By:
Henry P. Olsen
A
question often asked is how big of a carburetor
can be put on my engine, what is too big
and what will be lost if the carburetor
is too big for the engine's needs? On a
recent trip to the SEMA show in Las Vegas,
I asked several of the leading suppliers
of high performance carburetors and intake
manifolds these questions, many of the answers
were quite insightful.
What
A Carburetor Does
The
carburetor and intake manifold work together
as a team to supply a mixture of air and
air to the engine so the engine can convert
this air/fuel mixture into energy to power.
This air/fuel mixture will vary from 14.7
to 12 parts of air to 1 part of fuel depending
on the power demands of the engine, the
carburetor must mix the fuel into the air
in a fine atomized mixture if the engine
is expected to be able to convert this mixture
into power. The carburetor by the use of
a power valve or metering rods will vary
the air/fuel mixture from about 14.7 to
1 a/f at low engine power demand state such
as cruising down the highway at 65 mph for
fuel economy to a high power condition like
wide open throttle where the engine needs
a a/f mixture of 12/1 in order to supply
maximum power. A smaller airflow carburetor
will do a good job of mixing the fuel and
air together at normal/low power driving
conditions and also at higher power demands
of wide open throttle, a larger airflow
carburetor can supply more of the air/fuel
mixture to the engine, but this may be at
the expense of low rpm performance/power
and fuel mileage. As the carburetors airflow
is increased in an attempt to increase high
rpm power the carburetor will lose begin
to it's ability to properly mix the air
and fuel together properly at lower engine
rpm and power demands, the question for
you to decide is what do you want from your
engine high rpm horsepower, low rpm torque
or a combination of both.
Having
the proper fuel pressure and volume that
can maintain the correct fuel level in the
carburetor, as well as a ignition system
with the correct ignition advance curve
that will cause the combustion cylinder
pressure to be at the ideal 12 degrees after
top dead center are also items that must
be correct if you want to get all the power
out of the fuel the engine is burning. This
proper tuning of the ignition advance curve
is one of the most ignored and yet cheapest
method of increasing engine performance,
even if the box your distributor came in
said "ready to run" the advance curve that
it comes with probably is not correct for
your engine and the fuel you are using.
The
correct carburetor is a very important part
of a complete engine package consisting
of a intake manifold, cylinder heads, camshaft,
exhaust system and compression ratio that
all are designed to work with each other
to provide power in a defined rpm operating
range. A standard dual plane intake manifold
will be best for engine speeds from idle
to 5000 rpm, a single plane intake manifold
will provide more power at higher engine
speeds at the expense of lower engine speed
engine power and drivability. The newer
race inspired air-gap intake manifolds help
cool the air charge temperature in the attempt
to create more engine power, but this lower
temperature can cause the fuel/air mixture
to separate at lower engine speeds causing
a whole host of drivability problems. When
you are using an air-gap or other race inspired
intake manifold, I suggest using a high
performance carburetor designed with a tuner
in mind such as a Mighty or Race Demon carburetor
that is available from Barry Grant Inc.
Almost every camshaft, carburetor, intake
manifold or cylinder head manufacture will
have a tech line that you can call for advice
on which of their product will work best
for you and your engines needs, using this
tech advise can help you get the correct
product the first time and keep you from
wasting time and money in your search for
more power. First before you call a tech
line know how you will use your engine and
what the rpm range the engine will normally
be operated at, if your normal driving speed
is 65 mph what rpm is the engine turning,
how much does the car weigh, if a automatic
transmission is used what is the torque
converter stall speed, is the vehicle used
to tow a trailer or carry heavy loads, is
the vehicle used mainly at sea level or
at higher altitudes. All this information
is needed in order to make the correct decision
for almost any performance engine component.
A
good carburetor for a "stock" to "mild"
V-8 engine can be anything from a q-jet
to a afb style that is sold by Edelbrock
as the Performer series to a modular style
carb such as any of the Demon series of
carburetors from Barry Grant, Inc. or Holley
carburetor, all of these carburetors can
be tuned to run well on a normal engine
as long as you select a lower cfm carburetor.
In my experience, almost any carburetor
can be tuned to run well as long as you
stay below 650 cfm, many of the larger air
flow carburetors can have a host of stumbles,
hesitations and other drivability problems
due to the larger venturi necessary to increase
air flow not always supplying the correct
air/fuel mixture at part throttle driving
conditions. For me, the notable exception
of this 650 cfm rule is the Demon series
of carburetors from Barry Grant Inc., which
have been designed to avoid the stumbles,
and hesitations that are common on many
other types of carburetors. A larger cfm
carburetor will often help a engine to create
more power at the higher rpm ranges at the
expense of low rpm power and drivability,
a race designed carburetor can be used to
help overcome some of these problems, but
the carburetor still must not be too big
for the engines needs.
The
formula for determining the what the air
flow or cfm demands of an engine is that
I have read in many of the performance carburetor
books is, the engine cubic inch displacement
times the maximum rpm divided by 3456. This
formula would mean a 350 cubic inch engine
turning a maximum of 6000 rpm would need
a 600 cfm carburetor, this is a good starting
point, but there can be a lot of variables
that are added in such as the intake manifold,
the camshaft, cylinder heads and so on.
One rule of thumb told to me by one of the
excellent people that work for Barry Grant
is the correct cfm carburetor will cause
the engine to have one inch of vacuum at
wide open throttle with the engine at peak
rpm. If the engines vacuum is below one
inch, the air speed will be too slow thus
not mixing the fuel and air together properly,
if the engines vacuum is above one inch
the carburetor is too small and acting as
a restriction to the engines demand for
the air fuel mixture.
The
March 2002 issue of Kit Car has a very good
article by Ron Ceridono titled Tune-Up Tricks
which can be a good reference on how to
tune in your fuel and ignition systems,
or if you have internet access try www.automotiveu.com.
Source
Ole’s
Carburetor & Electric Inc.
120
El Camino Real
San
Bruno , CA 94066
650.589.7377
olescarbs@sanbrunocable.com
John
Bishop
Hot
Rod Tuning
808
Burlway, #2
Burlingame
, CA 94010
650.343.4860
jfb396@aol.com
OTC/SPX
Corporation
655
Eisenhower Dr .
Owatonna
, MN 55060
800.533.6127
www.otctools.com
Barry
Grant Inc.
1450
McDonald Road
Dahlonega
, GA 30533
706.864.2206
www.barrygrant.com |
