Selective Catalytic Reduction (SCR) catalysts have been demonstrated as an effective solution for controlling NOx emissions from diesel engines. There is a drive to reduce the overall packaging volume of the aftertreatment system for these applications. In addition, more active SCR catalysts will be needed as the applications become more challenging: e.g. lower temperatures and higher engine out NOx, for fuel consumption improvements. One approach to meet the challenges of reduced volume and/or higher NOx reduction is to increase the active site density of the SCR catalyst by coating higher amount of SCR catalyst on high porosity substrates (HPS). This approach could enable the reduction of the overall packaging volume while maintaining similar NOx conversion as compared to 2010/2013 systems, or improve the NOx reduction performance for equivalent volume and NH3 slip. In this work, systems consisting of SCR coated on high porosity substrates were evaluated in comparison to standard substrate based SCR systems used in typical 2010 applications. Performance testing was carried out on laboratory reactor and on engines over various test cycles (Federal Test Protocol (FTP), Ramped Modal Cycle (RMC) and Non-road Transient Cycle (NRTC)). The results indicate improved NOx conversion with high WCL SCR catalysts on high porosity substrate over standard washcoat loading (WCL) SCR on conventional substrate. In addition, NOx conversion is improved by increasing cell density from 400 to 750 cpsi. Overall, a volume reduction of 40-50% is achieved by coating SCR on high cell density, high porosity substrate, while maintaining similar NOx reduction efficiency. The higher amount of SCR catalyst in the HPS allows for higher NH3 storage and improved NOx conversion at lower temperature, thus improving performance under transient cycles. This work demonstrates that the higher performance achieved using highly loaded high porosity and high cell density substrates can enable significant volume reduction for future heavy duty and off-road diesel aftertreatement systems.