A Rotary Valve combustion System (RVS) is being developed which is a potential alternative to the conventional poppet valve combustion chamber systems currently in use on four-stroke reciprocating automotive engines. The RVS has been developed to operate in a Variable Valve Timing (VVT) mode, termed RVS/VVT. The system accomplishes variation of intake-valve-closure from 50 degrees After-Top-Center (ATC) to 250 degrees ATC. This broad range of variability is necessary to achieve throttleless power control from idle to full power.The RVS was evaluated for characteristics which were independent of its valve timing mode. These included: (1) system friction, (2) seal effectiveness, and (3) combustion performance at full load. System friction for the RVS valve train was measured by a pulley transducer on the drive-belt. Seal effectiveness was evaluated by static differential compression tests and dynamic blowby measurements. Combustion performance was similar to that of conventional systems, as indicated by the heat release histories which were similar for ignition and main combustion.The RVS/VVT system was tested for characteristics which were altered by implementation of the VVT mode. These included: (1) throttling-loss reduction at part-load, (2) combustion performance, (3) volumetric efficiency, and (4) idle stability. Results indicated that while producing the same Net Indicated Mean Effective Pressure (NIMEP), the “positive-loop” portion of the IMEP was reduced up to 12% due to the lower pumping MEP at part-loads. It is anticipated that this will translate into corresponding reductions in fuel consumption, assuming comparable valve train frictional losses. The combustion analysis for the VVT system exhibited faster burn rates than the conventionally throttled case, and registered low coefficients of variation in cycle-to-cycle indicated power. Volumetric efficiency was improved across the engine's speed range by virtue of the VVT's ability to tailor the timing of intake-valve-closing to the optimum point for each respective engine speed. As a result, the maximum IMEP attainable at low engine speed was significantly improved. The use of a simple tuned-pipe intake produced typical manifold dynamic tuning effects which, although not specifically optimized, combined with the airflow capacity and variable valve timings capability to produce volumetric efficiencies ranging from 85% at 2000 rpm to a peak value of 95% at 4500 rpm.