The experimental methodology consists of two different parts. The first one is the emission test and the other one is microstructure and materials composition analysis. The emission test was conducted by multi-gas analyzer for emission measurements. After completing the test, the converters were cut to extract the substrate or ‘honeycomb’ inside the housing and being analyzed for microstructure and materials composition using Scanning Electron Microscopy (SEM) and Energy Dispersive Analysis (EDX).Two ceramic catalytic converters (underbody type) of different vehicle models were evaluated. The first one is extracted from PROTON Wira 1.3L and another one from FIAT Punto Selecta 1.2L. Both converters had same substrate material, synthetic cordierite ceramic and substrate shape, square cell except they differ in size, chemical properties and geometrical attributes. Double substrate systems are as thermal mass, geometric surface area, and washcoat distribution (Presti et. al 2002). Per Marsh et. al (2001) have shown that less favourable mass transfer and higher flow velocity can be found as a results of the reduction of diameter, however, compensated with the cases of lower cell densities. implemented on both converters for separate reduction and oxidation purposes. Single heat shield is applied to Fiat converter while double heat shields that cover both the upper and bottom part are used in Proton converter. Also, for each converter, the inlet and outlet cones were fabricated identically. The engine used in the test is a spark-ignition (inline 4 cylinder) Proton engine coupled with eddy-current type dynamometer. It is controlled by AUTOTEST VI main controller located outside of the engine room with the aid of AUTOTEST VI computer software via numerous sensors. The gas analyzer employed in the setup was the upgraded version (MRU Delta 1600-L) which is capable to measure simultaneous reactions of carbon monoxide, carbon dioxide, hydrocarbon, oxygen and also nitrogen oxides. It also incorporates a probe, suitable to measure the gases at the inlet and outlet region, along with the thermocouple, to attain temperature values for ambient and the exhaust gas itself. Figure 2 shows the completeexperimental setup.The substrate materials, type of catalysts and their corresponding percentage loading for both converters were determined using the EDX machine that is coupled with the scanning electron microscopy (SEM) system.