Though some Trans Am owners...
Though some Trans Am owners strive to achieve maximum performance, not all are willing to alter the factory Shaker setup just to run an aftermarket intake manifold. If this describes you, read on-we compared several popular aftermarket manifolds with an original to determine which castings are compatible with a Quadrajet carburetor and original Shaker assembly.
The Comparison
Like most other Pontiac owners, we too were interested in testing a number of original and aftermarket intake manifolds on our Trans Am to find the castings that offered the best total performance. Though we could have purchased an aftermarket drop-base kit, we would rather retain the factory Shaker assembly for a couple of reasons-not only would it allow us to maintain an unmodified underhood appearance, but it might also make for quicker and easier manifold swaps in the future while on the dyno or at the dragstrip.
Knowing that not all aftermarket intake manifolds accept spread-bore-type carburetors, and that we wanted to retain our original Quadrajet, we limited our selection to those that do. We then installed each casting onto our Trans Am and compared the effects the aftermarket units had on carburetor positioning and how it subsequently affected Shaker placement. The group consisted of Performer, Performer RPM, and Torker castings from Edelbrock, and a Street Dominator from Holley.
The factory intake manifold we run most often is an aluminum '71 455 H.O. casting. Previous testing has shown that it's externally identical in every respect to a similar vintage cast-iron unit and locates the carburetor in exactly the same position. All of the castings used in our comparison have had the coolant crossover separated for easy installation, and though our original and a few of our after-market castings have been modified internally, none have had the cylinder head or carburetor flanges altered in any way.
To ensure maximum accuracy, we carefully replicated the same procedures when measuring the manifolds. Using two bolts-one on each flange at opposite ends-we secured the manifold to the cylinder heads. We then used a straightedge to record three specific measurements: down toward the block from the carburetor flange's front bolt holes (Point A), down toward the block from the carburetor flange's rear bolt holes (Point B), and from the carburetor flange's rear bolt holes toward the firewall (Point C). This should accurately provide us with the differing carburetor positions of each casting.
Analyzing The Results
Though any number of variables, including worn body bushings and/or engine mounts, can alter the Shaker's relative position to the hood of any Trans Am, ours are in excellent condition and maintain the correct body height in relation to the frame, and Shaker height in relation to the hood. Our measurements were, however, recorded using fixed points and should provide such owners the ability to accurately predict how an aftermarket intake manifold might ultimately affect the Shaker's placement regardless of its current position.
Externally identical to an...
Externally identical to an original cast-iron unit, we selected our '71 455 H.O. manifold as the basis for our comparison because of its aluminum construction-15 pounds of aluminum is certainly easier to lift over a fender than 40 pounds of cast-iron when making comparisons!
The Edelbrock castings are...
The Edelbrock castings are designed to accept this throttle bracket used on all '72-'74 Pontiac applications, without any modifications.
The Holley Street Dominator...
The Holley Street Dominator uses the Pontiac throttle bracket found on all vehicles from 1975 forward. This bracket can used on an Edelbrock casting, but it requires this adapter plate that's not only supplied with all new Edelbrock manifolds, but is also available from the company separately. Using any other type of throttle bracket may require some fabrication.
After reviewing the results of the single-plane castings, we found that the Holley Street Dominator appears to most closely replicate the positioning of the original manifold. So it wasn't any surprise to find that the factory Shaker assembly was correctly located after installing a Quadrajet and closing the hood. The Edelbrock Torker biased the Shaker so far rearward that we feared the hood would contact the rear portion of the scoop while closing. Fortunately it didn't, and otherwise, it positioned the Shaker suitably.
Carburetor positioning of the dual-planes seemed to deviate further from original than with the single-plane units. Though the Performer RPM didn't bias the carburetor rearward any, we unsurprisingly found that it was too tall for use with the Shaker and wouldn't allow us to even shut the hood. What did come as a surprise, however, was that the often-regarded stock-replacement Performer positions the rear portion of the carburetor significantly lower than the casting it was designed to replace.
Conclusion
With so many new and used aftermarket intake manifolds on the market, our selection for the comparison was limited to those that are compatible with the popular Quadrajet carburetor, and they seem to be common choices in Second-Gen Trans Am performance applications today. Determining which original or aftermarket intake manifolds perform best with your specific combination can be handled only on a case-by-case basis. But our comparison might provide Second-Gen Trans Am owners a better idea of the available choices if they want to retain the factory components.
Special thanks to Jim Hand for his assistance.