By: Henry P. Olsen
Setting
up the fuel and ignition systems to get
a blown engine to run properly may take
some time, but the results are worth it.
The engine must be built with the supercharger
in mind, the compression ratio, as well
camshaft should be matched to the application
and designed use. Always select the camshaft
to match the rpm band the engine will normally
be operated in, a race cam may make power
at high rpm but it will not run well at
lower engine speeds. There are a few �set-ups�
not used on a normally aspirated engine
that can make all the difference in how
the engine runs. The camshaft lobe separation
that seems to work best for a supercharged
engine is 112 to 114° along with a split
duration camshaft where the intake lift
and duration may be mild yet a more aggressive
exhaust cam profile. This type of camshaft
can be used on a supercharger since you
are blowing the air in to the cylinder;
this type of camshaft creates more horsepower
without hurting low rpm torque. Using tools
such the OTC/SPX PerformanceGas or MicroGas
exhaust gas analyzers and advance timing
lights allows us to tailor the fuel and
ignition advance curves to unleash all the
horsepower the supercharger is blowing into
the engine.
Blow
through carburetor supercharged set-ups
The
first concern for an engine using a blow-through
supercharger where the carburetor is under
boost pressure is fuel pressure. A special
fuel pressure regulator that is boost pressure
referenced along with a high pressure/volume
fuel pump to keep the fuel pressure correct
for this type of supercharger system must
be used. The boost referenced pressure regulator
senses the boost pressure and then regulates
the fuel pressure to keep the fuel pressure
at a set amount above the boost pressure.
The pressure that we use most is 5 � to
6 lbs. over the boost pressure, so when
you have 6 lbs. of boost the fuel pressure
would be 11 � to 12 lbs. The floats used
in the carburetor on a blow thru supercharger
must be made of nitrophyl or of some other
solid material, brass or hollow plastic
floats will collapse from the pressure the
supercharger puts into the fuel bowls. The
2 main ways of pressurizing or blowing through
the carburetor are #1 where the carburetor
is in a pressurized box and #2, is the use
of a hat in place of the air cleaner on
top of the carburetor. If the carburetor
is not inside of a �box� but using a hat
to pressurize the carburetor bowls &
venturii, the throttle shafts will have
to have seals installed to keep the fuel
from blowing out thru the clearance in the
throttle shaft. We always recommend a marine
type of flame arrestor be used inside the
air box or hat to defuse the air-charge,
if this is not used the air charge blowing
from just one side of the carburetor will
cause the air/fuel mixture to be incorrect
as the boost changes.
The
idle and off-idle systems are the hardest
part of the fuel system to get the fuel
curve correct with a blow-through supercharger
system, extreme care must be taken to properly
size the idle fuel and air restrictions
in order to obtain the correct idle and
part throttle fuel mixtures. The reason
this care must be taken is because this
type of supercharger is pressurizing the
idle air bleeds thus blowing the fuel into
the engine thru the idle system. The rest
of the carburetors fuel circuits operate
in a normal fashion. The fuel mixture at
idle speeds and cruise speeds seen most
are idle CO of 1 to 3%. The cruise mixture
on a low boost, mild cam engine would be
1 to 1.5% CO while the high boost pressure
hot cam cruise mixture is 3% CO. A power
mixture of 6.6% CO (12 to 1 air/fuel mixture)
is used on most engines, yet you can go
richer if needed to help control detonation.
Pull-through
Carburetor Supercharger Set-ups
�
An
engine using a carburetor on top of a supercharger
uses a standard set up for fuel pressure;
the fuel pressure we use most is 5 � to
6 psi. The most important point is to use
a high volume fuel pump along with fuel
lines large enough to keep fuel pressure
constant at all engine loads.
Selecting
the correct carburetor
Selecting
the correct carburetor for a supercharged
engine is very important; the fuel curves
for a low boost engine versus a high boost
engine make proper carb selection very important.
The Demon carburetor line from Barry Grant
Inc. offers carburetors designed for use
on supercharged applications; these carbs
are in their Race Demon series of carburetors.
