For a number of underhood applications such as throttle bodies, tight operating tolerances require dimensional stability of the entire assembly, including both the material and the part design, under a wide variety of chemical and temperature exposures. Using a prototype air conditioner compressor housing tool, a series of design of experiments (DOEs) were conducted to identify the key processing and material factors that effect final part out of roundness properties. The initial series of DOEs included both unfilled and glass filled polyetherimides and a glass filled polyphenylene sulfide. Thirty percent glass filled polyetherimide performed best dimensionally based on both low out of roundness characteristics as well as low variability. Conversely, the highest out of roundness and variability were evident with the glass filled polyphenylene sulfide. The key processing variables which seem to effect the polyetherimides' out of roundness were injection hold pressure and cure time. Polyphenylene sulfide was affected primarily by melt temperature. Additional DOEs were conducted to investigate a wider variety of materials (base resins, filler agents and levels) and evaluate the effects of post molding conditions such as thermocycling. Ultimately, this information can be utilized to optimize the combination of materials, design features and processing conditions with the goal of meeting or exceeding the application requirements while minimizing final piece costs.