plugs

[Todd King]

  <<<
  > The purpose of the plug is to ignite the fuel.  In some circumstances,
  >
  > different materials are more or less prone to oxidation in heated
  > environments, so yes, in theory, different plugs might make a
  > difference.
  >  Also, plugs come in temperature ranges, different gap settings,
  > different
  > insulator materials.  I've honestly never figured out technically the
  > reasons why, but if someone on the list can explain why a plug is not
  > a
  > plug, I'm all ears :)
  >
  The spark energy influences the power (by increasing the burn rate) more
  than the spark plug itself, and this is related to the stored coil
  energy and not necessarily the plug specification.
  >>>
  
  I just received some new SAE books last night; two of the papers in the 
  combustion book detail results of plug indexing, style (shape, number of 
  electrodes, etc) and gap experiments. All three plug aspects did indeed show 
  differences though the authors generally declined to make recommendations on 
  which way is "best". Interesting that as Andrew points out the spark energy 
  comes from what's stored in the coil, not really varying with the gap as is 
  sometimes implied. Point of diminishing returns is reached from widening the 
  gap when the spark begins finding alternate routes across the ceramic and 
  ignition efficiency drops. Indexing was interesting too; seems that getting 
  the ground electrode out of the "slipstream" of the small flame kernel showed 
  improvement in ignition. However the slipstream (mixture flow) direction 
  appears to vary with rpm, etc so nailing down a "correct" orientation is 
  difficult at best. However both results appear to imply that the ignition 
  process favors having hard parts out of the way of the initial flame kernel.
  Pretty good reading; it would help to be an ME though...
  
  Todd_King at ccm.co.intel.com