This external air filter is the primary element installed in the engine air-intake housing. It removes airborne particulate before the air reaches the turbocharger compressor inlet. A secondary internal filter is used inside this housing; the two elements are installed in series. A clogged outer element restricts airflow, which causes a drop in manifold pressure and increased fuel consumption. The manufacturer machines the sealing surfaces to match the housing geometry, ensuring the gasket seats correctly and no unfiltered air bypasses the element. Replace the outer element at the interval specified for the engine, and inspect the inner element at the same time.