Performance Exhaust Systems: How to Design, Fabricate, and Install. Mike Mavrigian
a single-plane manifold. A dual-plane is generally better suited to low-end torque and power for street use. Regardless of the application, pay attention to the advertised engine RPM operating range for both the manifold and the camshaft that you plan to use. Try to match the manifold to the cam based on those ratings."/>
A dual-plane intake manifold features a split plenum, providing an intake charge every 180 degrees of crankshaft rotation. (They are often referred to as 180-degree manifolds.) Intake runners are usually longer than on a single-plane manifold. A dual-plane is generally better suited to low-end torque and power for street use. Regardless of the application, pay attention to the advertised engine RPM operating range for both the manifold and the camshaft that you plan to use. Try to match the manifold to the cam based on those ratings.
If the cross-section area is too large for the application, a reduction in peak torque can occur and the RPM range of where peak torque is created can increase. On an engine dyno graph, peak torque generally indicates where in the RPM range VE is the greatest.
Note that these generalizations apply to naturally aspirated engines. When you consider an engine fed by forced induction, whether that involves a turbocharger or supercharger, intake manifold runner design becomes less critical because the air charge is forced into the cylinders under a positive pressure. Matching the intake manifold runners to the selected cylinder heads is the goal, in terms of port cross-section area, in order to maximize efficiency in terms of airflow.
Port matching (where the intake manifold exit ports match the size and shape of the cylinder head intake ports) is an important step to maximize airflow. The intent is to precisely mate the location, shape, and size of the intake manifold’s air exit to the cylinder head intake port entry. If the ports are mismatched (for instance if the port walls are not aligned or if the intake manifold ports are larger than the cylinder head intake ports), incoming air hits the exposed walls, creating unwanted turbulence. Mounting a poorly matched intake manifold to the best set of cylinder heads can easily reduce power and prevent taking advantage of the engine’s maximum potential. Intake manifold runners are commonly designed with a slight taper to promote airflow velocity, enhancing airflow speed on its way to the cylinder head.
Many single-carbureted intake manifolds feature unequal-length intake runners, and thus the front and rear runners are longer and narrower than the center runners, which may be shorter and wider in comparison. In theory, this equalizes or balances the volume of air distributed to the cylinders. In addition, airflow velocity must be considered. As a result, the shape of the intake manifold runners (runner tapering and the degree of turns in the runner) may be tuned in order to equalize airflow velocity to all cylinders.
Because of its split design, a dual-plane manifold is generally better suited to lower-RPM operation, enhancing overall driveability. That doesn’t mean that a dual-plane manifold isn’t suited for high-performance use, however. It depends on the powerband. In general terms, lower-RPM use is likely a better choice for a dual-plane and higher-RPM use is better accommodated with a single-plane manifold.
This cutaway view of a carbureted intake manifold provides a good view of the plenum dividers. (Photo Courtesy Holley Performance Products)
High-rise tunnel ram manifolds are best suited for extended high-RPM use and provide more torque and horsepower over a longer range (during higher engine speed). Modular high-rise intake manifolds (often called tunnel rams because of the long single-plane intake runners) are available that accommodate either a single 4-barrel carburetor or dual carbs. This example of a Holley Hi-Ram intake manifold is fitted with an upper plenum that allows mounting a pair of carburetors sideways, or in-line, depending on the carburetor dimensions and desired throttle linkage setup.
High-rise intake systems, by virtue of their design, raise the carburetor(s) well above the engine. Hood clearance issues are commonly solved by the installation of a tall hood scoop.
Certain electronic fuel injection intake manifold designs feature a central forward-mount throttle body and a series of equal-length intake runners directly to each cylinder. (Photo Courtesy BBK Performance)
In addition to cast-aluminum EFI intake manifolds, lighter-weight manifolds are also available made from advanced polymer material for better heat dissipation. Because of their design and construction, these are two-piece manifolds with lower and upper sections. A cooler intake charge provides a denser air charge, which in turn promotes increased power. Another benefit of the polymer manifold is its ability for designers to create smoother and more-laminar airflow more easily. (Photo Courtesy FAST/Comp Cams)
Carbureted intake manifolds commonly feature divider walls in the plenum to aid in directing the air/fuel charge to the cylinders. These walls are also designed to help in breaking up, or atomizing, the charge to prevent creation of large fuel droplets.
Custom sheet-metal intake manifolds are constructed of billet aluminum tubes and panels and welded together while stock manifolds are made of heavier-wall cast aluminum. Material and weld strength becomes a critical concern when dealing with a forced-induction setup, due to the repeated cycles of pressurized air within the manifold. This requires high-strength aluminum, such as 6061 billet.
In addition, when using a welded intake manifold, the plenum box volume becomes critical for a naturally aspirated application. Therefore, the plenum volume should ideally match the engine displacement to eliminate