Using asymptotic expansion and numerical analysis, we demonstrate how the step-response ignition time of an automobile catalytic converter depends on the ratio of the reaction rate to the interphase heat-transfer rate, as measured by a key Damkohler parameter chi and the degree of monolith subcooling eta. In the region of low reaction rate at small chi, the normalized ignition time t(ig) scaled by the homogeneous ignition time t(ig)(infinity) from the inlet gas temperature is (t(ig)/t(ig)(infinity)) = 1 + 2 chi(1/2)ln(chi(1/2)/2 eta)(1/2), and the ignition takes place at a thermal front deep in the monolith. At large chi when the reaction rate is high, ignition occurs at the leading edge of the monolith with (t(ig)/t(ig)(infinity))= 2.50 + chi(ln eta - 0.34). The delay in ignition time with increasing chi is due to a Taylor-Aris dispersion mechanism induced by interphase heat transfer. Although the small-chi ignition mechanism is faster, its downstream ignition location leads to a very slow upstream propagation of the thermal front that follows ignition. An optimal converter system that ignites in 13 s, 25% of the current value in a. standard step-response test, is then designed by placing a small igniter, which ignites by the small-chi mechanism, upstream to preheat the current converter which then ignites by the large-chi mechanism. The length of the igniter is kept small by bypassing 2/3 of the exhaust since, from our theory, t(ig)(infinity) is independent of the gas velocity.