Page 17 - Combined_141_OCR

SUPER CHARGING

DETONATION is the enemy of all supercharged applica­
Most hot rodders use Roots-type GMC blowers. They
mount direct on the engine ar use special low-protile in- tions ... because of the extra heat and pressures created.
take manifolds. It’s a plug killer and engine killer. To keep out of detonation:
a) the plug must be cold
b) proper fuel/air ratios must be maintained. (Remember
A "blower” drives 2 impel­ as the quantity of air entering a supercharged engine
lers (see Figure 1) which sim­ increases, the quantity of fuel must be increased to
ply compress or cram the maintain this proper ratio.)
fuel/air mixture into the cylin­ c) the spark advance must be retarded considerably.
ders. This, of course, rams a
denser mixture into the engine This applies to both gas, methanol and nitro blends.
... the denser the mixture ... MOST IMPORTANT:
the hotter the combustion ...
Do not use projected core nose plugs for supercharged
and, consequently, a colder applications.
plug must be used. Figure 1
All pump gas supercharged Do not use regular gap or “P” gap plugs for supercharged
engines generally use a 57R applications, as the side electrode cannot pass off com­
series. All methanol/nitro su­ bustion chamber heat rapidly enough. (There are a few
percharged engines generally exceptions.)
use a “54”I“52” series. There In addition to various engine compression ratios and
are a few exceptions based blower speeds, fuel can be injected directly into the cyl­
upon: inders or through the blower. There are many variations to
a) the compression ratio of feeding a supercharged engine. You can inject a certain
the engine percentage of fuel directly into the cylinders and the rest
b) the speed at which the of the percentage through the blower... or put it all thru
impellers are driven the blower. Each hot rodder has his favorite combination.
Variations or combinations The compounding of all the variables... nitro/methanol
of the above can cause dif­ mix... blower speed ... methods of ‘feeding”... camshaft
ferences in combustion cham­ choice... compression ratio selection ... jetting... calibra­
ber temperature ... and may tion of total spark advance... and spark plug heat range...
call for a heat range one step make supercharged dragster tuning one of the most complex
up or down. Figure 2 skills in all forms of racing.
Forget about hot rod lingo on "boost.” It is too confusing.
A blower is either overdriven or underdriven. This is known
as the drive ratio. Note in Figure 2 a pulley is mounted on SPARK PLUG INQUIRIES BY MAIL
the engine crankshaft and drives a belt which, in turn drives
a pulley connected to the supercharger. If both pulleys are The Champion Racing Division will answer techni­
the same diameter... the blower turns at engine crank­ cal inquiries and suggest a preliminary spark plug gap
shaft speed. However, if the blower pulley is smaller than style and heat range for racing engines. A precise rec­
the crankshaft pulley, the blower is overdriven. Conversely, ommendation is difficult by mail. Strictly stock en­
if the blower pulley is larger than the crankshaft pulley... gines may vary in heat range choice due to diverse
the blower is underdriven. combinations of jetting and spark advance (see pages
All hot rodders now refer to this set-up as their “drive 8, 9 and 29).
ratio." Consequently, if a hot rodder tells you his blower To establish some accuracy in initial reach, gap
is "18% overdrive” ... it’s turning 18% faster than the style, and heat range, the following information is
engine crankshaft... if it’s 22% overdrive, the blower is required*make and original displacement of engine
turning 22% faster than the crankshaft. Remember, the • present displacement • present compression ratio
higher the overdrive, the hotter it will be in the combustion • camshaft used • piston type (if custom) • plug
chamber as the impellers are cramming in a denser mixture. reach • if physical room exists for projected nose or
Roots-type blowers cannot be run on engines with
regular gap plugs (see page 5) • previous plug brand
extremely high compression ratios as destructive detona­ and type used • if burning or fouling was experienced
tion will occur. Engines utilizing Roots-blowers generally • degrees of total spark advance (see page 30) •
run with lowered compression ratios (from 4 to 1 up to 8 to brand and number of carbs • linkage (synchronized
1). If the engine compression ratio is low, more blower or progressive) • carb jetting or • injector or by-pass
overdrive can be utilized. If the compression ratio is high, jet size • fuel or fuel blend • type of racing • nature
then less overdrive can be sustained. In some cases, the of the course • name of race sanctioning organization.
blower may be underdriven. If the engine is supercharged, the following addi­
In 99% of all applications, regardless of drive ratio and tional information is requested: blower brand, model,
"feeding arrangements,” the "57” range will suffice with and type • percentage underdriven or overdriven •
pump gas and the "54” range will suffice with a meth­ percentage of fuel introduced thru blower and per­
anol/nitro mix. Only in very rare instances with a very high centage of fuel directly injected into the ports • in­
overdrive ratio (or high C.R.) would the "54” be out of heat jector brand.
range and a "52” needed. Regardless of percentage of nitro
added to methanol, a "54” range is usually “in the ball
park.”
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