Confirmation
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 |
| ’69 Cast-iron | 3.50 |
| ’71 H.O. | 3.50 |
| ’72 H.O. | 3.38 |
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.
Conclusion
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.
Factory Cast-Aluminum
Intake Manifold Casting Numbers |
| Number | Application |
| 9796614 | '69 A-body R/A-IV |
| 9799084 | '69 F-body R/A-IV |
| 9799084 | '70 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...  The airflow capacity of both manifolds is measured using a Superflow 110 flow bench and a ported cast-iron cylinder head flowing 255 cfm at 28 inches of pressure. Testing consists of measuring the bare cylinder head, connecting it to an intake manifold, limiting airflow to a specific runner, and remeasuring. Calculating the result yields an intake-to-head flow percentage, which-after averaging eight runners-provides a single percentage for each manifold. That can then be used to predict performance effects. |  Measuring downward from the...  Measuring downward from the secondary bore of the carburetor flange to the floor of the deep plenum, we compared a typical '68 cast-iron casting (No. 9790140), an original-dated '71 455 H.O. (No. 483674) casting, and a '72 455 H.O. casting (No. 488945). The plenum floor of the '72 455 H.O. (and all S/R units) is roughly 0.1 inch higher, subsequently reducing airflow by 1 percent. |  Though these two manifolds...  Though these two manifolds (see next image) both contain the same casting number for '71 455 H.O. applications (No. 483674), the unit on the right was produced in 1975 and subsequently contains the same deep-plenum floor height as the revised unit introduced for the '72 model year. |
 Airflow through the unit on...  Airflow through the unit on the left (cast in 1970) is slightly better, suggesting that it may be a better choice when using such a manifold in maximum performance applications. |  What About Casting No. 485640?...  What About Casting No. 485640?
Few are aware that two different casting numbers were used for the cast-aluminum manifold found on '72 455 H.O. engines: Nos. 485640 and 488945. It's rumored that No. 485640 was an experimental piece used during Super-Duty 455 development. Our research suggests that No. 485640 was in fact a production 455 H.O. piece used early in the '72 model year. It was replaced by No. 488945 midyear for unknown reasons. A visual inspection of the two manifolds reveals that the castings are identical in every respect, and that No. 485640 contains the lesser deep-plenum floor height. -RR | |