Few hobbyists will deny that from the mid-'70s forward, the Second-Gen Trans Am was Pontiac's performance leader. Its front and rear spoilers, wheel flares, and fender-mounted air extractors all helped solidify that image. But the rear-facing Shaker scoop peering up through the hood could be the most identifiable Second-Gen Trans Am characteristic of all time. Whether making it lean right as a driver or watching it nervously dance from the opposing lane, the Trans Am's Shaker provides plenty of visual appeal and, if functional, it can provide a performance boost, too.
In an effort to maintain its cutting-edge performance image and boost output, owners have for years installed a wide range of aftermarket components onto their Trans Ams. While carburetor, distributor, and exhaust-system modifications are commonplace, it seems that the components most commonly replaced are the cylinder heads and intake manifold. One factor some Trans Am owners contend with when considering an aftermarket manifold for their combination is whether it'll work with the Quadrajet carburetor and factory Shaker and air-cleaner assembly.
While some owners have no reservations about bolting their T/A's Shaker to thehood to reduce installation hassles and/or gain maximum clearance, others want their scoops to "shake" and opt for a modern drop-base kit when using certain castings. Still others want to maintain a stealthy appearance by retaining the factory setup. Since we tend to fall into the last category, we gathered several aftermarket intake manifolds thought to allow this and physically compared each with an original to determine the effects they have on Shaker placement. Follow along as we explore how they measure up-literally!
Intake Manifold Basics
The intent of our comparison is not to suggest specific intake manifolds for certain applications, but rather to discuss the various after-market castings that are compatible with a Trans Am's original Shaker assembly. No article about intake manifolds would be complete, however, without providing at least a general explanation of the different types and the operating characteristics one can expect from them when swapping manifolds.
Dual-plane intake manifolds...
Dual-plane intake manifolds such as Edelbrock's Performer (left) and Performer RPM (right) contain a split plenum with each half feeding four cylinders at alternate intervals. Though airflow through the carburetor at idle and low speed is high, a typical dual-plane's unequal runner length and overall lack of plenum and runner volume can limit its effectiveness at high rpm. The RPM is noticeably improved and has consistently proven in independent testing to rival the performance of a single-plane.
Single-plane castings such...
Single-plane castings such as the Edelbrock Torker (left) and Holley Street Dominator (right) typically generate higher peak-power numbers at high rpm. Notice the open-plenum design and near equal-length runners-these characteristics are critical factors that were considered by the manufacturers when determining the intended operating range during the design process.
Though both of these manifolds...
Though both of these manifolds are out-of-production, either can be purchased on the used market. The new Tomahawk manifold available from Pacific Performance Racing in San Pedro, California, is essentially a modern version of the Street Dominator boasting improved airflow, and it could be considered as a replacement in any application where the Holley unit is being considered.
Whether original or aftermarket, an intake manifold is designed to operate within a specific rpm range on a given combination, and factors such as plenum volume, cross-sectional run-ner area, and overall runner length contribute to that. Though any manifold can certainly operate at any practical engine speed, they do, however, become increasingly less efficient at some point and typically degrade performance as the intended operating range is grossly exceeded.
Intake manifolds can be broken down into two major groups-single-plane and dual-plane. Since both contain specific operational characteristics, one isn't necessarily any more valuable than the other in every application. Single-plane castings are generally associ-ated with increased high-rpm power. They fea-ture an open plenum where each of the eight runners is capable of pulling from the entire carburetor, typically improving cylinder fill at high rpm. A trade-off to this is lesser draw on the carburetor at idle and low speed, which can negatively affect throttle response and low-speed performance when compared with a dual-plane.
Dual-plane manifolds, including those produced by Pontiac, tend to generate maximum average power over a broad rpm range by incorporating a split-plenum design where four cylinders feed off one half of the carburetor, while the remaining four feed off the other half. This typically improves throttle response and low-speed performance as the high-speed air passing through the carburetor promotes better fuel atomization. Though a dual-plane manifold is highly efficient at low- and mid-rpm, high-rpm operation can be adversely affected because only half of the carburetor furnishes air to each cylinder.
Proper intake-manifold selection for maximum performance in any application depends on such variables as displacement, cylinder-head flow, camshaft duration, and the intended purpose and operating range of the subject vehicle. The only accurate method for determining this is through a series of controlled back-to-back tests measuring performance on a dragstrip or dyno, and not necessarily what seat-of-the-pants testing may indicate. So, what aftermarket manifolds do Trans Am owners have to choose from while retaining the factory Shaker? We wanted to know.