An intake manifold transfers the mixture of fuel and air from the carburetor to the cylinders. The plenum, or large voluminous area directly under the carburetor, gathers that combustible mixture before it’s distributed to the cylinders through individual runners that connect to the intake ports.
It’s tuned to provide maximum cylinder fill in a general RPM range and the design style and shape, and plenum and runners sizing strongly influences it. Engine displacement can also influence it. Larger engines tend to pull the power range downward, so a particular manifold that produces greatest average power from 3,000 to 6,000 rpm on a 400-ci, may work best from 2,500 and 5,500 on a 455-ci.
Intake manifolds can be classified into two distinct types, and it’s directly related to plenum style. A dual-plane features a split plenum, with each half branching into four runners. The design allows half the engine to draw from half the carburetor at all times. A dual plane is most commonly associated with a stock manifold because it improves carburetor efficiency, which subsequently improves economy, emissions, and low-speed performance. It does, however, adversely affect peak performance at very high RPM.
The induction system of a Pontiac V-8 typically consists of a carburetor and intake manifold. Each is designed to maximize performance in a particular range. Larger race engines that operate at high speed require a significant volume of fuel and air, and that requires a unique intake manifold with multiple carburetors.
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A dual-plane manifold splits the plenum in half. To maximize performance, the runners are staggered so the carburetor sees an equal amount of draw at all times, and that requires a top and bottom plane. The design improves air velocity through the manifold and carburetor, which improves engine efficiency at low to moderate speeds. A dual-plane is an excellent choice for a street engine.
A single-plane manifold contains a large, open plenum in which all eight runners draw from at all times. The design causes air velocity within the manifold to suffer at low engine speeds, affecting engine effi ciency. Generally speaking, a singleplane manifold is best suited for engines that regularly operate at moderate to high speeds, and spend little time loaded at very low speed.
Pontiac used a dual-plane intake manifold constructed of cast iron and a 4-barrel carburetor on many of its passenger car engines in the 1950s and 1960s. The manifold provided excellent performance, but Pontiac used multiple carburetors and specific manifolds for its high-performance engines of the era. The original 4-barrel produced through 1967 is best left for restoration applications.
The use of multiple carburetors was banned by General Motors for 1967. Pontiac developed an all-new cast-iron intake manifold for its high-performance engines that model year. It was combined with the new Rochester Quadrajet carburetor to equal the performance of the Tri-Power it replaced. It performed excellently and remained in production through the late 1970s.
A single-plane manifold contains a common plenum in which all eight intake runners draw. While it gives the engine greater voluminous area to pull from at high RPM, and can show strong performance gains in higher revving applications, a single-plane tends to degrade idle quality and throttle response in the process, and that impacts engines that are primarily street driven and operate at relatively low speeds. Which intake manifold type is best for a particular engine largely depends upon intended usage and operating range.
For its production engines Pontiac developed and used intake manifolds that accepted 2- or 4-barrel carburetors, and even those accepting multiple carburetors. Generally speaking, most were constructed of cast iron, but some special examples were cast in aluminum to shed weight and reduce intake charge temperature. There were even experimental intake manifolds produced for the early 1960s Super Duty that bolted together and contained a large common plenum that hobbyists referred to as the “bathtub.”
Factory Cast-Iron 4-Barrel
When Pontiac introduced its V-8 in 1955, a 2-barrel carburetor was only available. An optional 4-barrel carburetor and corresponding dual-plane intake was introduced shortly after for performance minded enthusiasts. It continued on through 1966 for all 4-barrel applications and was originally topped by a square-bore-type Carter or Rochester carburetor depending upon the application and model year. Its coolant passages and mounting points were revised for 1965, making it compatible with popular Pontiac cylinder heads commonly used today.
