The steady flow-field inside a monolith catalytic converter was examined by means of water-flow visualization. These tests, conducted with transparent, full-scale converter models with several different header geometries, showed that flow invariably separated from the inlet-header diffuser walls. A constant-diameter jet proceeded to the front monolith face, where it impacted and expanded to cover the substrate frontal area. For some visualization tests, the jet was constrained within a transparent tube which was translated toward the front monolith face, simulating shorter and shorter headers. The monolith internal flow field and pressure loss were found to be unaffected until the tube was within a few centimeters of the substrate. A converter with very short inlet and outlet headers is termed a truncated converter.The aged conversion-performance characteristics of monolith converters with truncated and conventional (tapered) inlet and outlet headers were evaluated in engine-dynamometer tests. Twelve single- and double-brick converters with volumes of 1.52 and 2.38 L and with Pt/Rh catalyst concentrations of 58 and 37 g/ft3, respectively, were aged on a relatively severe aging schedule to simulate European aging conditions. Ten double-brick converters of 1.8-L volume and 27 g/ft3 Pt/Rh catalyst were aged on a less-severe schedule appropriate to the United States. For both the high-loaded European applications and the lower-loaded U.S. applications, truncating both the inlet and outlet headers caused little or no deterioration either in CO, HC, and NOx warm-up times or in traditional measures of fully warm performance.