Nearly three decades have passed since a Pontiac V-8 was last installed in a production vehicle, but researchers and hobbyists are gaining new insights to the engine to dispel common misconceptions, clarify gray areas, or bring totally new information to light. As exciting as such events can be, that newfound knowledge can sometimes contradict past reports-some of which were even printed in the pages of HPP.
We reported on the topic of factory intake manifolds a few years ago and have since found additional information that wasn't available at the time. Since it's our duty at HPP to share these most recent findings with you, follow along as we discuss what we've learned about Pontiac's cast-aluminum intake manifolds.
Our Original Findings
You may recall the series "Intake Uptake" that appeared in HPP over the past few years. In addition to detailed physical comparisons, the articles often included the measured air-flow capacity of various factory intake manifolds. Using a bare cylinder head and our SuperFlow 110 flow bench, we then surmised how each might affect performance in certain applications.
The Oct. '05 issue of HPP contains a comparison of a typical Pontiac cast-iron four-barrel manifold and a factory cast-aluminum unit used in R/A-IV and 455 H.O. applications. The subjects used for testing have correct part numbers for the '68 and '71 model years, respectively. We recorded measurements in unmodified form to establish a base, modified both units, and measured again.
We reported that the cast-aluminum unit's larger diameter runners in stock form seemed to be less restrictive than those of the cast-iron piece when combined with a moderately ported, cast-iron, D-port cylinder head. When modified similarly, however-which includes enlarging the runners of both to maintain constant cross-sectional area-the cast-iron unit seemed to flow slightly better, suggesting slightly more performance potential.
Intrigued by the results, we sought a cause. After measuring the plenum floor depth of the two manifolds, we determined that the deep-plenum side (driver-side, feeds the No. 1, 7, 4, and 6 cylinders) of the cast-iron piece is roughly 0.1 inch deeper than that of its aluminum counterpart. Though seemingly minute, the difference clearly affects airflow.
After inspecting a few other cast-aluminum manifolds while compiling information for the article, we deduced that all of Pontiac's aluminum units from the era contain this characteristic. And that's what we conveyed in the article.
A Reader's Response
Not long after the article's release, hobbyist Tom Donovan of Houston, Texas, contacted HPP and shared some important information about his No. 483674, cast-aluminum, '71 455 H.O. intake manifold. It seems the deep-plenum depth of his unit is the same as other iron manifolds in his possession. We immediately began wondering if Donovan's casting was unique or if our initial assumption wasn't entirely correct.
Donovan states that his aluminum unit was cast in 1970. We found that the '71 H.O. unit we'd modified for the article was cast in 1975, suggesting that it was purchased over the parts counter or utilized as a service replacement (S/R) piece. We also discovered that the other '71 cast-aluminum manifolds we inspected for the article were of the same vintage as ours; however, the '72 units were originals.
Something was amiss, and we were determined to find out what.
We contacted other hobbyists who have owned factory cast-aluminum intakes, requesting such information as casting numbers, cast dates, and measured plenum depth. As the results trickled in, a trend became apparent. Units from the '72 model year and any R/A-IV or 455 H.O. S/R unit (which contains a cast date after that timeframe) were cast with a shallower deep-plenum floor immediately under the carburetor, unlike original units from the '71 model year and possibly earlier.
Though many assume (and HPP...
Though many assume (and HPP has past conveyed) that all Pontiac cast-aluminum intake manifolds are created equal, we recently found that they're not. Both these units were originally installed on 455 H.O. engines, and while they may appear identical, the deep-plenum floor on the '72 unit (No. 488945, right) is shallower than the '71 piece (No. 483674, left), inhibiting airflow.
Pontiac's standard-production,...
Pontiac's standard-production, cast-iron, four-barrel intake manifold is a formidable performer in myriad applications. When compared to the larger-diameter runners of the division's high-performance, cast-aluminum versions, the cast-iron piece doesn't flow as well in stock form, but it can be modified to flow as well as, or even slightly better than, its aluminum counterparts.
Here's what one can expect...
Here's what one can expect to see beneath any original-dated '69-'70 R/A-IV or '71 455 H.O. intake manifold. Notice that the deep-plenum side contains a uniform floor shape. A separate cast-iron heat-crossover aids in cold weather operation.
The cast date of a factory...
The cast date of a factory cast-aluminum manifold is located on the bottom side of the deep plenum. The "8.5.70" found on this particular '71 455 H.O. piece indicates that it was cast on August 5, 1970.
Beginning with the cast-aluminum...
Beginning with the cast-aluminum intake manifolds originally installed onto '72 455 H.O. engines, the floor of the deep plenum was revised directly beneath the carburetor. It may have been an attempt to gain clearance for the cast-iron heat-crossover. The same basic casting mold was then utilized to produce all service replacements.
This photo reveals just how...
This photo reveals just how far the deep plenum's floor deviates from the original design. This characteristic is common to all '72 455 H.O. manifolds and any service replacement R/A-IV or '71 H.O. casting. It appears that all modern cast-aluminum reproductions are also this way.