Lets talk about spark plugs heat range.

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I have been running Autolite 3924's for the last couple years. Every time I change them they still look new. As in they look like I just put them in absolutely no color change in the electrode.

I think I must be running too cold of a plug. I see they also sale Autolite 3923's and 3926's.

I think its the higher the number the hotter the plug is this correct?

What determines the heat range of spark plug that is best for an engine? I know you want a colder plug for power adder cars but on a naturally aspirated car how do you determine the correct heat range for your engine?

Thanks,