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GAP SIZES PLUGS FOR STOCK DRAG RACING
Wide “street” gaps generally have no advantage in racing Stock or moderately modified cars running Vs or iA-mile
service. (A possible, exception to this rule is stock quarter­ drag competition can use warmer plugs than oval track or
mile drag racing.) In fact, wide gaps can invite a high speed road racing vehicles ... as full engine powers and tempera­
miss due to decreased coil saturation time. Wide gaps can tures are of short duration.
also stress coils and cause ignition harness breakdown. Extremely cold plugs cannot tolerate return line running
Champion race plug gap settings are based on a simple without some fouling. Subsequent runs can result in an in­
principle . .. cylinder pressures are directly proportional to crease in E.T. and a decrease in top speed. Warmer plugs, on
temperatures developed in the combustion chamber ... the the other hand, will remain cleaner but may invite preignition
higher the compression ratio, boost pressure, or more, nitro near the end of the speed traps.
added, the greater the heat and the higher the pressure at­ An ideal heat range stays clean on the starting line
tempting to “quench” the spark. and return line but does not enter the preignition region at
Consequently, factory gap settings decrease as plugs be­ the traps. The ability of the plug to tailor to this thermal
come progressively colder in heat range. spectrum is dependent upon: proper plug heat range selec­
When transistorized or capacitance discharge ignition tion, ideal jettings, and correct spark advance.
systems are used, gaps should be set to the ignition manu­ Strong acceleration off the line but a falling-off of power
facturer’s specifications. (or RPM) near the traps may indicate the heat range is too
Extremely wide gaps, fired by C.D. ignition systems, are warm, detonation or preignition is occurring, mixture is
not prudent as an output weakness developed during a race too lean, insufficient fuel pressure, incorrect spark curve,
can invite total failure. A closer gap might enable the sys­ marginal ignition output, or plug gaps too wide.
tem to “finish” the race. Poor acceleration off the line but increasing power down
the strip may indicate fouled plugs, weak accelerator pump,
overrich mixture, improper spark curve, wrong gearing, weak
SURFACE GAP PLUGS
ignition, or plug gaps too close.
Surface (polar) gap plugs, identified by the suffix “V”,
cannot be bar graphed in heat range as they are extremely RETURN LINE FOULING . . .
colder than conventional racing types. Surface gap plugs
have blunt insulator noses. They fire across the area of the PADDOCK FOULING
insulator tip, between the center electrode and the shell. Stock car drag racing imposes an extra burden on spark
Use of these plugs should be restricted to specific ca­ plugs ... known as “return line fouling.”
pacitance discharge ignition system factory approved by the When driving back to the return line, the engine should
engine manufacturer. not be “burped” continuously. The car should be kept in
Extremely cold “R” (retracted gap) plugs may be used to the lowest gear possible, utilizing only the primary system
an advantage. of the carburetor to make the return run.
Colder plugs cannot perform efficiently if the following
CROSS-REFERENCE abuses are practiced:
Tuning the car in the pits; e.g., setting valves,
This booklet depicts heat range by bar graph. Plug position
is uniquely pinpointed. This method of thermal reference Adjusting spark advance and idle settings.
Backing off and on trailers.
removes the guess work of selecting heat ranges from a
Running the battery into a state of discharge.
vertical column of numbers and letters which give no indi­ (Partially discharged batteries and loose grounding cables
cation of precisely how much warmer or colder each plug are responsible for many cases of spark plug misfire. The
is in relation to the other. practice of running cars with loosened generator or alter­
Brand cross-referencing is an approximation. In addition, nator belts, or without any charging system whatsoever,
the recent usage and profusion of regular, projected nose, makes it essential that batteries be in a state of full charge.)
and retracted gaps used in racing leaves many heat range
“holes” in cross-reference charts. “HOT” IGNITION SYSTEMS----
Overzealous comparison (due to absences within the ther­
mal spectrum) and “extra-wide” heat range claims lend in­ “COLD” PLUGS?
accuracy and conjecture to plug cross-reference selection.
A plug is mated to a race engine-IT IS NOT CROSS-REF­ “Hot” coils, special magneto, and transistorized ignition
ERENCED. Extra wide heat ranges encompassing three to systems do not usually require changes in spark plug heat
four competitive ranges are risky choices and almost non­ range. It is not the temperature of the spark but the
existent (the cost of component fabrication and exotic ma­ temperature of the combustion chamber that influences heat
terials would price plugs far beyond the means of the average range. The heat of the spark is insignificant. Spark erosion
racer). cannot be materially reduced by colder heat range substitu­
tion. If the engine timing is changed, heat range substitution
It is more difficult to precision-tune a race engine with
extra wide range plugs. Plug reading is difficult, as the may be in order.
“elasticity” of heat range may be camouflaging plug inter­ Capacitor discharge ignition-systems have the ability to
pretation. An analogy is like over-stretching a rubber band fire fouled plugs. “Super cold” race plugs do give better
to hold too large a package. preignition protection when fired by capacitor systems. How­
This booklet lists cross-references but all are mere evalu­ ever, plugs thermally matched to combustion chamber tem­
ations. No claims are made to precise thermal accuracy. The peratures are usually superior to colder plugs mated to
spark plug industry has not internationalized on heat range “exotic” ignition systems.
procedures and techniques, consequently, no claim is made Setting extra-wide plug gaps because of exotic ignition
to pinpoint accuracy of cross-references used in this booklet. systems presents some risk ... if ignition becomes marginal
during a race, a closer gap might have prevented a break­
down. Some magneto systems cannot tolerate wide gaps, as
starting ability suffers. Transistorized or capacitor discharge
systems have very fast “rise times” and highly stress igni­
tion system components. Consequently, perfect ignition sys­
tem maintenance is a must.
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