Tuning in to Manifold Design

Ever wonder why so many plastic air intake manifolds are showing up under the hood lately? In addition to lower weight and cost, one of the big advantages to molding a plastic manifold vs. casting an aluminum one is the design flexibility plastics afford automotive engineers.

For example, "tuning" steady-state airflow within the manifold involves manipulating geometry. With injection molded plastics, the intricate curves and smooth shapes required for tuning are a given. Tuning affects both engine performance and sound. And in the automotive industry, how an engine sounds and performs equates to car buyers' opinions of overall vehicle quality, an important factor in any decision to buy.

Case in point: Ford Motor's Split Port Injection manifold for the 1996 Windstar Van 3.8 liter V-6 engine. Unlike many other manifolds, this one was designed from the ground up as a plastic component. Ford design teams chose a material they are familiar with: 33 percent glass-filled nylon 6/6, a Zytel grade from DuPont. Each manifold consumes 8 lb of resin, saving 5 lb over previous aluminum versions. The material also stands up to core melt-out temperatures (150C), impact, and structural load bogeys of 10g, as well as resists creep and cyclic loads.

For a performance boost, designers included two sets of six runners each-shorter versions for high rpm operation and longer ones for low rpm. Ford Engineering tells IMM that the split port technique makes it possible to use two runners per intake valve on a two-valve-per-cylinder engine for better tuning without adding the cost and complexity of a four-valve engine. More efficient charges of air mean engine performance peaks during both conditions and overall engine performance improves by 29 percent over last year's model.

Noise generation was also a major consideration. Plastic manifolds have less mass than aluminum, a factor that actually increases vibration. To counteract the tendency, designers tuned airflow via geometry and included isolator bushings and rubber gaskets pressed into molded-in grooves to eliminate any structure-borne noise transmission. Again, the ability to mold in the grooves and eliminate a separate gasket carrier gave plastics a clear advantage.

CMI International Inc. performed tool design and now molds the mammoth horseshoe-shaped manifold using a metal core overmolded with Zytel at its facilities in Nuevo Laredo, Mexico. According to a recent IMM interview with John Haley, CMI project manager-polymer engineering, molding the manifold is no easy task. "Wall thicknesses of 3 mm with an inner diameter of 1 to 2 inches make central gating locations critical to prevent core shift," says Haley. "The mold contains a single core, four slides, including one with a core keeper at the throttle body, and six ejector pins." At CMI's Tech Center (Ferndale, MI), engineers performed mold filling and finite-element analyses as well as vibration testing.

Two out of six cells at the Mexican facility are kept busy molding the Windstar manifold for a total capability of 300,000 to 400,000 units per year. Cells contain a core-making machine (designed by CMI), a curing oven for the molded tin-bismuth core, a 1000-ton molding machine (there are four Ube and two Battenfeld machines) with valve-gated hot runner systems, and a core melt-out tank.