The paper compares quantitatively the back-pressure characteristics of the metal-, thinwall and standard ceramic substrates in coated and uncoated condition. This is done under ideal flow conditions and real flow conditions in a series production converter.The influence of the different types of substrates on the back-pressure characteristics and on the performance of the engine is limited. Of more importance is the wash coat layer which in even minimum thickness variations affects the engine performance. However, by far the most significant impact is from the optimization of the system design for minimum pressure loss and uniform flow distribution. Ideal gas flow results in both increased engine power output and reduction in the conversion efficiency degradation factor.