Engineers at Delphi Automotive Systems, Troy, Mich. (www.delphiautomotive.com), are now molding the engine-air control valve, or throttle body, of an outboard engine in plastic instead of machining it from aluminum. "The biggest concern for this type of application," says David Evans, senior design engineer at Delphi Automotive, "is the roundness of the body's bore. The butterfly must seal tightly against the bore to minimize air leakage around the valve. A good fit at low airflows is critical to controlling engine speed at the low end, particularly at idle."

The plastic is Amodel A-1565, a mineral and glass-reinforced grade of PPA (polyphthalamide) from Solvay Advanced Polymers LLC, Alpharetta, Ga. (www.solvayadvancedpolymers-US.com). The material has a heat-deflection temperature of 271°C and a flexural modulus of 2,600 kpsi. Delphi engineers can mold the throttle body and hold tight tolerances on both the size and roundness of the bore.

Injection molding the plastic part leaves a small gap at the valve seat for efficient engine operation. Moreover, "The costs of casting aluminum and injection-molding plastic are about the same," claims Evans, "but the molding results in a net shape which eliminates the cost of machining the aluminum part to its final dimensions." The plastic-throttle body better resists corrosives, such as salt water, and works over a -40 to 125°C temperature range.