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Old 07-16-2012, 07:29 PM
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NGK Spark Plugs (my personal preference, BTW) has a bit of a blurb on spark plug design.

Spark Plug Basics

The spark plug has two primary functions:
  • To ignite the air/fuel mixture
  • To remove heat from the combustion chamber
It is important to remember that spark plugs do not create heat, they can only remove heat. The spark plug works as a heat exchanger by pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine's cooling system. The heat range is defined as a plug's ability to dissipate heat.

The rate of heat transfer is determined by:
The insulator nose length
Gas volume around the insulator nose
The materials/construction of the centre electrode and porcelain insulator
A projected style spark plug firing tip temperature is increased by 10C to 20C.
So ... to answer your question, I would go with the recommended plug right fom the catalog for a "bone stock" engine. If the engine has been modified, the recommendation is to go to a "colder" plug.
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