Intake Manifold - Between The Sheets

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 All
So 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 than 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.