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 ScienceThe 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.
Sheet Metal MythsAs 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 FactsThere 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."
In addition to replicating the shape of the intake port, the runners on a sheetmetal intake can be positioned to find the ideal angle to achieve a direct shot into the intake port. Likewise, the fuel injector bosses can be precisely angled to spray fuel right on the back of the intake valves. While many carbureted manifolds feature longer runners on the corners, resulting in uneven air distribution from cylinder to cylinder, the runners on a sheetmetal intake can each be made the same length with relative ease. Designers can also experiment with runner taper to help speed up the intake charge velocity near the intake port while maintaining the target runner volume and position the carburetor or throttle-body directly above the runners to keep the air charge velocity up. In carbureted motors, since the air/fuel mixture doesn't have to transition around tight bends before reaching the port, the direct path down a straight runner reduces the potential of fuel puddling. While the size and shape of runners is the primary tuning device, altering plenum volume also affects where a motor produces peak power and torque. "A general rule of thumb is that the plenum volume should be equal to the displacement of the motor," says Beck. "A smaller plenum gives you more torque, while a bigger plenum makes more power at high rpm."
One Size Doesn't Fit AllSo far there haven't been any astounding revelations, just insight into the basic principles of building horsepower that a sheetmetal design incorporates. However, perhaps the biggest advantage of a sheet-metal intake is its customizability for specific applications. "People don't understand that an intake manifold is like a camshaft and is infinitely adjustable," says Keith Wilson. "We can alter where the carburetor, injectors, and nitrous nozzles are placed, and we can also customize the runner length, runner taper, runner volume, plenum shape, and plenum volume."
Engine specs, such as displacement, compression ratio, rpm range, cylinder-head flow, carburetor size, spacer height, and fuel type all influence how a manifold will be built around an engine. However, just as important are vehicle specs, such as transmission type, rear gear ratio, torque converter stall speed, shift rpm, and vehicle weight, since they determine where in the rpm range power needs to be made to optimize acceleration. "We're more concerned with going down the track than making power on a dyno," says Keith Wilson. "One intake might make more power on dyno, but won't get you down the track any faster if the car has a Powerglide and only shifts once."
In essence, a sheetmetal intake is built more around the entire vehicle package rather than just the engine. "People don't realize what goes into designing and building a custom intake manifold," says Wilson. "The first step is determining if a motor is moving enough air to justify a sheetmetal intake in the first place. We have customers fill out and fax in a questionnaire with detailed engine and vehicle specs. We then study that information, come up with some preliminary CAD drawings, and combine that with our track experience to come up with a final design before any actual fabrication begins. The only way we'll build one is if your heads are here, because we don't use jigs. Your engine is our jig." It is all these factors combined-using airflow dynamics and customizing manifold design for a specific engine and chassis combination-that harmoniously coalesce to make more power.
Do You Need One?Building a sheetmetal intake manifold is an arduous process that consumes roughly 60 hours. The customer is the one who gets to pay for all that labor, and a custom manifold can easily ring up a bill of a couple thousand dollars. It would be wise to do lots of research before handing out your Visa number-but how do you know if you need one?
Unfortunately, there is no simple formula, no horsepower cut-off level, no head flow figure that gives a clear-cut answer. That said, sheetmetal intakes are most common on ultra high-end motors bound by tight rule restrictions in competitive race classes (like Pro Stock) where finding an extra 3 hp is a very big deal. Likewise, they're also found on engine platforms neglected by the aftermarket or one that is too new to have sufficient aftermarket support. Generally, sheetmetal intakes are more for race cars than street cars. "Honestly, we turn away more business an we accept," says Wilson. This should give you some idea of what type of combinations warrant a sheetmetal intake, but the only way to know for sure is talking with a manufacturer.