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Selecting a Carburetor
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How Big?
Prior to the development of the WCFB, the installation of multiple carburetors was the only effective
means of increasing intake airflow capacity. Early attempts at high-performance V-8 carburetion
typically consisted of an aluminum intake manifold and two, three, four, six, or eight Stromberg two-
barrel carburetors. Airflow ratings were not generally available, but back in the days of the small-
displacement Ford flathead V-8 and Chevrolet 265 and 283 cubic inch small-block, there was little
need for exceptionally high flow ratings.
Later efforts at multiple two-barrel carburetion used the larger Rochester 2GC carburetor, and in
the late ’50s even General Motors got into the act with triple two-barrels on the Oldsmobile “J-2,”
Pontiac, and Chevrolet 348 engines. But the most unusual (although ill-fated) development in
multiple two-barrel manifolds was the handiwork of a company by the name of Man-A-Fre. By
positioning a Rochester two-barrel directly over each pair of cylinder head ports, Man-A-Fre
marketed “direct port induction” as the state-of-the-art in manifolding. If nothing else, the four
carburetors looked impressive, but for most users, performance never lived up to the enthusiastic
promises of the marketers.
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When awareness of carburetor airflow ratings became commonplace, “bigger” was typically
equated with “better” (and too much was just enough), because even the largest four-barrel of the
time was not capable of over-carbureting a highly modified overhead-valve engine. The early
WCFB flowed only about 385 cfm. Consequently, a dual-WCFB installation would flow only 770 cfm.
(It is interesting to note that dual-WCFB four-barrels first appeared as original equipment on the
225-horsepower 1956 Corvette; however, a factory triple two-barrel installation wasn’t offered until
1957, when three Rochester 2GCs were fitted on certain Oldsmobile and Pontiac engines.)
By comparison, the largest-capacity Edelbrock AFB and AVS four-barrels currently produced flow
800 cfm. Obviously, with the general availability of such high-capacity carburetors, bigger no longer
necessarily equals better. In fact, a smaller four-barrel is often preferable to an extremely large
one. While a 625-cfm unit may not offer the glamour of dual AFBs on a ram-type manifold, the
former will provide better drivability, greater fuel economy, and crisper throttle response than the
latter, on all but large-displacement super-high-performance and race engines. A single four-barrel
is also considerably lower in price and easier to maintain.
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How to Determine Flow Requirements
Choosing the correct performance carburetor for a particular installation involves careful analysis.
The first step is to determine the maximum airflow requirements of the engine. On the surface, this
appears to require little more than converting engine size from cubic inches to swept volume (air
required to “fill” the cylinders) in cubic feet, then selecting a carburetor that offers a corresponding
airflow capacity. However, to arrive at an optimum selection, induction efficiency, useable RPM
range, the intended use of the engine, plus the total number of throttle bores must be taken into
consideration.
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Factory-installed, dual Carter
four-barrels first appeared on
the 225-hp Corvette engine of
1956. Combined airflow
capacity was only 770 cfm.
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Airflow: Ratings vs. Reality
Carburetor flow ratings are based on a theoretical vacuum in the intake manifold under maximum
load (at wide-open throttle). These pressure-drop values are intended to relate flow bench data to
“real world” airflow through a carburetor when mounted on an engine. However, as total throttle-
bore and venturi areas increase, maximum theoretical manifold vacuum at wide-open throttle
(WOT) decreases (as does maximum airflow potential). On the other hand, as engine size
increases, so does the potential for airflow demand to exceed a carburetor’s flow capacity—in which
case actual manifold vacuum will not drop to the levels at which carburetors are rated (and the
engine will be airflow restricted or “throttled,” even when a carb is wide open). As a result, it is
entirely possible that a Performer Series carb rated at 500 cfm will flow almost 600 cfm, and that an
800-cfm model will not flow more than 700 cfm. Actual airflow through any carburetor will match its
rated flow capacity only if manifold vacuum at WOT is the same as the pressure differential at which
the carburetor was rated.
Whether a carburetor never meets its rated airflow capacity or exceeds it is to some degree
irrelevant. The key point is that for maximum performance a carburetor must deliver the airflow
capacity demanded by the engine. As a general point of reference, if manifold vacuum exceeds 1.5
in/Hg (at WOT) on an engine equipped with a four-barrel, the carburetor is too small; if it falls below
1.5 in/Hg, the carburetor is too large. Obviously, there is at least one hole in that theory because
any manifold vacuum level above zero represents a throttled condition, which means an engine won’
t produce maximum power. However, if a carburetor is large enough to drop WOT manifold vacuum
levels to zero, drivability issues at part throttle may result and low airflow velocities may negatively
impact power. The key is to find that magical airflow capacity that keeps air velocities high, without
excessively restricting flow.