Pontiac introduced a new 4-barrel manifold when multiple carburetion went away for 1967. It was a takeoff of the early 1960s Super Duty manifold developed for classes were only a single carburetor was allowed. It features long, smoothly contoured runners to maximize midrange torque and provide strong peak horsepower without compromising low-speed performance. Along with the new Rochester Quadrajet, the combination was used in all 1967 performance applications. The existing 4-barrel manifold and Carter carburetor remained for select standard performance applications, however.
The basic intake manifold casting was modified to accept emissions equipment over the years. An exhaust gas recirculation valve (EGR) was added to its passenger side in 1973. It remained in that position until traditional V-8 production ceased in 1978. Despite the negative perception, EGR didn’t affect the performance potential of the castings.
The new manifold proved to be so well suited for myriad applications that it became Pontiac’s only 4-barrel from 1968 forward, and the original design was discontinued. Over the years the casting saw the addition of an exhaust gas recirculation (EGR) system to quell certain emissions, and even physical deviations. It was discontinued after the 1978 model year, but several thousand were stockpiled for use in the optional 400 available 1979 Trans Am.
Factory High-Performance 4-Barrel
A specific cast-aluminum manifold was used with the 1969-1970 Ram Air IV and 1971–1972 455 H.O. An essential copy of the cast-iron manifold, the runners were raised about .125 inch to match the cylinder head intake ports and improve airflow. To isolate the manifold from excess heat, a separate cast-iron exhaust crossover connects the cylinder heads. Pricing can range from $400 to $1,500 depending upon the casting and if the original exhaust crossover is included. If considering one for your build, the 455 H.O. castings are typically most affordable.
A cast-aluminum intake manifold was developed to complement Pontiac’s highest performance production engines of the late 1960s and early 1970s. Its runners were enlarged to accommodate the increased airflow capacity of the Ram Air IV and 455 H.O. engines it was used with. The aluminum construction improved heat dissipation. A separate cast-iron exhaust crossover (not shown) housed a divorced choke stove.
Proper runner dimension and core shift can be common issues with the castaluminum manifolds, particularly the service-dated units sold though Pontiac’s parts departments. Correcting takes little more than an hour with the grinder, and it’s a process that’s should be performed when using any intake manifold in a performance rebuild. The 1973–1974 SD-455 cast-iron manifold is otherwise externally identical to a standard production casting that year, but its runners taller to improve airflow. It’s easiest identifiable by the large “LS2” cast onto its coolant crossover and/or underside. It rather rare and quite valuable, and an asking price of $2,000 or more isn’t uncommon. Considering that it performs similarly to the cast-aluminum manifold, I suggest leaving these manifolds for purist restorations.
Factory Tri-Power
The Tri-Power manifold is quite likely Pontiac’s most popular. A regular production option during the 1950s and 1960s, it performed consistently and reliably on the street. The center 2-barrel is used for idling, low-speed acceleration, and lightthrottle cruising while the end carburetors open progressively as demand increases.
While a dual 4-barrel carburetor package was limited to Pontiac’s competitions engines only, Tri-Power was a popular option that improved the output of its high-performance street engines during the late 1950s and 1960s. Fitted with three Rochester 2-barrel carburetors, the center unit is responsible for idling and low-speed driving. The end carburetors open progressively as engine load increases.
A reproduction of Pontiac’s most desirable Tri-Power intake manifold has been on the aftermarket for a few years. It’s a cast-aluminum copy of the 1966 unit that accepts all original equipment and is compatible with popular Pontiac cylinder heads. While it flows as well as an original in as-cast form, the aluminum construction allows for easy porting, which improves airflow capacity.
Most Tri-Power manifolds were constructed of cast iron. The 1965 and 1966 units are compatible with the original and aftermarket Pontiac cylinder heads commonly used today. The center 2-barrel carburetor is larger on the 1966 manifold, which increases its desirability and can improve performance, particularly with larger engines. Depending upon condition, a bare early-vintage manifold can bring $100 or more. A 1965 manifold generally cost a few hundred dollars while the 1966 units can bring several hundred. Expect the 2-barrel carburetor trio and associated linkages and hardware to add several hundred dollars to that cost.
