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 has past conveyed) that all Pontiac cast-aluminum intake manif
Pontiac's standard-production, cast-iron, four-barrel intake manifold is a formidable perf
Here's what one can expect to see beneath any original-dated '69-'70 R/A-IV or '71 455 H.O
The cast date of a factory cast-aluminum manifold is located on the bottom side of the dee
Beginning with the cast-aluminum intake manifolds originally installed onto '72 455 H.O. e
This photo reveals just how far the deep plenum's floor deviates from the original design.
Locating manifolds originally installed on '71 or '72 455 H.O. engines isn't overly difficult, but those originally installed on '69-'70 R/A-IV engines can be a daunting task. We've only had one hobbyist respond with that information.
Considering the rarity of R/A-IV engines, availability of reproduction manifolds might further skew our research. Needing additional facts to corroborate with our latest finding, we sought it from a source whose cast-aluminum manifolds are undoubtedly factory originals.
Retired Pontiac engine-builder Nunzi Romano of Brooklyn, New York provided us with all the information we needed. Romano has countless factory cast-aluminum intake manifolds in either good/used or N.O.S. condition, and all are original-dated units. After explaining our objective, Romano willingly measured his R/A-IV and 455 H.O. castings. The results confirm what we found-the original '69 and '70 R/A-IV and '71 H.O. manifolds are roughly 0.1 inch deeper in the deep plenum than the '72 H.O. unit or any S/R piece.
A Follow-Up Comparison
Knowing that there is now gray area in what we initially reported, we want to set the record straight. We borrowed a period-dated No. 483674 ('71 455 H.O) cast-aluminum intake from hobbyist Bob Chumley of Blue Springs, Missouri, for a direct comparison to our period-dated No. 488945 '72 455 H.O. casting. Both units are completely unmodified, and after performing the same testing procedures outlined in the Oct. '05 installment, we now have an understanding of the exact differences between the two castings we once thought were identical.
The first step was to measure the deep-plenum floor depth of the manifolds. For reference, we included an unmodified '68 cast-iron unit in this portion. As with the iron piece, the original '71 455 H.O. unit's deep-plenum floor depth is roughly 3.5 inches, while the original '72 455 H.O. piece measures approximately 0.1 inch less. There is no discernable difference in shallow-plenum floor height of the manifolds, however.
|Deep-Plenum Depth in Inches|
Measuring the airflow capacity of the two cast-aluminum units was next. Replicating the exact procedure in our initial test, we placed a cylinder head averaging 254.5 cfm at 28 inches onto the flow bench and measured airflow through each manifold runner. Unsurprisingly, the individual results prove that shallow-plenum airflow is practically identical; however, deep-plenum airflow differs noticeably. After averaging the results, the intake-to-head flow percentage of the original No. 483674 is roughly 1 percent better than the No. 488945.
|Average Airflow at 254.5 cfm at 28 inches|
|No. 484674||'71 cast-aluminum||90.8% |
|No. 488945||'72 cast-aluminum||89.6% |
Analyzing The Results
It appears that Pontiac Engineering revised the cast-aluminum manifold's mold somewhere along the way, altering the floor height in its deep plenum for the '72 model year. It then seems that the same casting mold was used to produce all S/R units from that point forward. Those units contain limited airflow, not the original '69-'70 R/A-IV and '71 H.O. units.
While it might appear that the information presented here discredits the airflow results we shared in HPP Oct. '05, nothing is further from the truth. Those results remain accurate, but we now know they may only apply to original '72 units and all S/R R/A-IV and 455 H.O. castings.
As years pass and the hobby grows, we'll certainly come across similar anomalies in the future. We must always remember that information is only as accurate as what's available at that time. We at HPP are committed to spreading as much accurate information as possible, and as those advancements arise, we'll undoubtedly share them with you.
Intake Manifold Casting Numbers
|9796614||'69 A-body R/A-IV|
|9799084||'69 F-body R/A-IV |
|483674||'71 455 H.O.|
|485640||'72 455 H.O. (early)|
|488945||'72 455 H.O. (midyear)|
The airflow capacity of both manifolds is measured using a Superflow 110 flow bench and a
Measuring downward from the secondary bore of the carburetor flange to the floor of the de
Though these two manifolds (see next image) both contain the same casting number for '71 4
Airflow through the unit on the left (cast in 1970) is slightly better, suggesting that it
What About Casting No. 485640? Few are aware that two different casting numbers were used