Page 5 - Combined_141_OCR

Basic Heat Range (Continued) MIXTURE
Rich mixture—warmer plug? Lean mixture —cooler plug?
Having found a suitable plug (heat range ... gap style),
can spark plug heat range be precisely tailored to engine
mixture settings by going another step up or down the heat
range scale?
AVERAGE INSULATOR TIP TEMPERATURE IN HIGH PERFORMANCE ENGINES power is also a risk, as there is less protection from
In an unblown engine not using oxygen-bearing fuel addi­
tives, there is a spectrum of efficient carburetion. This
spectrum, or speed, ranges from best power to full rich.
Generally, lean-best-power yields maximum horsepower,
but the setting is dangerous if intake manifold design cannot
supply every cylinder with the proper F/A ratio. Lean-best-
detonation and preignition. Cylinder temperatures are also
increased. Consequently, most racing engines are enriched
slightly to avoid cylinder starvation and dangerous
temperatures.
In most instances, the mixture spread between lean-best-
power and full rich can raise or lower combustion chamber
temperatures as much as 300°F. As the spark plug firing
end resides within this thermal environment, it is also
affected. (Its job is to “bail” heat out of its nose into the
cooling medium of the engine.) Consequently, ’in some in­
stances, spark plug heat range must be changed one step
Spark plug tip temperature should not exceed 1600° F. up or down.
(preignition) or fall below 600° F. (fouling). The chart plots There is another tuning adjustment that has a profound
the tip temperatures of a street plug and a race plug from influence on combustion chamber temperature. This is the
idle to full power. The "street plug” is immune to fouling spark advance setting.
at low speed but overheats rapidly at high speed. The race The cautious combination of mixture and spark advance
plug requires a longer time to climb out of the fouling settings can act as a “thermostat” on combustion chamber
range but does not overheat at full engine power. temperature, and, in some cases, heat range substitution
When fuels other than pump gas are used, racing spark may be in order. Again, prudent and careful choice is rec­
plug heat range choice must be carefully selected. To avoid ommended.
preignition, plug temperatures must be held down to about In the various types of plugs shown in this booklet,
1600° F. when using gasoline and 1350° F. when using (categorized by size and gap style), some heat range steps
methanol. For racing, a further safety margin of 200 to 250° are far apart, some are closer together. All heat ranges in
is recommended, or maximums of about 1400° for gasoline this booklet are "graph style” to show you how much rela­
and 1100= for methanol. tively warmer or cooler each heat range step is in relation
Supercharging, of course, requires a colder plug. Nitro to the plug above or below it. This graph style reference is
mixes are notoriously unstable and have pressure and heat an invaluable aid in selecting plugs for a racing engine.
limitations in which the mixture can ignite without normal
ignition.
JETTING WARM-UP PLUGS
Fuel/air ratios are usually referred to as being "rich” or It is good practice to warm up a racing engine with
"lean.” A rich mixture is one that has a comparatively small hotter plugs. The warm plugs will assist the ignition and
volume of air... a lean mixture has a comparatively large carburetion to bring the engine up to proper operating
volume of air in relation to the fuel volume. temperature.
Lean mixtures are “dangerous” because they burn more It is a fallacy to believe hot plugs assist initial starting
slowly and require a longer time to conduct heat from the while cold plugs cause hard starting. When plugs are in­
combustion chamber, plugs, cylinder walls, and piston stalled "out of the box” and initially fired, both are the same
crowns. This can lead to detonation and preignition. temperature... whether cold plugs or hot plugs. Warm-up
Although some power is sacrificed, a slightly rich mixture plugs merely bring the engine up to temperature more
is a safety measure in racing for the following reasons: smoothly and rapidly. They also save the expense or in­
absorbs heat from the air and surfaqes ansund it.. .offers convenience of oil or carbon fouling race plugs.
detonation protection at full throttle.. .reduces heat trans­ If room exists within the combustion chamber, a projected
ferred to the surfaces involved .. satisfies the leanest cyl­ nose plug should be used for warm-up, as it has a wider
inder. heat range and “lights” the cylinder more easily as the
If in doubt as to proper Jetting A~en o'.eriean conditions spark is deeper in the combustion chamber. If projected
may exist), always enrichen considered small mixture nose plugs cannot be used, regular gap plugs are "second
adjustments. best.” If the engine, because of physical considerations, can
In an engine running excessively lean, combustion cham­ only fit a retracted gap plug, use the warmest retracted gap
ber temperature will rise as the engine accroaches lean-best- heat range available in the line.
power. This dangerous area must be “jumped.” Go exces­ Many engines have been destroyed when operators have
sively rich .. .then work toward the correct’fuel/air ratio. forgotten to remove warm-up plugs during practice, qualify­
ing, or in racing usage. It is sometimes expedient to tag the
NOTE: A fuel pressure gaLge = a . ze nstrument on a steering wheel as a reminder that warm plugs are in the
race car. If abncr—a z’a= = Jres exist, proper engine.
jetting cannot be ach e.ec

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