Factory Reproductions
A cast-aluminum reproduction of the original R/A IV and 455 H.O. manifolds is available from a number of sources. I haven’t been impressed with the quality for performance use, the machining process leaves behind many sharp edges internally and plenum volume is reduced. A significant amount of grinding is required to achieve the flow capacity of an original. When considering the amount of time and money invested in buying and modifying a reproduction, it may negate the cost saving compared to buying an original, which might only require minor cleanup.
Any 1968–1972 Pontiac cast-iron intake manifold provides excellent performance in as-cast form. They can be purchased relatively cheaply and provide sufficient hood clearance with any Pontiac chassis. Castings from 1972 are most desirable, since its exhaust crossover is compatible with those on Pontiac cylinder heads produced during the late 1970s.
A cast-aluminum version of the 1966 Tri-Power manifold was produced in recent years. It’s a near-exact copy of the original and accepts all corresponding factory accessories and hardware. The quality is generally very good and it’s available from a number of sources. If you’re considering a Tri-Power manifold for your build, I suggest using this piece. Ames Performance Engineering and Performance Years are two companies presently stocking new examples. Expect to spend around $400 for the bare manifold.
Selecting a Factory 4-Barrel
Any 1968–1972 cast-iron 4-barrel makes an excellent performance piece for Pontiacs operating up to 5,500 rpm or so. Just be sure the exhaust crossover passage is compatible with your particular cylinder heads. Prices generally range from 50 to $100.
The 1973–1974 manifold uses an EGR valve and contains a large passage beneath the exhaust crossover to feed it. Beyond being 5 pounds heavier, it performs as well as its predecessor and can often be found for less than $50.
The EGR system became an integral part of the 4-barrel manifold for 1975, and the secondary openings gained a pronounced “D” shaped appearance because of it. Despite the inhibiting appearance, it doesn’t noticeably affect performance, but the negative perception makes such manifolds undesirable. They can oftentimes be purchased for $25 or less!
You probably noticed that I haven’t mentioned the 1967 manifold, yet that‘s the year the new casting debuted. Its carburetor flange contains an exhaust passage intended to improve carburetor operation a feature eliminated for 1968. The passage can be problematic if the appropriate carburetor gasket isn’t used. I recommend plugging the holes and welding the corners to eliminate potential flange leaks. Since it performs identically to the 1968–1972 piece, I don’t see spending any money or effort modifying a 1967 manifold unless it’s required for a numbers-matching application.
Modifying Factory Manifolds
A few years back I compared the airflow capacity of several factory manifolds using a professional flow bench to determine which is best in stock form. Since a manifold and cylinder head work in conjunction, I measured the airflow effects an intake manifold had upon it cylinder head that otherwise flowed about 250 cfm. It could provide the flow potential of a particular manifold, which can be used to predict how it might affect performance.
Not surprisingly, 1969–1972 the high performance cast-aluminum and 1973– 1974 SD-455 cast-iron manifolds affected cylinder head airflow the least, flowing about 90 percent of the bare head. I found that all 1967–1978 standard production intake manifolds flowed similarly, regardless of flange shape or appearance. That included late-1970s casting with “D” shaped secondary bores. Total airflow of the standard performance castings averaged to more than 88 percent.
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Smog-era manifolds are quite undesirable from a performance perspective, but don’t discount those from the late 1970s if you’re building on a budget or if class rules require it. An EGR valve was added in 1973 and the carburetor flange was modified in 1975, and the secondary bores gained a distinct “D” shape, but neither greatly affected flow capacity.
I measured the airflow capacity of many Pontiac intake manifolds on my flow bench. I used a cylinder head to establish a baseline figure and re-measured airflow with an intake manifold connected to it. In as-cast form, the high-performance aluminum manifold contained the greatest airflow capacity. The standard cast-iron manifold flowed slightly less.
