snapshot 05-01-2022 15:53

How to determine spark plug heat range

How to determine spark plug heat range and why is it important?

I’m glad you asked!  It gets hot inside an engine right?  How hot depends on many things but if you have modified your engine away from OEM chances are it is making more power and more heat.  

Spark plugs are much like Goldilocks and the Three Bears with regards to temperature.  They don’t want to be too cold or hot, but just right.  Why is this though?  Too cold and the plug surface does not reach a sufficient temperature to clean carbon (assuming no AFR problems with your fuelling strategy) and too hot and heat builds up potentially creating a source from pre ignition or erosion of the plug components as it melts away.

Heat flow diagram demostrating spark plug heat range (courtesy NGK Spark Plug Co. LTD)

How to test spark plug heat range?

How to determine spark plug heat range is simple in my mind.  My test involved a variety of plug heat ranges tested in the field.  Plugs used in my test were BPR6ES, BPR7ES and BPR8ES all gapped to 1.1mm.

Part of my method was to keep the engine in a ‘hot’ condition with as little idling as possible and no restarting.  It is normal for an engine to inject additional fuel during starting and this can contaminate the results.  I exposed each set of plugs to a minimum of 10 wide open throttle pulls from near idle to redline.

There are a number of methods to verify correct spark plug heat range.  In this test I use to visual indicators on the spark plug itself. 

1. Cleaniness of the insulator

When the electrode insulator is operating at the correct temperature, carbon deposits are burn off.  Too clean also indicators too much heat.  Typically a light dusting of a redish brown colour is somewhere in the ball park.

2. Heat marking on the earth strap

Spark plugs are normally electroplated to extend life.  This plating will burn off when the parent metal heats to a given value. 

Wide open throttle engine conditions normally generate the most heat and will burn off the plating along the earth strap to a point.  This shows as a distinct band on the earth strap.  If the band is close to the spark plug thread then the earth strap is too hot and conversely if the band is close to the earth strap end, there is insufficient heat.

Having this band located close to the bend in the earth strap, or middle third, is considered optimal.

The results

Out of the three heat ranges tested NGK heat range 8 scored the best.  I suspected that the factory NGK heat range 6 would not be suitable as my engine produces twice as much power as factory.

NGK heat range 8 showed the cleanest insulator as well a heat band close to the bend of the earth strap.  The yellow arrows point to the earth strap heat band and you can see the insulator is clean on the BPR8ES plug.

Yellow arrows indicate the earth strap heat band

I have created a YouTube video of this as well if you’d prefer to watch rather than read.

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