Rocky Rotella Responds:
A detailed comparison of the factory spread-bore intake manifolds used from '67 forward can be found in "Intake Uptake" (HPP, Feb. and Mar. '05). In that two-part story, we recorded airflow measurements in stock form and discussed such physical attributes as accessory bracket mounting and exhaust crossover size at that time.
We learned from the airflow portion of our testing that all factory manifolds contain similar flow capacity, but the cast-iron '73-'74 SD-455, with its enlarged runners, was best in unmodified form. It was followed closely by the cast-aluminum R/A-IV and 455 H.O. units, while the various standard production cast-iron units trailed marginally behind.
You'll be pleased to know that any production four-barrel manifold featured in our test will fit under your Trans Am's Shaker, but it's unlikely that you'll want to spend the hundreds to thousands of dollars required to acquire such a highly desirable piece as a cast-iron SD-455 or aluminum R/A-IV or 455 H.O. unit. If you prefer taking an economic approach, any standard production cast-iron unit from '67-'78 will provide excellent performance at a reasonable cost while working within your stated parameters.
The '67-'72 cast-iron units are generally accepted as the best performers, and with careful hunting one can be purchased for well under $100. EGR was added in 1973, and the same basic casting received additional plumbing for the emissions-control device. The extra material adds weight, but it seems that performance is unaffected. The carburetor flange took on a unique appearance with D-shaped secondary openings in '75, and while it might appear that airflow was significantly affected, airflow testing surprisingly shows otherwise.
After testing various factory and aftermarket intake manifolds, it seems that most moderately built street engines don't recognize the miniscule airflow differences amongst the stock castings. While without modification the SD-455 and cast-aluminum units might possess more potential than a standard-production cast manifold, careful porting such as that outlined in "Intake Uptake, Part 3" (Oct. '05)-which includes enlarging each intake runner to full gasket size to maintain the constant cross-sectional area into the plenum to eliminate restriction-is an easy way to maximize performance from it.
The question then remains, is your engine modified enough that it will recognize the additional airflow at high rpm? If you feel it might but lack the time required to accomplish the task, Dave Bisschop of SD Performance in British Columbia, Canada, has created a CNC-porting program to increase airflow through the factory manifolds. You can learn more about that service at www.sdperformance.com.
If you reach the performance limits of a stock manifold and decide to try an aftermarket unit, consider reading "Shaking Out The Details" (June '07). We gathered several aftermarket intake manifolds and recorded a series of measurements to determine the effects each has on Second-Gen Shaker placement. If you don't have that particu-lar issue, the story is available online at www.highperformancepontiac.com.