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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.

Pressure-loss characteristics of a variety of single- and double-substrate metal-foil and ceramic-substrate converters with tapered and truncated inlet and outlet headers were measured in room-air flow, hot-gas flow, and engine-exhaust tests. Test data in the three different media correlated with the inlet-pipe Reynolds number when expressed as a loss coefficient, i.e., pressure loss normalized by the inlet-pipe dynamic head. Because restriction measurements made in different media correlate well as a Reynolds number-dependent loss coefficient, inexpensive room-air test data can be used to estimate converter pressure losses in the engine environment. The normalized losses in the substrate varied inversely with inlet-pipe Reynolds number, ranging from, e.g., 6 at Re = 30 000 to 2 at Re = 200 000. The remainder of the losses occurred in the inlet and outlet headers and in the section between the substrates.