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READING CHAMPION RACING SPARK PLUGS

ALL PLUGS SHOULD “READ” ALIKE
PLUG “COLORING” TIME
Every Set of plugs removed from a racing engine should How long does it take to color a plug? This question can­
look alike... in color and condition. Any difference in color not be answered with any rule-of-thumb. The coloring pro­
or condition among a set of plugs is an indication the com­ cess depends upon the heat range of the plug, the position
bustion chamber temperature or fuel/air ratios are not the of the plug within the cylinder, the richness of the mixture,
same in every cylinder, or that related engine components and the gap style of the plug.
need attention. We do not regard plug coloring as an accurate means to
If differences exist in firing end condition, they can interpret heat range. In racing, the time function (or mile­
generally be traced to the following: age) is insufficient to color a plug. (Plug color charts are
unequal cooling of the plug ports useful for street plug analysis as mileage is adequate to
intake manifold/unequal distribution achieve ttre full coloring process.) In fact, pump gas is slow­
a weakness in the ignition system est to color a plug. Methanol is usually quickest to color
poor oil control a plug, especially when nitro speeds up the coloring pro­
weak compression cess. (7 to 11 second acceleration runs are an exception.)
unequal valve timing In some cases, “chocolate brown” plugs present a decep­
distributor cam indexing tion. If oil control is poor, the presence of oil helps to color
Close inspection of the respective systems may reveal the the plug.
following: A plug “sees” engine temperature very rapidly... such
COOLANT —steam pockets, leaking head gaskets, cracked indications are best evidenced by “cement boil” or electrode
header tank, water pump cavitation, insufficient radiator discoloration, disfiguration, or oxidation.
area, restricted water passages. Some combinations of fuel and oil will never color a plug,
CARBURETION-unbalanced multi-carbs, throttle valves or the plug can be so hot that all color is scavenged from
out of sync, air leaks in the induction system, disturbances the insulator.
in air flow, improper ram tube length, exhaust obstructions, The best procedure is to set up the engine with the jet­
dirty fuel, unequal float levels, dirty nozzles (fuel injec­ ting and spark advance representing the best power as evi­
tion), or air cleaner restrictions, mismatched intake manifold. denced by dyno runs or “seat-of-the-pants” tuning in prac­
IGNITION-weak battery, points out of sync, distributor tice sessions ... protecting the engine with a cold plug. (It is
cap arcing, defective rotor, “cross-fire,” defective spark much cheaper to foul a plug than burn a piston.)
plug leads, grounded resistors. The only concern during these tests should be if the plug
POOR OIL CONTROL —new rings not seated, broken rings, is too hot. The engine should be tuned to deliver maximum
worn valve guides or seals, excessive oil pressure, improper power and good acceleration characteristics. When this has
crankcase ventilation. (Detonation can also cause poor oil been accomplished, a more suitable heat range (and possi­
control.) bly gap style), can be selected as a compromise with the
VALVE TIMING —improper valve lash, worn cups, defec­ course, weather, altitude, and tuning techniques.
tive springs. Large displacement engines of 250 cubic inches (4,100
UNEQUAL CYLINDER PRESSURES-piston deck heights cc), or higher, are not as sensitive to course or atmospheric
unequal, rod-crank-piston tolerances not the same, leaking conditions as commonly supposed. When compression ratio,
cylinders, defective or unseated rings, unequal C.C.’s, im­ cam design, and fuel choice are “locked in” and properly
proper valve lash. tailored to engine timing and jetting, environmental condi­
Note that some cylinder temperatures or F/A ratios may tions assume less importance.
be so extreme among cylinders that it would require two In very small displacement engines, turning extremely
spark plug heat ranges to succesfully “satisfy” the engines high RPM, environmental conditions assume greater im­
in various cylinders. If such differences exist, it is impossible portance.
to expect top engine performance... conditions must be A probe light is very helpful in ascertaining mixture con­
corrected to insure equal compression, across-the-board ditions. The probe is inserted in the spark plug hole and
temperatures, equal F/A ratio, ignition equal in all cylinders. the piston top inspected for color and condition.
This is most essential in race tuning, as wide difference
in plug “readings” cannot be tolerated. A racing engine that
reflects this condition will never perform at peak efficiency
and will be in frequent spark plug “trouble.” TESTING RACING PLUGS
Some race mechanics will condemn a spark plug if it does
DUAL PURPOSE PLUGS­ not spark in a plug tester at an air pressure equal to or
NOT ADVISABLE greater than the engine’s cranking pressure. This idea is
entirely wrong and falls into the category of “Old Wive’s
Best performance in “strictly stock” drag racing or sports Tales”. Air pressure as read on a plug tester has no direct
car racing, cannot be obtained using the same plugs for relationship to engine cranking pressure for several reasons:
street driving and racing.
• The plug under test is cold. In the engine, the electrodes
A plug in daily use picks up deposits during start/stop operate at high temperature and require les.s voltage to
driving. These deposits, however slight, will bleed away volt­ fire than when cold.
age under strenuous racing conditions and penalize per­ • The tester does not present an air-fuel mixture.
formance ... under high speeds and sudden accelerations, • Spark plugs normally do not fire at the point of maximum
the normal deposits do not fluff off the plugs... instead, cylinder pressure, but well ahead of TDC.
they melt and may form a conductive coating causing • The quench point of a plug under test is determined by
misfire. gap setting.
Best policy is to use two sets of plugs ... one set for the • The line voltage may vary, thus affecting tester output.
street, one set for racing (usually colder). The race set Because of these and other variables, the efficiency of a
should be gapped more closely... the idle may suffer but spark plug just cannot be measured in terms of “pounds per
smooth idle characteristics are of no consequence in racing. square inch”.
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