It's hard not to believe engine intake manifolds made of plastic eventually will take over under the hood. Proponents argue convincingly that plastic manifolds - usually glass-reinforced nylon 6/6 - are substantially lighter and less expensive than those made of metals such as aluminum. The interior surfaces of plastic intake runners also are smoother. That translates into less turbulent airflow through the tubes and - bingo - more horsepower.

What's more, the technology is well established, and featured on many of the world's best engines, including several of Ward's Auto World's 10 Best picks. BMW, Mercedes-Benz, Volkswagen, General Motors, Ford, Chrysler and others have been replacing metal with plastic intakes for almost a decade.

European automakers have been particularly aggressive in this area. Plastic intakes have been used on Porsche engines since the early 1970s.

So it was no small surprise to plastic suppliers - and a few grease-stained automotive journalists - to find that Mercedes' much heralded new V-6 has an intake manifold made of - gasp - magnesium. Especially since plastic is expected to dominate metal in this application on most new engine programs.

This is not a case of cost-is-no-object engineering or German engineers simply showboating. Those days are long gone at Mercedes. And this isn't a low-volume engine, either. First full-year production should hit 300,000 units. A

V-8 version of this modular engine - also featuring a magnesium intake - will add another 100,000-odd units.

What's more, Mer-cedes happily used plastic for the intake manifold on its previous inline 6-cyl. engine and uses an innovative multishell plastic design for the intakes of its 2L and 2.3L 4-cyl. engines.

So what gives?

A Mercedes engineer says that magnesium was deemed necessary for weight reduction, but was used primarily to reduce noise and to achieve a "more desirable quality" for the intake sound.

Ah, a fly in the oatmeal. Despite all of its obvious benefits, the plastic manifold is not perfect; it actually gets a little noisy and rambunctious on occasion. "That's really the only thing you could stick in the negative column on the nylon manifold," says Jim Best, president of Market Search Inc., an automotive plastics research company.

Despite this drawback, Mr. Best and most nylon producers still expect the plastic manifold juggernaut to continue, but it won't completely displace metal because of quibbles over such things as noise control. Today thermoplastics (essentially nylon 6/6 and nylon 6) have a 35% share of global air intake manifold applications, with aluminum accounting for almost all of the rest. Broken down by region, plastics have the highest share in Europe (50%), the smallest in Asia (less than 5%) and around 40% in North America.

By the year 2000, global use of thermoplastics for air intakes is projected to increase to about a 50% share vs. aluminum. North American and European shares will be between 60% and 70%, and about 30% in Asia/Pacific. DuPont and BASF AG are the major suppliers of nylon 6/6 for intake manifolds, but applications for nylon 6 also are expected to grow, which makes suppliers such as DSM, Bayer AG and AlliedSignal Inc. bigger players.

But that doesn't mean complaints about noise are falling on deaf ears.

DuPont Automotive, the world's largest producer of nylon for intake manifolds, acknowledges noise sometimes can be a problem, mainly because wall thickness tends to be thinner in plastic than metal.

In fact, DuPont is opening an acoustical analysis laboratory at its Troy, MI, headquarters during first quarter 1998. It will include sophisticated laser and acoustical holography equipment designed to predict potential noise problems very early in the design phase.

Kenneth W. Nelson, senior technical consultant at DuPont automotive, says intake manifolds are very much like musical instruments, and are just as difficult to tune properly. With so much air whooshing through their complex vortices, it's easy to get unpleasant noises. Plus emission-control devices such as exhaust gas recirculation valves often are required to stick up into the airstream like an open Coke bottle. Blow across the open mouth of a Coke bottle and you know what kind of problem this represents.

Yong Yu, a technical specialist in acoustical analysis at DuPont, says the noise problem is further compounded by the highly subjective nature of intake sound quality. One person's noise is another's music. Check your local music store for details.

Nevertheless, DuPont engineers say the plastic intake on the sweet-sounding Porsche Boxster engine forever shatters the idea that automakers must give up sound quality to gain the advantages of a thermoplastic intake manifold.

Early testing of the Boxster air intake revealed "an unwanted resonant frequency that threatened to distract from the engine's characteristic sound quality," DuPont says. Translation: It sounded like a lawnmower when the variable camshaft timing switched from the torque to the power mode at 2,250 rpm.

Using a combination of finite-element modal analysis, acoustic holography and laser-scanning analysis, DuPont experts pinpointed the source of the noise and corrected the problem and delivered prototypes for testing in just seven days.

If you ever have the pleasure of cranking up and winding out a Boxster, you'll know the fixes worked.

Only months after The Budd Co. subsidiary Waupaca Foundry began full production in its new $60 million castings plant in Tell City, IN, it is announcing expansion plans. Budd Chairman Siegfried Buschmann says the strong market demand and excellent performance of the plant warrants an additional $65 million investment, which will be used to produce large ductile iron castings for the automotive industry. Budd produces gray iron and ductile iron castings for 100 current vehicles.

BASF doubles nylon capacity BASF is launching commercial production of a 70-million-lbs./year (31,750 t) expansion of its Freeport, TX, nylon 6 engineering resin plant. The expansion brings the company's total NAFTA region capacity to 620 million lbs./year (280,000 t). The Freeport startup is the latest in a string of nylon 6 expansions by the company. Earlier this year BASF brought on stream a new 70-million lbs./year plant in Belgium. It also has major nylon-producing complexes in Germany and England, plus there are plans for building a major resin plant in China. "Currently there is very strong demand for our Ultramid nylon resins in virtually all markets in which we participate," says Dr. Reinhard B. Katz, group vice president, BASF Plastics.

Nissan shifts to hot-dip steel In an effort to cut costs, Nissan Motor Co. will phase out electrogalvanized steel sheet in its passenger cars over the next two years and shift to less-expensive continuous hot-dipped galvanized steel, American Metal Market reports. Hot-dipped galvanized reportedly costs $32 to $40 less per ton. Honda, Mitsubishi, Toyota and numerous other automakers already have made similar moves, even though EG steel is claimed to have superior formability and rust resistance.