The carb and intake sit atop the engine to supply the correct amount of fuel and air for ideal combustion. Whether you’re performing an original restoration and need factory-matching external parts or performing a highperformance build, you need to select a compatible intake manifold and carburetor combination.
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Like most Mark IV big-block parts, intake manifolds and carburetors are now of limited performance value unless you are pursuing a full-on restoration. While newer internal components are easily hidden, external components must not only have the correct look, but also the correct casting number and physical features to match the original application. Intake manifolds are among the most visible of these components and some of the original Chevy intakes still offer good performance as long as you don’t push them too far past the original performance curve.
Four-barrel cast-iron and aluminum intakes comprise the bulk of original factory intakes. With the exception of tall-deck engines, all intake manifolds interchange between Mark IV, Gen V, and Gen VI engines as long as they are correctly matched to the cylinder heads according to intake port shape. They can also be used on tall-deck blocks with aftermarket spacer adapters. They were made in three different configurations to match cylinder heads on the original applications: large rectangular-port performance versions, standard oval ports, and small oval “peanut” ports. Peanut ports are rarely used except on trucks and some full-size passenger cars so the high-performance manifolds are rectangular-port and oval-port versions. They were cast primarily in iron and aluminum and the carburetor flange was configured for either a square Holley bolt pattern or a spread-bore Quadrajet pattern. There are also some EFI intakes from the 1990s, but these are largely ignored because better aftermarket manifolds are readily available.
The important factors in manifold selection include matching the cylinder head port shape and carburetor flange configuration in the right material, either iron or aluminum. If you’re doing a restoration project, you also want the correct casting number and possibly a reasonably believable date code. There are many variations and, in many cases, finding the desirable casting number helps a lot because it ensures that all the correct accessory mounting bosses are already there. Cast-iron manifolds have the casting number and casting date located on one of the rear intake runners behind the carburetor flange. All aluminum manifolds are cast by the Winters foundry, an outside contractor that casts many of GM’s aluminum engine parts. Winters castings are identified by a prominent snowflake logo with a “W” in the center. The snowflake logo on cast-aluminum manifolds is located on one of the front intake runners or on the manifold flange directly adjacent to the thermostat-housing flange. These manifolds also have a casting date on the bottom, but it is covered by a metal oilsplash/heat shield secured under the plenum by rivets.
Almost all Mark IV big-block intake manifolds are 4-barrel intakes, but Chevy did offer a 2-barrel, oval-port cast-iron manifold on full-size passenger cars in 1969. They are rare and seldom sought even by restorers. Aluminum three 2-barrel manifolds were used on select 435-hp 427 Corvettes from 1967 to 1969. There are both rectangular- and oval-port versions depending on the horsepower rating. Additionally, cast-iron and aluminum intakes were made in both high-rise and low-rise versions to accommodate hoodline clearance on various models. There are no cast-iron intake manifolds with rectangular intake ports; they are all oval-port intakes.
The majority of aluminum big-block manifolds have rectangular intake ports and the square Holley-style carburetor flange. The exception is the aluminum low-rise, oval-port, 4-barrel intake used on 1968–1969 390-hp 427 Corvettes to provide adequate hood clearance. These manifolds carry casting numbers 3947801, 3919849, or 3937793 and are relatively rare and useless for performance applications unless of course you’re restoring one of the 390-horse Vettes. The last rectangular-port factory aluminum intake was offered on the 1971 454-ci LS6. It can still be found at swap meets, but better alternatives are available from the aftermarket. All factory 4-barrel intakes made after 1971 were cast-iron oval-port units. However, since all manifolds interchange among standard-deck big-blocks of any generation (except cathedral-port Gen VII) all you have to do is match the port shape to the cylinder head and you’re good to go.
In some cases you may also need to select a manifold that has the correct accessory mounting bosses to support any EGR (exhaust gas recirculation) or choke-stove configuration you might require for your application.
Temperature sending units were installed in a boss at the front of the intake manifold until late 1968 when they were relocated to the cylinder head and subsequently returned to the manifold in the mid-1970s. Some, but not all, big-block intakes have raised numbers above each runner indicating the cylinder number that the runner services. Additionally, 1973 and later manifolds all have an EGR mounting flange.
The last year for high-rise big-block manifolds was 1970. All subsequent manifolds were of low-rise configuration except for select high-performance replacement manifolds that were never installed on production engines. Production big-blocks from 1987 on are configured for TBI. They all use a Rochester Products Model 220 2-barrel throttle body incorporating twin fuel injectors. These oval-port aluminum intakes were used on Mark IV and Gen V engines through 1995. They are not performance intakes and are not widely sought. In 1996 Gen VI engines were fitted with circular tuned length port fuel injection (PFI) manifolds. In factory configuration they offered more torque and better fuel mileage than TBI units, but they are not performance units. TBI systems are low fuel pressure (9 to 13 psi) units while PFI systems require 43 to 45 psi of rail pressure to support eight individual injectors.
