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Champion Spark Plug Identification System In addition to the numbering system used in cataloging,
this booklet uses bar graphs to illustrate precisely how much
colder or hotter each race plug is in relation to the other.
Basic Heat Range Numbers This system is used throughout the booklet to enable the
race mechanic to choose plugs and heat ranges more pre
Example: cisely. NOTE! The longer the bar graph, the hotter the plug.
57 50
All Champion spark plug numbers fit into four categories
defining application and heat range.
1 Cold Cold 51
Automotive & l 1
Thru Thru ... Racing
Hi-Performance
25 Hot Hot 75 Since the insulator tip is usually the hottest part of the
spark plug, its temperature can be related to preignition
and fouling regions. In 4-cycle engines or non-supercharged,
26 Cold Cold 76 gasoline-burning engines, preignition is likely to occur if
♦ Special
Aircraft... Thru Thru . anything in the combustion chamber exceeds approximately
▼ I •• & Racing 1750° F. On the other hand, fouling or shorting of the plug
50 Hot Hot 99 due to carbon is likely to occur if its insulator tip tempera
ture drops approximately 600° F.
“HOT” OR “COLD” SPARK PLUGS F.
The words “hot" or “cold” when used in reference to 1800
spark plugs are often a source of confusion since normally
a hot plug is used in a cold engine (low horsepower) and a 1600
AVERAGE INSULATOR TIP TEMPERATURE IN HIGH PERFORMANCE ENGINES 1200
cold plug in a hot eng’-.e (high horsepower). The terms
actually refer to the hea: rat:"g or thermal characteristics
of the plug—more spec f'ca y tc the plug’s ability to transfer 1400
heat from its firing end into the engine cylinder head.
1000
800
600
400
Racing engines develop very high combustion tempera
tures. “Street plugs” cannot survive this thermal environ
ment, especially when full power is sustained. Consequently
(colder) racing plugs are fitted. Preliminary heat range selec
tion is tailored to the pressure and heat the engine basically
develops, but final heat range selection is more precise when
the nature of the course, weather, and stage of tune of the
engine are considered.
A cold rur,r.;-g z z? ze* - : ~esz 'ac'diy Heat range selection consists of choosing a plug that will
from its firing e"z 3"z = _sez :: z : z : -g .-."ere “ride” thermally between the borders of fouling and preig
combustion chamber or cylinder head temperatures are rel nition at all engine powers and course conditions.
atively high. A hot running plug has a much slower rate of It may not appear difficult to choose a race plug with an
heat transfer and is used tc a.z'd fou _g wnere combus allowable 1000° of latitude (between fouling and preigni
tion chamber or Zj/rze' "ezc :e~ze's:_'es are relatively tion), but the rigordus conditions of racing narrows this
low. latitude considerably.
Length of core nose and electrode a z, material are the Proper heat range selection should position the plug’s
primary factors in eszaz s~ ~g e=z "g zf a particular operating temperature well below the preignition zone as
spark plug design. Hot p _gs see ustrat on) have relatively any abnormalities in engine, timing, or carburetion during
long insulator noses with long heat transfer paths. Cold the race could send plug temperature toward the preigni
plugs have much shorter insulator nose lengths and thus tion region ... inviting not only plug burning but severe
transfer heat more rapid;*. engine damage.
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