Selecting the Proper Intake Manifold

Selecting an intake manifold is just as important as selecting carburetor or fuel injection, although it’s sometimes overlooked, because the focus is on the piece that sits on top. Like carburetors, intake manifolds also fall into the “bigger isn’t always better” category. Let’s take a look at the different types of intake manifolds and how each one suits particular engines with different requirements.

There are three main types: single plane, dual plane and high or tunnel ram, and each of these has its place on the street and on the track. The very first consideration is what type of fuel delivery will be used. A carburetor and throttle body injection application can use the same style intake with a standard square-bore mount on top. A multi-port injection system will need bungs welded in place for fuel injectors to enter the runners close to the cylinder head. The second consideration is the rpm and power levels of the engine. An engine that makes high horsepower at high revs won’t typically use the same type of intake manifold as a low-rpm torque monster. This rpm and power variable will determine the runner design, plenum volume and which of the three types listed above will be used.

Dual Plane

This is the most common style intake on the street and on engines whose max power outputs don’t peak above 6,500 rpm. The design can be pictured as two intake manifolds cast into one. With a standard four-barrel carburetor, two of the barrels feed four cylinders through one plane, and the other two carb barrels feed the other four cylinders through the other plane. These intakes are also known as 180-degree intake manifolds because the four cylinders fed by each manifold plane draw at intervals of 180 degrees of crankshaft rotation. By splitting up an eight-cylinder engine into two four-cylinder systems, the firing frequency in each half supports a greater volumetric efficiency than if it were combined. These intakes provide excellent mid-range horsepower and torque levels. They also dampen the negative effects of an oversized carburetor because the strong signal is maintained from the cylinder to the plenum. This generally short and less impressive-looking dual-plane type will often be dismissed as a high-performance-style intake, but it’s often the best choice for maximum output on these sub-7,000-rpm engines.

Single Plane

A single-plane intake starts with an open plenum under a four-barrel carburetor or throttle body. Each runner makes a straight shot to the intake for minimum turbulence and restriction. They are often taller than their dual-plane counterparts to allow for this. This means hood clearance is a consideration. Because of this design, they are ideal for engines making peak horsepower in the 7,000-9,000 rpm range. At times, total airflow of the single-plane intake compromises good drivability, throttle response and cylinder-to-cylinder distribution throughout the engine rpm range.

Hi Ram/Tunnel Ram

These bathtub plenum intakes are tall single-plane designs with nearly straight runners to the cylinder heads. You’ll often see these sky-high intakes with more than one carburetor or throttle body on top. They’re designed for maximum air and fuel, and have less regard for low-rpm torque or drivability. Although that’s not to say that neither is possible.

Multi-port Fuel Injection

Any of these three intakes can be fitted for use with multi-port fuel injection. Many styles come with pre-drilled fuel injector bungs, others with a boss in place to guide milling, and others require a start-from-scratch retrofit. Holley’s new single four-barrel single-plane LS intakes, like the one seen above, come in pre-drilled versions for fuel injection. With port injection, some of the downsides of a single-plane intake become less of a problem. Since fuel isn’t being carried down the runners, a larger port size and throttle body size can be used without adverse drivability problems. That’s why both single-plane and hi-ram-style intakes are largely used with multi-port fuel injection.

 

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