Hobbyists typically regard the mid-to-late '70s as a low point for Pontiac performance. Not helping that image was an intake manifold with D-shaped secondary bores. And like most other hobbyists who own a Pontiac originally equipped with this manifold, we decided to replace our '76 Trans Am's with a '67-'72 cast-iron unit expecting a noticeable performance increase.
After correcting the exhaust-crossover mismatch and locating the proper air conditioning and throttle brackets for our new intake, we couldn't wait to get our Trans Am back on the road. But within a few miles, disappointment set in. There was no detectible performance difference from the mildly built-but-relatively stock 455 in our Trans Am at that time.
We rationalized that even though we had not felt a performance increase, the early-cast manifold simply had to flow better! How could it not? Logic says restricting the size or shape of an opening should negatively affect flow. What we had not considered, however, is that if the opening is already large enough to accommodate the required volume, increasing its diameter may not have a positive effect. We then realized that our 455 didn't recognize its original intake manifold as an airflow restriction.
You might be wondering what effects the uniquely shaped secondary bores have on overall flow. In the Feb. and March '05 issues of HPP, we evaluated several different Pontiac intake manifolds, including a '75-'78 unit, and found virtually no flow variance amongst them when used in near-stock applications. But as cylinder-head airflow increases, what effect do the D-shaped bores have on total flow, and is there any benefit to opening them? Follow along as we find out and share the results.
Many hobbyists have heard a number of reasons for the uniquely shaped secondary bores. Complicating matters are the differing explanations from a few engineers within Pontiac at that time. Ranging from attempts at reducing advertised horsepower to combating intake-tract reversion, we can only speculate that a combination of reasons prompted the casting change for 1975.
While testing the '75-'78 casting in our initial evaluation, we noticed another prevalent characteristic besides the D-shaped secondary bores: a recessed area around the primary bores containing the EGR outlet port. Unlike the '73-'74 casting which has two EGR ports, one on each floor of the respective plenum, the '75-'78-style manifold has a single EGR port located within the recession that feeds both plenums.
Closer inspection revealed that the primary recession might have subsequently affected the normal primary-to-secondary dividers found on earlier castings. To increase overall flange strength and reduce any chance of fatigue, especially when hot exhaust gas is involved, it is possible that engineers added material to the critical areas, giving the once-round secondary bores a D shape. Since most engines of that era never revved much past 4,500 rpm, it may have been determined that the structural benefits outweighed any negative performance effects.
Without detecting a performance increase after swapping manifolds on our 455, we began to wonder if the intake manifold that so many considered a low-performance, restricted-secondary unit wasn't nearly as bad as perceived. The results from our initial evaluation agreed: There was little flow difference between the early and later castings when used in near-stock applications. But what effect might the smog-era manifold have on total airflow when combined with modified cast-iron cylinder heads?
To accurately determine a manifold's effect on total flow, we must first measure the cylinder head, then attach an intake manifold to it, and measure again. The difference between the two values is calculated into an intake-to-head flow percentage for that particular port. Once all eight runners are measured, the individual results can be averaged to produce a single intake-to-head flow percentage for that manifold. That average can then be used to predict how similar manifolds might affect performance when used in similar applications.
Previous testing indicates that any unmodified intake manifold, including the aluminum Ram Air and H.O. units, should flow around 91 percent of the bare head when combined with cylinder-head flow around 210 cfm at maximum valve lift, like that of an unmodified D-port casting with 2.11-inch intake valves. As head flow increases towards 250 cfm, a typical cast-iron manifold acts as a larger restriction, and the average will likely drop to roughly 88 percent.
Typical Intake-to-Head Flow at 210 cfm - 91%
Typical Intake-to-Head Flow at 250 cfm - 88%
Our latest evaluation began by visually comparing a group of unmodified '75-'78 manifolds. We determined that with the exception of various exterior details, all castings from that era are functionally the same. And when compared to a typical '67-'72 cast-iron unit, we found that the runners of both appeared to share the same basic internal shape and size.