Using a cutting bit and high-speed grinder, I modified the runners of a few different Pontiac manifolds, enlarging them to full gasket dimensions in an attempt to improve flow capacity. A follow up session on the flow bench revealed definite improvements. The cast-iron unit showed the greatest gain, rivaling the airflow of the cast-aluminum unit.
If you’re interested in improving the airflow capacity of your stock Pontiac intake manifold, but are uncomfortable performing the work yourself, SD Performance offers an excellent solution. It uses its CNC mill to enlarge the runners and modify the plenum of many factory cast-iron or aluminum manifolds. Optional hand finishing further enhances the effect. (Photo Courtesy SD Performance)
I then modified certain castings in an attempt to maximize airflow. Using various grinding bits and sanding rolls, I enlarged the intake runners to Ram Air gasket dimensions and maintained cross-section area toward the plenum to prevent any flow-inhibiting restrictions within the runner. I was able to improve the cast-aluminum manifold slightly to 92 percent, but the modification significantly improved the flow capacity of the standard iron casting, which totaled nearly 92 percent.
The result proved that after proper modification, any Pontiac 4-barrel manifold is a capable performer. If you’d rather forego the time and effort spent porting your own manifold, SD Performance has an excellent solution. It can CNC machine the runners and plenum area of any original iron or aluminum casting, improving airflow. With pricing that starts at $175, the cost is quite reasonable, and there are a few options for even greater potential.
There’s some debate to whether Tri- Power actually performed better than a 4-barrel in certain original applications. Pontiac actually installed a small baffle into some 4-barrel carburetors to limit airflow, giving Tri-Power a performance edge. I won’t argue which is better, but am confident in saying that the Tri-Power manifold responds similarly to porting. If you wish to use Tri-Power on your engine and casting numbers aren’t important, I strongly recommend the aluminum reproduction. It can make the porting process much easier than modifying an iron original.
Aftermarket Manifolds
There’s a wide variety of castaluminum Pontiac V-8 intake manifolds available on the aftermarket. Generally speaking, they’re designed to improve the output of modified Pontiacs, particularly at high engine speeds. While those accepting multiple carburetors were once rather common and certain examples are still available, single 4-barrel examples are most popular today.
A few aftermarket manifolds that are no longer in production are still popularly discussed on enthusiast web forums. Those include the Warrior, Edelbrock P4B and original Torker, and Holley Street Dominator. While still offering excellent performance, used examples can be found with some hunting. This book, however, only covers the popular aftermarket manifolds that are currently in production.
Edelbrock has been producing aftermarket Pontiac intake manifolds since the 1950s. Its stock-replacement Performer is a 50-state-legal dual plane that’s compatible with most factory equipment and accepts both spread-bore and square-bore carburetors. Constructed of cast-aluminum to reduce weight, two specific variants are available—one with EGR for applications where it’s required, and another without EGR. Pricing starts at less than $250.
The Performer RPM from Edelbrock is a high-performance dual-plane that provides excellent throttle response and low-speed street manners while allowing for excellent engine horsepower. It’s much taller than a stock manifold and that can be an issue in certain applications, but it’s the best aftermarket dual-plane ever produced for the Pontiac V-8.
Edelbrock’s Torker II is a single-plane manifold that features a relatively small plenum with long, narrow runners that provide strong mixture velocity. The result is the ability to mimic dual-plane performance at lower speeds while maintaining excellent high-speed performance. The plenum volume is a bit small for larger engines. A carburetor spacer measuring at least 1 inch generally yields a performance improvement. (Photo Courtesy Edelbrock, LLC)
The Edelbrock Victor is a max-performance single-plane designed to promote maximum high-RPM horsepower. Its plenum and runners contain a significant amount of material that allows for port matching to a particular cylinder head. It’s significantly taller than stock and requires some modification to the stock hoodline, but it’s an excellent manifold for race engines with limited street use.
