a plug in a cold enough heat range to cope with sustained high boost use you will be risking the plug 'over cooling' when off boost (e.g. driving in town traffic), the result will often be fouled plugs and misfiring. A sensible 'trade off' is required to keep the engine running reasonably well under all operating conditions. The heat range required depends largely on the amount of additional boost pressure used and the type of driving you will be doing. It is a lot more sensible to err on the side of caution by using a plug which is potentially a little too cold, may 'foul up' and subsequently need to be replaced rather than one which is too warm and destroys your engine.
  3. It is almost always necessary to use a plug with a smaller gap setting when running increased boost. The additional charge in the combustion chamber dictates that more voltage is required to jump the standard spark plug gap. Options are to close the original gap of the standard plugs by approx 0.2mm, this will relieve the strain on the ignition system, meaning a smaller voltage will be required when off boost and when on boost enough voltage should be available to fire successfully and reduce the risk of misfire. For some plugs, equivalents with a smaller preset gap are available (e.g. original part BKR6E-11 (1.1mm gap) can be replaced by BKR6E which has a preset gap of approx 0.8mm) - this is preferable to adjusting a larger gap which can put the electrodes out of true and also weaken the ground electrode. A similar effect can sometimes be had by replacing a standard copper plug with a fine wire precious metal plug (e.g. platinum or Iridium). A fine wire type plug may be able to maintain the standard (larger) plug gap setting due to the inherent lower firing voltage requirement, this benefits the engine, larger gap = larger spark - providing enough voltage is available to produce a spark!

In summary, a trade off is usually required. Priority should always be to protect the engine. Installing a plug which is suitable for high boost use will always detract from off boost/low speed ignition performance. If you want an engine that runs perfectly under all conditions then don't modify it from manufacturer's spec!


For computer controlled engine management systems, it is sometimes possible to reprogram various settings (fuel injector duty cycle, ignition advance etc) or simply to remove manufacturer's restrictions such as speed or rev limiting. Reprogramming may be done with suitable EPROM reading and writing equipment but this operation should be left to the experts. For most people either the chip within the ECU which holds the critical settings may be replaced or the entire ECU unit may be replaced - often on an exchange basis. ECU programming is usually carried out in addition to other modifications, often to more safely accommodate other upgrades (e.g. to increase fuel delivery when running higher than standard turbo boost). Because reprogramming can be carried out for a huge variety of reasons and with extremely varied results we are unable to offer any specific advice, you should perhaps consider which modifications have dictated that the ECU needed to be reprogrammed. If reprogramming alone has resulted in an increase in horsepower then perhaps consider using a single heat range colder (e.g. if using an NGK '6' change to an NGK '7') for an added 50BHP, two heat ranges colder for 100BHP or more, and then an additional step colder for subsequent 150BHP increases. This is a general estimate and requires very careful consideration to avoid fouling if too cold and, even worse, detonation if too warm.


The first stage of modifying is usually air intake and exhaust replacement or de-restriction. This will not usually result in a huge increase in horsepower unless the original equipment items were extremely restrictive. A change of spark plug is rarely required as any change in combustion chamber temperature will be very small and may even result in a lower temperature (this is most common when removing a catalytic convertor). Some drivers may find that they drive their vehicle harder due to improved exhaust note and better engine response (due to improved engine aspiration), in this circumstance a slightly colder plug may be beneficial but is unlikely to be necessary.