Intake Manifold - Between The Sheets

Ensconced atop thousands of serious street and race motors during its time, the Holley Dominator carb-with its four severely engorged throats-has been a formidable visual element associated with performance for generations. Its mere presence draws ogles and speculation of what components beneath it warrant all those cfm, but the Dominator's losing a good chunk of its gawk market share to sheetmetal intake manifolds. Big boosts in power numbers aside, their lustrous slabs of contoured billet aluminum, stitched together with surgically precise welds, form stately sculptures worthy of residence at the Guggenheim. There are certainly many applications that can benefit from sheetmetal intakes, but just because the meanest and most affluent dogs in the kennel run them doesn't necessarily mean they're right for you. We talked to experts John Beck and Keith Wilson to bring you the lowdown.

Intake Science
The purpose of an intake manifold is to distribute air evenly to the intake ports. Air first enters through the throttle body or carburetor and then reaches the plenum before traveling down the intake runners that lead to each individual intake port. While that seems relatively rudimentary in concept, intake manifold design plays an enormous role in determining the shape of horsepower and torque curves, in addition to affecting how much power a motor produces. Flow benches are great for measuring the potential flow of cylinder heads in a controlled environment under ideal circumstances, but impressive flow numbers don't mean squat if there's a mismatched intake manifold choking the air supply.

The two basic methods of tuning an intake manifold are varying the length of the intake runners and the volume of the plenum. Longer runners improve low- and mid-range torque at the expense of top-end horsepower. At low rpm, increasing the distance the air must travel also increases the inertia of the column of air within the runners, which promotes cylinder filling and torque output. However, that same runner length becomes restrictive at high rpm, where the intake valves stay open for far shorter durations. Likewise, a smaller cross-sectional diameter increases the intake air velocity and low-end torque while laying over in the upper reaches of the powerband. A perfect example is the Chevy TPI motor, which produces tremendous torque down low but falls on its face shy of 5,000 rpm.

Nonetheless, with modern performance cars approaching 4,000 pounds, and displacement limited by fuel economy concerns, late-model engines, such as the GM LS1, Ford mod motor, and Chrysler Hemi, employ long runners for brisk off-the-line performance. Considering most consumers spend very little time winding out the tach, sacrificing some power for bottom-end torque is appropriate, but piling on internal engine modifications like ported cylinder heads and increasing the compression ratio can quickly exceed the limit of a factory intake manifold. Porting the runners helps, but today's composite manifolds make the procedure a bit more delicate. Granted, there are usually aftermarket solutions that work reasonably well, they're still a power liability in particularly demanding applications, and the need for high-performance intakes precipitates a much faster turn-around than vendors can provide with a new engine platform. Case in point: The early years of LS1 tinkering saw hot rodders develop reverse-split cams with more intake duration than exhaust duration as a crutch for a weak factory intake manifold. Under such circumstances, a custom sheetmetal manifold may be the only viable alternative.

Sheetmetal Myths
As exotic specimens most enthusiasts aren't too familiar with, sheetmetal intake manifolds are surrounded by misconceptions that need debunking. Sheets of billet aluminum aren't inherently superior to cast aluminum, and the performance advantage of sheetmetal intakes has nothing to do with the material itself. In reality, sheets of metal greatly simplify the production process of engineering and constructing an intake manifold because it eliminates the costs associated with tooling, casting, and operating a foundry. Moreover, even if overhead weren't an issue, since most sheetmetal intakes are custom one-offs designed specifically for each application, the standard casting process would be far too time- consuming. Hence, sheetmetal reduces costs and streamlines the production process while billet aluminum is the material of choice for its low mass and strength.

The Facts
There really is no magic as to how sheetmetal intake manifolds can make more power over a standard cast intake. The truth is, designing a manifold around a specific engine combination makes it easier to take advantage of basic engineering principles that more efficiently supply air to the cylinder heads. Since no two CNC programs are the same and even a skilled machinist probably can't identically hand-port two sets of heads, the intake-runner openings and the intake ports on the heads rarely line up. "People don't realize it, but all heads are different because of the way they're ported," says John Beck. "To ensure a smooth transition of airflow from the runner to the port, we first measure the intake- port openings and feed the measurements into a CAD/CAM program. We use that data to program the CNC machine, which then cuts openings that perfectly line up with the intake ports."