The Northwind manifold from KRE has a large plenum and runners, but its carburetor flange is only slightly taller than stock. It provides excellent performance without significant hood clearance issues, and plenty of room for porting. It’s another excellent choice for competition engines.
Certain cylinder heads have an intake port arrangement that requires a specific intake manifold design. Some manufacturers offer matching cast-aluminum intake manifolds, but others like this unit from KRE (for its Warp-6 heads), must be custom fabricated for the exact application. As long as hood clearance isn’t a concern, the possibilities are limitless, including one or more carburetors.
In as-cast form, the Performer’s runners are rather small. It’s intended to maximize throttle response when used with smaller engines, but that can limit peak horsepower with larger engines producing more than about 400 hp. A significant airflow increase is possible by enlarging the runners to Ram Air gasket specifications, and while it’s a modification I recommend when using the Performer in any high-performance engine, Edelbrock’s Performer RPM may be a better choice in those instances.
The Performer RPM is a high performance dual-plane that features a large plenum and very long, smoothly contoured runners. It provides an excellent balance of throttle response, lowspeed street manners, and top end charge. The Performer RPM’s carburetor mounting flange, which accepts both spread- and square-bore types is positioned about 1.25 inches higher than stock, and that can present hood clearance issues with certain Pontiac models. With a price starting at less than $250, I consider the Performer RPM among the best aftermarket intake choices available for street/strip applications today.
Edelbrock’s original Torker was introduced in the mid 1970s as a high performance single-plane compatible with the stock Quadrajet. Edelbrock replaced it with the redesigned Torker II in the late 1980s. The Torker II’s carburetor flange is raised nearly .50 inch over stock. Its smoothly shaped runners connect to a moderately-sized plenum, which allows dual-plane-type throttle response and street manners at low speed, yet strong peak horsepower to nearly 6,000 rpm on larger engines. The Torker II only accepts a square-bore carburetor. It’s an excellent manifold and sells for less than $250.
Pacific Performance Racing (PPR) introduced its single-plane Tomahawk a few years back. It’s essentially a modern interpretation of Holley’s Street Dominator, but with internal modifications to improve as-cast airflow. It’s an excellent performer for engines operating up to 6,000 rpm or slightly more. Its low-rise design and carburetor flange that accepts spread- and square-bore carburetors make it a popular option with second-generation Trans Am owners for its Shaker compatibility. Available from many vendors including PPR, pricing starts at about $200.
The Edelbrock Victor is a race-only manifold that can sometimes be used on the street. The high-rise single-plane was developed using input from professional Pontiac engine builders. It’s often considered the best max-performance manifold available today. Its large plenum and runners contain plenty of material for custom porting so it can be tailored to the exact application. Two versions are available: one that accepts a typical squarebore carburetor and a one for use with a larger Dominator-type carburetor. Expect to spend at least $350 for either model. The Northwind from KRE is another high-performance single-plane that’s proven to be an excellent choice for max-performance applications. Its rather large plenum and runner sizes provides Victor-like performance, but its carburetor flange is lowered by more than an inch, which allows for greater adaptability to Pontiac models where hood clearance issues might otherwise exist. Available from many vendors including KRE, pricing starts at about $300.
The Hurricane and Crosswind from Professional Products are basically copies of Pontiac intake manifolds previously mentioned. I don’t have any direct experience with their quality or performance level, however. There are a host of specialty manifolds specifically designed for use with certain aftermarket cylinder heads too. Those include the BOP, Tiger, Gutsram, CV-1, and Ram Air V units. While some may be compatible with traditional Pontiac cylinder heads, it’s best to discuss compatibility with a Pontiac engine building specialist. Some companies can even produce a custom unit tailored to your application with single or multiple carburetors.
Written by Rocky Rotella and Posted with Permission of CarTechBooks
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