Modern engine hardware and controls are very complex, requiring equally complex testing methodologies when performing powertrain benchmarking. Tethered benchmarking connects an engine in a test cell to a complete vehicle through an extended wire harness, enabling evaluation of stock powertrain calibration without manufacturer support. This test method can be used to develop brake specific fuel consumption maps and evaluate control strategies. However, this testing is limited to factory “on calibration” test points. To enable the evaluation of off-calibration powertrain operation, the selective interrupt and control (SIC) test capability was developed as part of an EPA evaluation of a 1.6 L EcoBoost® engine. A control and data acquisition device sits between the stock powertrain controller and the engine; the device selectively passes through, or modifies, control signals while also simulating feedback signals. This paper describes the development process of SIC that enabled a test engineer to command off-calibration setpoints for intake and exhaust cam phasing as well as ignition timing. Results are presented demonstrating the impact of ignition timing and cam phasing on engine efficiency. When coupled with combustion analysis and crank-domain data acquisition, this test configuration provides a complete picture of powertrain performance. Future applications of SIC could enable evaluating the impact of cam phasing on trapped residuals, examining knock tolerance, or studying the impact of splitting direct fuel injection into multiple pulses – all on a stock powertrain platform.