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How to read spark plugs

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  • ja70cad
    This was posted on the Century Performance Message Board by Larry Meaux of Meaux Racing Heads. BTW, also a very good message board. How to read spark plugs;
    Message 1 of 1 , Jul 9, 2002
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      This was posted on the Century Performance Message Board by Larry
      Meaux of Meaux Racing Heads. BTW, also a very good message board.

      How to read spark plugs;
      *** Mostly valid only for full-load dyno tests ,
      or if engine is quickly turned-off after
      finish line, and then towed back to pits,
      and spark plugs removed for readings there !
      Plugs, best viewed with a 10x power illuminated mag. glass
      Heat Range = Ground Strap , the ground strap indicates the
      heat-range of the spark plug. If the "color" of
      the ground strap "changes" too-close to the ground-strap's
      end, (which is above the center electrode), then the
      heat-range is "too-cold" , meaning that the strap is
      loosing heat too-quickly to the base-ring, and is not
      able to burn-off deposits until near its end.
      If the "color" of the strap changes near where it is
      welded/attached to the base-ring(last thread ring),
      then it means that the plug heat-range is "too-hot",
      because heat is not being tranferred/cooled from the
      strap to the base-ring quickly enough !!!! The strap
      might begin to act like a "glow-plug", eventually
      causing preignition/and/or detonation later on.
      Proper heat-range is when the "color" is at the
      half-way point on the strap, neither too-cold or
      too-hot.
      (Color= meaning the evidence of heat/or lack of heat
      by the appearance dark vs lightened color of metal)
      Jetting = the air/fuel mixture ratio shows up on
      the base-ring (the last thread ring, it has the strap
      welded to it).
      You want 1 full-turn of light-soot color on the base
      ring!!! If you want to tune for max. power, then you
      want 3/4 to 7/8ths of a full-turn of light soot color
      to show-up on the base-ring, but this is on ragged-edge
      of being too-lean, but will make the most HP on most
      engines. To be safe, leave it at a full turn of light
      soot color.If the base-ring has a full-turn of color,
      but there are "spots" of heavy build-up of "dry-soot"
      on top of color, then jetting is too-rich .
      NOTE> If the base-ring has a full turn of color with
      some spots of heavy dry-soot, then jetting is too-rich,
      REGARDLESS,if porcelin maybe "BONE-WHITE", jetting is
      still TOO-RICH !!!
      NOTE> Do not look at the porcelin to read jetting !!!
      Porcelin = the porcelin shows up preignition/detonation,
      it will not accurately reveal jetting/air/fuel ratios.
      To look for the first/beginning signs of detonation,
      search the white porcelin for tiny black specks or
      shiny specks of aluminum that have fused to the porcelin.
      When detonation occurs, part of the air/fuel mixture
      explodes instead of burning, the explosion is heard as a
      "metallic-knock", this audible knock is the result of a
      sound shock-wave, this shock wave travels back and forth
      across the clearance volume "disrupting" the cooler
      boundary layer gases that cover the entire clearance volume
      area,this disruption allows "more" heat to be transferred
      into parts, especially, domes/piston tops,...along with the
      very rapid rise in pressure like a hammer like blow, pistons
      can get torched/melted sides/holes !!! With the early signs
      of detonation, the shock-wave will also rattle rings causing
      the tiny amounts of oil that now gets by rings, to be fused
      to the white porcelin as tiny black specks, also fused as specks
      are soot that was clinging to clearance volume surfaces
      in the relatively "still-air" of the boundary layer.
      One step beyond the black specks,will be tiny specks/balls
      of alumimum coming off-pistons that will be fused to white
      porcelin,....the next step to be reached is occasional pieces
      of the porcelin being broken-off as detonation gets worse,ETC.
      Soon after that, are holes, blown head gaskets, broken
      connecting rods, ETC.
      NOTE> additional signs of beginning detonation are piston
      rings. By comparing ring's "free-diameter" to "out-of-box"
      free-diameters of new unused rings will reveals beginnings
      of detonation before much harm is caused !!! This is a result
      of the above previous explanation about heat being transferred
      more readily because the of the relatively insulating/cooler
      boundary layer being disrupted by the shock-wave.
      A piston ring is a simple spring,
      when a spring is over-heated it will loose tension.
      Center Electrode = the very tiny sharply defined porcelin -"ditch"
      that encircles the center electrode, will also showup early
      signs of preignition/detonation/wrong heat-range. Look for signs
      of the ditch beginning to be filled up with melted porcelin,
      you will need a 5x or 10x mag. glass.
      The "Ground-Strap" = Heat Range
      The "Plug's Base-Ring" = Jetting
      The "Porcelain" = signs of preignition/detonation
      --------------------------------------------------------------
      Larry Meaux (meauxracing@...)
      Meaux Racing Heads
      MaxRace Software
      ET_Analyst for DragRacers (software)
      Support Israel - Genesis 12:3
      http://www.mindspring.com/~meauxracing/
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