To measure airflow through the manifold, we used our SuperFlow 110 flow bench and a professionally ported, cast-iron, D-port cylinder head. After proper bench preparation, average airflow of the head measured 254 cfm when converted to 28 inches of pressure. We installed the intake manifold, then measured airflow through all eight runners by repositioning the intake manifold on the cylinder head and the cylinder head on the flow bench.
Unmodified '75-'78 Casting Intake-to-Head Flow at 254 cfm 88.4%
Modifying The Restricted- Secondary Manifold
After establishing a base, we had to determine how much material to remove to give the secondary bores a concentric shape. We affixed a bare Quadrajet baseplate to the carburetor flange and began to trace the overhang area. But what we found was a little shocking: There wasn't as much material blocking secondary airflow as one might assume! We could see very little improvement.
Our initial thought was to simply open the secondary bores, but we found it easier to remove the entire primary-to-secondary divider. And since previous testing had no adverse effects, we removed the divider from each respective plenum. The modified manifold was reinstalled onto the cylinder head and measured again.
Modified '75-'78 Casting Intake-to-Head Flow at 254 cfm 88.4%
Analyzing The Results
Even though there were slight airflow variances from the individual runners, we were amazed to find that the unmodified and modified intake-to-head flow percentages averaged identically. After visually comparing the '75-'78 manifold to a '67-'72 unit, we were not surprised to find that the later unit (modified or unmodified) had no more effect on total flow than an earlier unit.
Wanting to know if the results were reasonable, we contacted two individuals synonymous with Pontiac performance. One is a renowned Pontiac engine-builder with nearly 50 years experience, and the other is a retired Pontiac engineer whose accomplishments in his 44 years with the company include the Pontiac V-8, the Super Duty 389 and 421 packages, and the four-barrel intake manifold. Both listened to our objectives and outcomes and were asked to share their comments.
Nunzi Romano of Nunzi's Automotive in Brooklyn, New York, said his experience with the later manifolds was limited, but his testing agreed with ours. He recalled no noticeable performance increase after modifying the carburetor flange of the smog-era casting. Romano noted, however, that because of its emissions-related perception by hobbyists, he found it easier to install a '67-'72 casting as opposed to modifying the original when working on a customer's engine.
Retired Pontiac engineer Malcolm "Mac" McKellar said he wasn't surprised at the outcome of our test. Because the original design was so efficient, such changes didn't seem to adversely effect overall engine performance. He could not, however, provide an exact reason for the 1975 casting change, but he did share a little known fact about intake manifolds of that era.
An important point must be clarified when speaking of intake-manifold airflow. When a manifold is installed onto a cylinder head, its runners become an extension of the cylinder head's intake ports. In most instances, the intake runners restrict a specific amount of airflow into the head, but because that restriction is proportionate to cylinder-head airflow, we can't rate an intake manifold in cfm form.
According to McKellar, emissions took priority over performance in those days. And because the manifold contained an emissions-control device, the EGR valve, it and a few other engine components were controlled by the emissions engineering team. Any changes to the manifold during that time were likely related to EGR. McKellar added that our theory of increasing overall flange strength after relocating the EGR port was probable.
Conclusion
Few will argue that from its initial appearance, the '75-'78 intake manifold is a loser, but no one can deny that its overall design has a proven history. After spending several hours cutting and flowing only to find no improvement and hearing the supporting comments from two individuals so closely related to Pontiac performance, it's now possible to view the restricted-secondary casting differently.
The '67-'72 castings will surely remain the stock intake manifold of choice for many hobbyists, including us. But to originality-conscious owners, our results might prove that the smog-era manifold isn't necessarily affecting horsepower. The results may also appeal to those on budgets since the later castings could be a low-buck alternative to the higher-priced early castings in situations such as replacing a two-barrel. So whether numbers-matching or low-buck/ go-fast is your goal, don't discount the lowly restricted-secondary casting. Chances are one can be had for less than what you spent on lunch today!
Special Thanks to Jim Hand, Pete McCarthy, Mac McKellar, and Nunzi Romano for their contributions to this story.