How to read spark plugs
- 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
(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
ET_Analyst for DragRacers (software)
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