An exhaust collector system for discharging a flow of exhaust gases from an internal combustion engine. The exhaust collector system includes a plurality of primary headers having their discharge portions merged together at a merge angle into a substantially oval or obround-shaped manifold outlet, and a substantially-round secondary pipe having a pipe inlet with a diameter greater than the diameter of the primary headers. The exhaust collector system also includes an oval-to-round collector flowtube which has a substantially oval-or obround-shaped collector inlet that is coupled with the manifold outlet and a substantially-round collector outlet that is coupled with the pipe inlet. The collector flowtube has a constant circumference along its entire length and a transverse cross-sectional area that continuously increases from the collector inlet to the collector outlet to cause a continuous expansion of the flow of exhaust gases.