Since the days of relieving flathead Fords, we've known about the advantage of porting. Grind those critical passages with the right touch, and airflow through the engine improves-and with more air comes more horsepower. There was a time when a set of ported heads was considered exotic, the realm of high-end race power. Back then, a set of nicely ported heads on a street machine was enough to have a leg up on the vast majority of regular Joes with their typical street brawler or Saturday night race cars. Times have changed. With the explosion in aftermarket cylinder heads and easy CNC porting, these days it seems like any serious engine build will include a set of ported heads. Does that mean the job is done? Not according to Bryce "Manifold Man" Mulvey of Dr. J's Performance.
Consider the complete engine as a flow system, and it becomes clear that the pathway does not begin and end at the cylinder head. Bolt an intake manifold to a fully prepped cylinder head, and the two become one as far as airflow is concerned. Guys tend to take it for granted that most any aftermarket intake manifold will be more than up for the job, and it can be-but not always. Sometimes the intake flow is not even close, and in most cases there is room for improvement, especially in high-powered applications where the heads are set to kill. Here is where Manifold Man Mulvey persuasively argues that custom manifold work can up the power ante. We asked him to put his manifolds where his mouth is and show us the numbers, and he did, convincingly.
Using a Dr. J's adapter and blowing into the plenum, Manifold Man Bryce Mulvey prepares to test an Edelbrock Super Victor intake. The flow adapter simulates the throttle bores of a 4150 carb.>
Raw Flow
Very few performance shops flow-test intake manifolds, primarily because the setup can be awkward and time-consuming. To simplify things, Dr. J's manufactures flow adapters that make this a quick and easy procedure; they're available for mounting 4150, 4500, and spread bore intakes on a SuperFlow bench. The arrangement sets the manifold up in a blow-through configuration, allowing each port in an intake manifold to be tested in rapid succession. While some perceive this flow procedure with misgivings, intuiting that the flow bench is blowing into the manifold while the engine sucks, from the standpoint of airflow volume the only thing that matters is the pressure differential across the manifold. The bench and the intake manifold runners don't care which side of the flow system is providing the force creating the pressure differential.
Flow-testing intakes on the Dr. J's adapter sets the manifold upside down on the bench, allowing seven of the eight runners to be blocked to isolate any individual runner for flow evaluation. The configuration also allows easy access to the runner for velocity probing, greatly aiding in the development process in search of improved flow. Mulvey points out that the manifold adapters are designed with four radiused-entry orifices that mimic the placement of the throttle bores on the corresponding carburetor configuration, thus simulating the airflow entering the manifold with a carburetor. The placement of the throttle bores has an influence on the airflow into the plenum, as well as runner-to-runner airflow distribution.
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