These carburetors come with removable sleeves;
this feature allows you to change the carburetor
airflow (cfm), so if you change boost pressure,
engine size, or cam, you can resize the
carb for the new engine without buying a
new carb. The Race Demon series carb also
has changeable air bleeds and idle feed
restrictions; this feature makes it easy
for your tuner to dial in the fuel mixture
curve for the engine�s needs. �
The
blower Mighty and Race Demon carburetors
have power valves with vacuum tubes that
allow the power valves to be boost referenced,
just connect this port to boost pressure
and the power valve can see engine load
or vacuum instead of the vacuum created
by the supercharger. A boost referenced
power valve helps make it possible to get
the fuel mixture curve correct for all engine
loads and should be used on any high boost
street driven engine. One of the more common
problems I have seen is with the engine
at idle speed; the engine will go into a
supercharger roll (the engine speeds up
the slows down then speeds up�.). This supercharger
roll is caused by the superchargers vacuum
signal to the power valve not being the
actual engine vacuum, but the vacuum created
by the supercharger. The power valve reads
the higher vacuum created by the supercharger,
this vacuum closes the power valve, causing
the fuel mixture to go lean, the engine
then slows down due to the lean air/fuel
mixture so the vacuum signal from the supercharger
causes the power valve to open as the superchargers
vacuum drops, so again the engine again
speeds up due to the richer air/fuel mixture,
rolling from rich to lean causing the engine
to speed up then slow down. �
The
fuel pressure we use most is 5 � to 6 lbs.
and we always suggest a high volume fuel
pump that can supply enough fuel to keep
the pressure constant at all engine loads.
We have seen Holley style carbs where we
had a fuel leak from the plugs on the metering
block; this was caused by a carb air scoop
creating a high pressure in the fuel bowls
pushing fuel past poorly fitting metering
block plugs. The cure for this was to epoxy
the plugs in the metering block and putting
a few vent holes in the carb scoop to help
lower the air pressure (this problem has
not been seen by us on Demon carbs or on
AFB style carbs).
�
AFB
style carbs on a low boost pressure - mild
blown engine can work well, but since they
do not offer a boost referenced power system,
care must be taken to avoid overly lean
fuel mixtures, especially at off-idle to
part throttle. A street driven dual 500-cfm
afb set-up can be a very good running set-up
on a mild blown engine that gives a great
appearance and also can run well under normal
street driving conditions. The 500 &
600-cfm carbs can be modified to avoid an
off-idle lean condition, so they can work
well on a low boost pressure single or dual
carburetor set-up, but we avoid the 750
and 800 cfm AFB�s on street driven blown
engines because of a lean at part throttle
problem that is inherent with this type
of carburetor.
Jetting
a supercharged engine with the MicroGas
��
The
method we use to determine and then obtain
the correct air/fuel mixture (jetting) is
on an engine is the use of a tool from OTC/SPX
tool company, the PerformanceGas and MicroGas
portable exhaust gas analyzers, both units
have provided us with accurate data that
allows us to obtain great results. The CO
reading that an exhaust gas analyzer provides
is an accurate indicator of the air/fuel
mixture (jetting). Jetting or obtaining
the correct fuel mixture on a supercharged
engine is different on a low boost engine
when compared to a high boost application.
� On a low boost application the air/fuel
mixture is almost the same as a normal hot
rod fuel curve, but when a set-up is used
with a lot of boost, the fuel mixtures used
can be much richer. On a low boost engine
the power mixture we use is a CO reading
of 6.6% or a 12 to 1 air/fuel ratio, but
when using a lot of boost a power mixture
0f 11.5 to 1 air/fuel or CO reading of 8.0%
is not uncommon. This richer mixture can
help in controlling detonation created by
the high boost pressure, but if you go too
rich you will lose power. The cruise mixture
on a mild cam low boost pressure engine
would be 1% TO 2% CO (14.1-13.8 TO 1 air/fuel
ratio) but on a hot cam high boost engine
the target mixture would be 3% CO (13.4
to 1 air/fuel ratio).
��������������������������������������
A
blown engine will require the accelerator
pump to be tailored to its needs; some engines
will need a quicker accelerator pump squirt,
others will need more accelerator pump volume.
Always avoid drowning the engine with too
much accelerator pump. A street driven supercharged
engine with 2 Holley style carbs will very
seldom need 50cc pumps on the primary side
of the carbs.