The primary concern with intake manifolds is matching the intake port shape to the intake ports on the cylinder heads. Intake port velocity is a key consideration. Designers sized the ports according to their production application to maintain good flow velocity within the RPM range of intended use.
Peanut ports (small oval ports) were kept quite small to ensure good low-RPM port velocity and thus efficient low-speed cylinder filling along with improved mixture quality. These heads were never expected to see much over 4,000 rpm and were designed for optimum efficiency in moving heavy vehicles in city traffic and on the highway. They have no value in a performance application.
Oval-port intakes were fitted to most standard production vehicles and they have often proved to be suitable for moderate performance use and even some racing applications depending on the displacement and engine speed. Oval-port intakes can often support up to 550 hp with snappier low- and midrange performance than square-port intakes. They are particularly suitable for heavier vehicles like trucks and fullsize passenger cars because they provide more low- and mid-range torque than rectangular-port intakes.
Rectangular-port manifolds are intended for high-performance use only. They have the largest ports and require higher engine speeds to make power. These intakes make good top-end power, but may feel lazy at lower engine speeds depending on the engine size.
Inspect any potential manifold purchase carefully. Iron manifolds are pretty tough customers and they are pretty difficult to damage except for corrosion and wear around the exhaust-heat trough on Q-jet carburetor flanges. Aluminum manifolds sometimes have broken bolt flanges at the front or rear and they often suffer serious corrosion around the thermostat-housing flange and the temperature-sending boss. Manifold ears can be re-welded and corrosion can be repaired with patience and good welding skills, but this should only be required if you’re performing a restoration and simply cannot find another intake.
If you find a factory aluminum manifold with the center divider cut down further or completely cut away you should probably pass on it. It won’t provide the performance you’re looking for unless you’re seeking better high-RPM breathing; and if that’s the case an aftermarket manifold is going to suit your purpose much better. This is particularly true of aftermarket dual-plane manifolds configured to reduce the performance disparity between long and short runners (or good port/bad port runners as they are often called). If you are performing anything other than a restoration project, you should probably seek an aftermarket intake or a Chevrolet Performance intake from a dealer.
Big-blocks came from the factory with Holley 4-barrel carburetors or Rochester Quadrajet carburetors. Both were used in standard and high-performance applications on cast-iron and aluminum intake manifolds. Two-barrel Holley carburetors were used on the three 2-barrel 427 Corvette engines; a Rochester 2-barrel was used on some lowperformance 396-ci engines in 1969.
Holley 4-barrel carburetors are identified by the “List” number stamped into the choke horn. Factory carburetors also include a date code and a seven-digit Chevrolet part number stamped above the List number. Holley replacement carburetors sold over the counter at dealerships did not have a factory part number. Although not as prolific as Chevrolet, it should be noted that Ford and Chrysler also used Holley carburetors on selected applications and they applied their factory part numbers to them. Surprisingly these are rarely seen at swap meets so you’re not likely to run across them.
The accompanying chart shows the “List” numbers for big-block Holley carburetors. Note that factory carburetors have a three-digit date code denoting the year, month, and week that the carburetor was manufactured. The first number indicates the last number of the year of production. It does not specify the decade, but this is not problematic because Holley carbs were only used from 1964 through 1972. The second digit indicates the month and the last digit indicates the week the carburetor was manufactured (it is always a number from 1 to 4). For example, a list number of 754 equates to 1967, May, 4th week.
All 4-barrel big-blocks that did not use a Holley carburetor were equipped with spread-bore Rochester Quadrajets incorporating small primaries and extra-large secondaries assisted by a spring-loaded air valve. They are easily recognized by the smaller choke horn with two large air-valve doors immediately behind it. They have two forms of identification: a number stamped vertically on the throttle lever side of the body or a stamped metal tag attached to one of the air-horn bolts.
Some Quadrajets were contracted to Carter Carburetor to help meet production requirements. These carbs have a small sheet-metal disc on the driver’s side of the carb with identification numbers. A date code is stamped on the base of the carburetor on the throttle lever side near the rear bolt. It is a four-digit code with the first three numbers indicating the day by Julian date, i.e., 138 = 138th day of the year. The last digit indicates the year, which in this case could be 1965, 1975, or 1985. From 1965 to 1975 ID numbers used seven digits. From 1976 on they used eight digits beginning with a “1.” This makes it relatively easy to identify the correct decade.
For example, 1385 could indicate the 138th day of 1975 or 1985, which could be confusing except that the 1985 version was a partially electronic carburetor identified by an electric metering solenoid on top of the air horn ahead of the choke housing.
Written by John Baechtel and Posted with Permission of CarTechBooks