plugs

[Frank Piccolo]

Todd King wrote:

>   <<<
>   > 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

  You fail to take into account the timing issue.  The wider the gap the
greater the impedance across it, the greater the impedance the more
energy is needed to cross that gap.  It takes longer for the coil to
accumulate this and therefore affect the time advance curve.  At low RPM
it wont be much but a six grand the difference will offset any benefit
gained by the larger kernel.  Keep the gap around the close to original
unless you plan on taking control of the spark timing.