Intercoolers
/ cooling the charge
The
act of compressing the air charge into the
engine creates heat; it is not uncommon
to see air charge temperatures in the 200
to 250 degree plus range. This heating of
the air charge hurts engine performance
by reducing the air density. By cooling
the air charge you can gain back about 1%
in horsepower for every 11 degrees you can
drop the air temp, by using an intercooler,
it is easy to get a 10% increase in power
or more.
Fuel
injection with an add on blow through supercharger
If
factory fuel injection is used and you do
not use an intercooler the vehicles computer
may �see� air charge temperatures that are
not in the computers program. If a computer
sees data that is not in it�s program, it
may cause the engine to run poor or not
at all. In most cases when adding on a supercharger
to a fuel injected car, higher flow fuel
injectors be needed and the computer may
need reprogramming for the engine to perform
properly. A mass air flow equipped fuel
injection system will be the easiest type
of fuel injection to add a supercharger
to since this type of injection can �see�
the extra air being feed to the engine and
therefore can calculate the correct amount
of fuel needed. Speed density fuel injection
systems will require reprogramming for a
supercharger.
Ignition
system set-ups with a supercharger
The
ignition system used on a supercharged engine
must be up to the job; the higher the boost
pressure, the more ignition output is going
to be required to fire the spark across
the spark plug gap. This higher ignition
output will also require the wires and the
rest of the ignition system to be up to
the job of delivering the extra ignition
output to the spark plugs. Always consult
the ignition supplier of the product you
are using for the recommended spark plug
gap when using a supercharger; use these
specs as a guideline for where to start.
Some experimentation may be needed to find
the spark plug gap that works best for your
engines demands and the air-fuel mixture
inside the cylinder. � The use of a engine
ignition analyzer to check the condition
and output of the ignition system can confirm
if the ignition system has enough spark
output to handle the extra ignition output
needed to fire the sparkplugs because of
the higher combustion chamber pressures
created by the boost from the supercharger.
The ignition system analyzer we use
to check a ignition system is the Vision
Premier or the Genisys Scope Module from
OTC/SPX, either unit allows us to easily
check the output and condition of the ignition
system.
An
MSD ignition system such as a #6btm (boost
timing master) may use a spark plug gap
of up to .042� with 6 psi of boost. The
higher the boost (combustion chamber pressure)
the smaller the plug gap, we have seen spark
plug gaps of as small as .018 - .022� on
a high boost engine with a Msd ignition
system. (Note a 9 to 1 non-supercharged
engine may use a plug gap of up to .055�.)
The
ignition advance curve, in general, is quicker
yet shorter than an engine without a supercharger,
this helps give better throttle response
until the boost pressure kicks in. The advance
curve most seen is an initial timing of
18 degrees with a total of 32 degrees; this
is just a starting point and must be tailored
to each engine. MSD can supply a boost retard
ignition system, which allows an advance
curve that can supply enough advance to
give good throttle response yet, as the
boost comes in, it can retard the timing
to avoid detonation. The boost retard system
can retard the timing from 1 to 3 degrees
per pound of boost and total retard can
be as much as 20 degrees. A vacuum advance
should only be used in a low boost application
and should be limited to 10 degrees of advance
with engine vacuum above 10 to 12 inches
of vacuum. The best way to check and set
the advance curve is on a distributor test
stand, then confirm the results with an
advance timing light such as the type we
use from the Black Light series of timing
lights supplied by OTC/SPX Tool Company.
So
there�s the long and short of it. If you
are going to run a blown engine, the right
carburetors and ignition system that are
tuned for your engine with the proper tools
will make your hot-rod experience enjoyable
and trouble free.
Ole�s
Carburetor & Electric �
120
Camino Real �
San
Bruno, CA 94066
650.589.7377
�
olescarb@aol.com
�
Reggie
Jackson�s High Performance Engines �
1137
San Mateo Ave�
San Bruno, CA 94066 �
650.873.7492
�
reggiejacksonhp.com
John
Bishop � Hot Rod Tuning
808
Burlway#2
Burlingame,
CA 94010
650.343.4860
�
jfb396@aol.com
��
SPX/OTC
Corp.
655
Eisenhower Dr.
Owatonna,
MN 55060-0995
507.455.7000������
www.otctools.com
��
Barry
Grant Inc.
1450
McDonald Road
Dahlonega,
GA 30533
706.864.8544
www.barrygrant.com
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