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The Pontiac GTO has always
enjoyed an excellent
performance reputation
because its engines would dish
out buckets full of low-speed
and mid-range torque. The
base engine was equipped with
a Carter AFB, properly sized to
avoid over-carburetion. (Robert
Genat)
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Obviously, carburetor airflow ratings are not intended as absolutes; rather, they are relative
guidelines. Depending on engine displacement and volumetric efficiency, airflow may exceed or fall
below the rated capacity of a particular carburetor. In most instances, the manufacturer’s rating will
be a very usable approximation, so it really doesn’t pay to become entangled in long calculations
involving venturi shapes and air-flow peculiarities—even if you do have the latest-model electronic
calculator or modeling software. The best bet is to accurately assess an engine’s airflow
requirements and choose the appropriate carburetor. Manufacturer recommendations are also
helpful. As an example, the Edelbrock catalog contains a chart of suggested carburetors for street
and combination street/race usage. Each of the categories assumes a specific level of engine
modification (i.e., a certain degree of volumetric efficiency). As the degree of modification rises, so
should the level of volumetric efficiency and therefore the flow capacity of the carburetor.
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Too much of a good thing is still
too much. The modest-size
carburetors used in lower Stock
and Super Stock classes
demonstrate that “big number”
carburetors are not essential
for impressive performance.
The key is matching airflow
capacity to specific engine
requirements. (Robert Genat)
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Too much of a good thing is still too much. The modest-size carburetors used in lower Stock and
Super Stock classes demonstrate that “big number” carburetors are not essential for impressive
performance. The key is matching airflow capacity to specific engine requirements. (Robert Genat)
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Previous | Next
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This has been a sample page from
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How to Rebuild and Modify
Carter/Edelbrock Carburetors
by Dave Emanuel
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If you want to modify your carburetor-equipped vehicle for
performance today, you can choose from three major carburetor
families – Holley, Demon, and Edelbrock. While Holley and Holley-
style carburetors like Demon have been popular for years with
their well documented successes and quirks, there has never
before been a book covering the ins and outs of the emerging
Edelbrock line of carburetors.
How to Rebuild and Modify Carter/Edelbrock Carburetors reflects
the emergence of Edelbrock carburetors as the predominant
Carter-style carburetors in the market today. A revision of the
best-selling title Super Tuning and Modifying Carter Carburetors,
this book contains more than 300 color photos, illustrations, and
diagrams, covering rebuilding, tuning, and modifying Carter and
Edelbrock carburetors.
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Click below to view
sample pages!
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1 - History of Carburetors
2 - Carburetor Types
3 - Selecting a Carburetor
4 - How a Carburetor Works
5 - Carburetor Modifications
6 - Carburetor Tuning
7 - Rebuilding Tips
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This book also features the history of Carter as well as the history
of the AFB and the AVS since the purchase by Edelbrock. Author
David Emanuel outlines carburetor types, gives a thorough look at
carb selection and carb function, and offers detailed information
on modifications, tuning, and rebuilding Carter/Edelbrock
carburetors.
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This is a great book
that anyone who
owns or works on
Carter or Edelbrock
Carburetors will love!
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8-1/2 x 11"
Softbound
136 pages
159 color and 156 b/w photos
Item # SA130
Price: $22.95
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Click here to buy now!
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Rebuild and Modify Quadrajet Carburetors
How to Rebuild and Modify Rochester Quadrajet
Carburetors, seeks to lift the veil of mystery surrounding the
Q-Jet and show owners how to tune and modify their carbs for
maximum performance. This is the only book to truly cover
step-by-step instruction on performance building the
Rochester Quadrajet!
A complete guide to selecting,
rebuilding, and modifying the Q- Jet, aimed at both muscle
car restorers and racers.
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Price:
$
22.95
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Holley Carburetor Handbook
The Holley Carburetor Handbook explains the basics of fuel
inlet, idle, main metering, power valve and metering,
accelerator pump, secondary and choke systems on Holley
carburetors. It also covers carburetor repair and adjustment.
Bonus section describes how to choose a carburetor based on
engine size and volumetric efficiency. More than 100 photos,
charts and drawings show you how to do it right the first time.
This is a must have book for anyone working on the Holley
model 4150 or 4160 Selection.
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Price:
$11.95
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Super Tuning and Modifying Holley Carburetors
Super Tuning and Modifying Holley Carburetors shows you
how to select, install, tune, and modify all popular Holley
performance carburetors. This book gives a detailed view of
basic carburetor functioning, modifying for performance
applications, custom tuning for street, racing, off-road,
turbocharging, economy, and other special uses.
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Price:
$
18.95
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Payment, Shipping & Sales
Tax: Iowa
residents must pay 7% sales tax. Items usually ship
within one
business day of receipt of payment! Standard shipping is a flat rate of
$4.95 to
anywhere in the United States with USPS Media Mail.
Priority Mail shipping is available for an
additional $2.95, or
$7.90 for shipping. Shipping is combined and discounted for multiple item
purchases as follows: first item regular shipping price, add
$1.95 for each additional item. For
purchases of 3 or more items
shipping is automatically upgraded to Priority for no additional
charge! We offer world wide shipping and ship to Canada and Mexico
with USPS Priority Mail
International for $11.95, and to most
locations in Europe, Australia, Asia, Japan and South
America for
$14.95. Satisfaction is Guaranteed. Our store has a NO HASSLE RETURN
POLICY
within 7 days of purchase.
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