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Stoichiometric air-fuel ratio control during transient operation requires an accurate estimate or measurement of the instantaneous air flow rate in an engine. Two methods are commonly used for determining engine air flow rate: air-mass sensing and conventional “speed-density.” The lead air flow information provided by air-mass sensors helps compensate for manifold filling and other fuel system delays. However, the high cost (and sometimes lower reliability) of air-mass sensors has led many manufacturers to continue to use the less accurate speed-density method for determining air flow rate. This paper develops a model-based nonlinear manifold pressure observer that estimates the flow rates at the throttle and the intake ports of an engine using speed-density type sensors. The throttle flow rate estimate can be used instead of an air-mass sensor to provide the lead information necessary for accurate transient air-fuel control on TBI engines.