The substrate is used in a catalytic converter can to which a cylindricalinlet pipe leadsand has an inner catalytic zone portion, an outer catalytic zone portion andan insulationmaterial thermally separating the portions. The insulation extends through thesubstrate and has a uniform cross-section substantially defined by theintersection oftwo notional cylinders and the upstream face of the substrate, each notionalcylinderhaving: a nominal diameter that is between 1.08 and 1.20 of the diameter ofthe inletpipe; a thickness of 1-4 mm; and an axis aligned with the gas direction at thepoint ofmaximum velocity at the intersection of the inlet pipe and the can. One of thecylindersis associated with the gas flow at the lower limit of the operating range andthe other ofthe cylinders is associated with the gas flow at the upper limit of theoperating range.
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