In a gasoline engine, the cycle-by-cycle fresh trapped charge, and corresponding unswept residual gas fraction (RGF) are critical parameters of interest for maintaining the desired air-fuel ratio (AFR). Accurate fueling is a key precursor to improved engine fuel economy, and reduced engine out emissions. Asymmetric flow paths to cylinders in certain engines can cause differences in the gas exchange process, which in turn cause imbalances in trapped fresh charge and RGF. Variable cam timing (VCT) can make the gas exchange process even more complex. Due to the reasons stated above, simplified models can result in significant estimation errors for fresh trapped charge and RGF if they are not gas dynamics-based or detailed enough to handle features such as variable valve timing, duration, or lift. In this paper, a new air flow and RGF measurement tool is introduced. The tool is a combination of 1-D gas dynamics modeling and imposed, measured dynamic pressure signals in the intake and exhaust manifolds. With this method, the components to be modeled in the gas exchange system are reduced to a minimum, making it possible to model the gas exchange process with adequate detail and still maintain reasonable tool execution time. In this approach, the accuracy of the air flow and RGF estimation are not influenced by the location of pressure transducers and it is possible to “track” EGR flow from the exhaust to the intake system. The tool is demonstrated on a 4-cylinder gasoline engine powered vehicle with dual intake and exhaust cam phasing. Predictions from the tool are compared to AFR and other real-time signals measured during a variety of different real-world transient drive cycles. The instantaneous fresh trapped charge, RGF across all cylinders, as well as the influence of VCT actuation is discussed.