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SPARK PLUG HEAT RANGE
FACTORS INFLUENCING

CAMSHAFTS
COMPRESSION RATIO
Customized cams increase combustion efficiency and
An engine bored over-size will increase in compression
ratio. Higher compression heads, pistons, a stroked crank­ horsepower over a limited engine RPM band .. .within this
shaft, will also increase compression ratio. band, the cam will actually increase cylinder pressures and
The degree of heat produced within the combustion cham­ temperatures. If this RPM range can be maintained and sus­
ber depends upon the number of alterations effected. A tained over a long period of time, the engine may require a
colder plug will be required to tolerate the higher combus­ colder plug ... but this is usually not the case in most rac­
tion temperature. ing... especially drag racing, as full throttle is maintained
Special high-lift camshafts installed in engines with in­ less than 15 seconds. Custom cams cannot maintain their
creased compression ratios can raise combustion tempera­ full efficiency when used on the street or on courses that
ture even higher, particularly when the camshaft is within demand “on and off” throttle settings. Consequently, there
its “working” RPM range. is not a continuous or sustained increase in compression
(In extensively modified or full race engines, compression pressure resulting in ... higher combustion pressure ... and
gauge readings taken at cranking speeds are not indicative heat... therefore, the substitution of a colder plug may be
unnecessary. In fact, when engine RPM is consistently out­
of true cylinder pressure. At lower speeds, the camshaft side of the “working range” prescribed for a particular cam,
cannot “pump” high cylinder pressure because of excessive
valve overlap.) a colder plug could only lead to “loading up” or fouling
conditions.
Best bet with custom cams is a projected nose plug (pro­
viding room exists between piston and plug). Projected nose
NATURE OF THE COURSE
plugs of proper heat range perform well with high-perform­
Spark plug heat range (and occasionally gap style) can be ance camshafts for the following reasons:
tailored to a race course.
Tracks having long straight-a-ways or courses having high- a) at lower RPM, combustion pressure is less
banked, high-speed turns generally require colder plugs than b) combustion chamber temperature is lower
circuits with numerous esses and hair-pin turns or flat tracks c) a rich mixture persists
with short straight-a-ways. The ability of the projected nose plug to run warmer un­
Providing the engine is not highly modified or burning der these conditions minimizes plug fouling and provides
nitro ... or supercharged ... (and physical room exists), pro­ good acceleration characteristics when maximum power is
jected nose plugs of comparable heat range can be substi­ again applied. Projected nose plugs are not recommended
tuted on “slower” tracks. Fouling protection and engine re­ for supercharged engines.
sponse is generally improved. (See Gap Style, 10 and 11.) Because “performance” camshafts can alter effective
cylinder pressures at a given engine speed, it becomes
necessary to make corresponding changes in ignition tim­
STREET USAGE ing; not only in advance rates, but frequently in initial and
Best performance in competition cannot be obtained us­ total settings. When to fire a plug is going to be dependent,
ing the same plugs for street driving and racing. A plug in among other factors, upon when the intake valve actually
street use picks up deposits during start/stop driving. These closes (in terms of crank rotation). Altered intake valve tim­
deposits, however slight, will bleed away voltage under ing will require ignition timing changes, and along with
strenuous racing conditions and penalize performance. Best gearing, carburetion, and effective cylinder pressures, be­
policy is to use two sets of plugs ... one set for street, one comes one of the several variables whose effects are prob­
set for racing only. The set for racing should carry a .005" ably best handled by a certain degree of experimentation.
closer gap setting than the recommended street gap. As a starting point, duration increases to drop cylinder pres­
sures in the lower rpm ranges and require more ignition
“lead.”
ELEVATION
An engine loses horsepower and efficiency as elevation SPARK ADVANCE
increases. At 1,000 ft., it will lose almost 3% of its The factor of spark timing has one of the greatest effects
total horsepower... at 2,000, nearly 6¥2% ... at 3,000, on spark plug temperature. It becomes a more critical factor
about 9^2%. as compression ratio increases and engines are extremely
Such losses are reasonable because the cylinders can’t modified.
induct as much oxygen when air density decreases. To make There are “internal” environments that influence spark
matters worse, the thinner atmosphere changes the fuel/air advance settings:
ratio ... you get less air but the fuel fed to the engine stays
nearly the same... with the result... the carburetor (un­ 1) combustion chamber design
touched by human hands) delivers a richer mixture so more 2) compression ratio
horsepower is wasted by the carb’s inability to maintain 3) fuel/air ratios
proper fuel/air mixture. Consequently, if a race is held at 4) type of fuel
elevations considerably above sea level, the mixture should 5) engine speed
be leaned to compensate for nature’s automatic over­ There are also “external” environments:
nourishment.
A jet change should be effected on drag strips or race 1) elevation
courses of high elevation. Spark plug heat range should stay 2) humidity
the same as at sea level... unless racing at an extremely 3) temperature
high elevation (7,000 ft. or more)... in which case, one
step hotter would suffice. Many racers believe that extremely high RPM gives great
dividends in horsepower. This is true with small engines
Frequently, you can flog a few more horses at higher ele­ (Continued next page)
vations by increasing the spark lead a few degrees.
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