Design, testing, and implementation of new aftertreatment devices under various engine operating conditions is necessary to meet increasingly stringent regulatory mandates. One common aftertreatment device, the catalytic converter, is typically developed at a reduced scale and tested using predefined fluid compositions sourced from bottle gases and can undergo both species and temperature cycling in addition to steady-state testing. However, these bench-top conditions may differ from real-world operation in terms of flow-rates, species composition, and temperatures experienced. Transitioning from small-scale bench-top testing to full-scale engine applications requires larger monoliths that therefore have a significant amount of catalyst slurry to be washcoated, which increases cost and fabrication time. Being able to experience realistic emission streams under scaled flowrates would allow for a physically smaller catalyst testing at matched space velocities resulting in faster, more cost-effective determination of aftertreatment device effectiveness. This work documents the design and performance of an intermediary-scale (5-50 SLPM) setup to aid in the catalyst testing process. This is accomplished using a secondary exhaust branch to flow a variable percentage of exhaust from the main branch. The system siphons exhaust via a slip-stream approach driven by a venturi ejector, which is commonly used in automotive applications to dilute samples for emissions analysis. Instead, the pre-diluted flow from the ejector is routed through the catalyst, where post catalyst emissions testing occurs. The system is evaluated under a range of engine operating conditions with varied equivalence ratio and intake pressures to affect exhaust out temperatures / catalyst inlet temperature which is critical for testing catalyst activation. Emissions are recorded in both the main and secondary branch with no aftertreatment device installed to verify compositional parity. Initial results show that the two branches produce self-similar engine-out emissions, but with the ability to scale flow and modulate temperature through the secondary catalyst testing branch.