It does not take a lot of specialized equipment to port match, just a die grinder and an appropriate carbide cutter. The actual process begins by marking the manifold for a cut line using the gasket as a template. Once marked, there is nothing left to do but start cutting. The key to an effective job here is to blend the work properly into the port. The idea is to create a seamless transition from the gasket line into the port runner, and not to simply flare the port out to the cut-lines at the flange. On the dyno, we have seen 8-10 horsepower gains given a proper port match on a standard Edelbrock Performer intake to the full 340/360 gasket size.
From the basic port-matching modification described above, the sky is the limit on further intake porting. This can be as basic as a mild clean-up of the runner’s casting irregularities, to full-on flow bench porting. 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. Most guys like 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 a fully ported manifold work can up the power numbers.
Generally, major manifold porting begins at the plenum, with the key areas being the roof transition from the carburetor flange into the tops of the runner entries. This area has a significant effect on flow into the runner, and the typical form is to create a nice gentle rolled radius. Another area of attention is at the runners’ junction with the plenum, with some favoring a bull-nosed radius, while others work to a knife edge. The idea is to improve the transition into the runner and produce a more uniform cross-sectional area and, ultimately, airflow through the runners. While these are the most critical areas to manifold porting, sometimes the runners themselves are subjected to considerable rework, sometimes to enlarge, and always to streamline for more flow—just like porting a head. Also like head porting, different intake manifold designs have their own peculiarities. At the higher levels of manifold porting, a flow bench is an indispensible tool. Considerations beyond raw flow include the airflow distribution, manifold cross section and tapper, and wet flow which can influence fuel distribution.
Obviously some of these considerations are not the kinds of things most guys can address with a grinder in their garage, but at the professional level of manifold porting, these items are essential. That said, an enthusiast taking a common sense approach to manifold porting can generally improve a typical aftermarket manifold casting.
On The Dyno: Single-Plane vs. Dual-Plane
As much as the lower-rpm advantages of a dual-plane are discussed, we find many enthusiasts seem to refuse to believe it. Sure, in a high-rpm, or race application, the single-plane is going to have an advantage. Often such an installation will come with a high-stall converter in the transmission that will make the lower end of the rpm range even less relevant. However, in a typical street performance application, a good dual-plane like Edelbrock’s RPM AirGap will be a much better choice. To illustrate, we ran a typical street style 360 Magnum on the dyno, with both an AirGap dual-plane and an M1 single-plane. The results of our little test were typical of what we see over and over in these applications, where a single-plane is substituted for a well-designed two-plane—a tremendous amount of torque given up in the low- to mid-range for a small gain up top in peak power. If it's just the bragging rights of a higher peak power number that you are after, the single-plane delivers. For tire-smoking torque and far more average power over the full rpm range, the AirGap rules.
Our test engine is an old Mopar Performance Magnum 300 horsepower crate engine that has been modified to a hot street performance spec. The engine features box stock Edelbrock Magnum heads, and a custom Comp Cams hydraulic roller cam speccing out to 224/230-degrees duration at .050, with .573/.568-inch lift via a set of Comp "Magnum” stud mounted 1.6:1 ratio rockers. It is a pretty simple combination, but with the upgraded heads and cam combination, it represents a hot street 360. The numbers really tell the tale of which intake is the better choice.
Manifold Porting: What’s it Worth?
When considering porting an intake manifold, the first question that needs to be answered is Do you really need it? For a mild combination with production style heads, an out-of-the-box installation or a simple port match will probably do just fine. However, if the combo is seriously stepped up and the cylinder head moves big volumes of air, the intake manifold might become the bottleneck. In this situation, a well ported intake can pay surprising dividends in power. With the broader availability of serious cylinder heads for Mopar wedge and small-block engines, manifold porting is more relevant than ever before. We put together a stout 408 Magnum stroker, and with max ported Edelbrock heads, the intake flow was an issue. In this case we were running an M1 single-plane intake, and on the bench, the head flow exceeded that of the manifold. We have found through numerous flow and dyno tests that the manifold flow should exceed that of the peak intake port flow for best power.
We flow tested all eight ports of the M1 intake and found it to be better than many we have tested in terms of relatively even port flow distribution, with the best port flowing 296 cfm and the worst recording 289. Fully ported, the intake delivered airflow in the 320-cfm range, more than enough to keep up with our modified cylinder heads. These kinds of flow improvements take considerable expertise, and benefit from flow bench development, but illustrate the kinds of gains that can be achieved. We have seen even more dramatic increases with manifolds of a poorer design than this Mopar Performance unit, often with the flow distribution dramatically improved.