Spark Plugs.

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2 Questions.

1, If 3 spark plugs are identical on a 4 cylinder engine and 1 is different by heat range, how far off on the heat ranges before the engine starts running bad? 1 heat range? 2? More?

2, All spark plugs are same brand. 1 spark plug is 1 heat range hotter. Will this cause the cylinder to burn rich?

I know different ranges can and will cause different driveability concerns. However, I am in talks with someone saying 1 heat range off on same brand, same part number(minus heat range) plug will cause a bad cylinder. While possible....I do not think this is the case. Just wanna hear from some others their opinion on the subject.
 

Fire-medic

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I think that would be more of a 'thing' on a 2-stroke with an old-style ignition, probably a 6 volt system too.

In my experience, the heat range is specified in the spark plug # from what I recall during my vintage 2-stroke offroad riding.
 

MaxMidnight

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The heat range indicates how much heat will be dissipated through the plug.

This will be more critical on an air cooled motor and as Mr Medic says with two strokes in particular.
It is probably less critical on a water cooled motor as the amount of cooling can be altered by the thermostat.
IMO it wouldn't cause a significant issue on a water cooled four stroke. However as the incorrect plug would would make the overall coolant temperature cooler or hotter this would take the operating temperature away from optimum which could have an effect on performance, fuel consumption and emissions.
This would be more noticeable on a twin but reduce the greater the number of cylinders.
 

one2dmax

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Gap and with or without the platinum tips seem to have more effect for me. Mostly under boosted conditions. Higher boost needs smaller gaps to prevent being blown out.
 

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