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Understanding Spark Plugs.

All You Need To Know

What Is Spark Plug Heat Range?

When it comes to spark plugs, one of the most important specs is heat rating. Heat rating is a measure of the balance of the amount of heat received from the combustion chamber and amount of heat dissipated. You need to choose a heat rating at which the spark plug can be used normally by maintaining this balance; however, this varies between all types of engines.

Hot and Cold Type Spark Plugs

Spark plugs with firing ends that heat up quickly are referred to as ‘hot type’ plugs while those which don’t heat up quickly are called ‘cold type’ plugs. When choosing spark plugs, it is essential to use a plug with a heat rating that matches your engine and the driving conditions you subject it to. If you use spark plugs with the wrong heat rating, there are some issues which can occur within your engine. With the Repco REGO search tool, you can shop a range of spark plugs with heat ranges that are safe to use with your engine.

When To Use A Hotter Spark Plug 

If you are experiencing fouling issues with your spark plugs, this indicates the plug is not reaching the minimum operating temperature to burn off carbon deposits.  After eliminating all other possible causes, the final step is to choose a hotter heat rating plug to increase the plugs operating temperature and reach the self-cleaning temperature. Our recommendation is to change one heat range at a time as you do not want to run a plug which operates a too high of a temperature and put your motor at risk.

All spark plug application listings made by NGK are for unmodified applications. When making modification to your engine, it is important to consider what impact each modification will have on the cylinder operating temperatures which affects the spark plugs operating temperature.

In general, any modification which increases the cylinder pressures will result in an increase in the cylinder temperatures. This can be by increasing the compression ratio or adding/increasing forced induction with a turbo or supercharger. In each of these cases, the cylinder temperature will increase, which needs to be compensated by using a colder heat range plug.

Alternatively, modifications to the induction system or fuel type which results in more fuel entering the combustion chamber has a cooling affect, reducing the cylinder temperatures.
Even differences in the tune of identical setups can result in different operating temperatures.

Therefore, it is important to understand that every modified application is unique and the individual making the modifications needs to consider the impact of all the modifications and select a suitable heat range. Our recommendation is to be conservative and start with a cold heat rating plug as this does not put your engine at risk. A spark plug which is too cold will eventually build up enough carbon deposits to cause a misfire.

If experiencing fouling issues with your spark plugs this indicates the plug is not reaching the minimum operating temperature to burn off carbon deposits. After eliminating all other possible causes, the final step is to choose a hotter heat rating plug to increase the plugs operating temperature and reach the self-cleaning temperature. Our recommendation is to change one heat range at a time as you do not want to run a plug which operates a too high of a temperature and put your motor at risk.

Hot and Cold


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