Intake volumetric efficiency (VE) of a spark-ignition engine varies with valve timings, engine speeds, and manifold air loads. The existing approaches to reveal the underlying effects of these VE factors on instant valve flows remain complicated and expensive. In an effort to develop an applicable approach to analyze the detail valve flows, a naturally aspirated production engine with dual independent VVT was dynamometer-tested with fast in-cylinder pressure measurements and slow manifold pressure measurements. Both intake and exhaust valve flow was then reproduced using a new model, DQS model, in crank-angle resolution (CAR). One new flow mechanism, the flow wave subsidence, has been revealed to be one of the major drives of VE changes.We propose a dynamic quasi-steady (DQS) flow model to reproduce the valve flow profile from the measured pressure data. The DQS model features two manifold dynamics and a delay in the use of in-cylinder pressure measurements. The delay represents the effect of duct gas inertia on quasi-steady flow formation. In order to determine the delay amount as well as the model discharge coefficients, we construct an effective calibration method which matches the calculated valve flow profile with the estimated net cylinder flow. The DQS model results were validated with measured residual gas amount and overall charge amount in a large number of test runs with vastly different valve timings, engine speeds and loads.