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The prediction of tumble speed in the cylinder of spark ignition engines is discussed in this paper. Based on the experimental results from the steady state flow rig and the flow field measurement in the cylinder of a motored engine by LDA system, a model for the physical process of the tumble formation is proposed. That is: during the early stage of induction and before the piston reaching a critical crank angle, tumble can't be formed effectively. This critical piston position or crank angle should correspond to the time when the in-cylinder tumble has to be distorted and destroyed by the upward moving piston during the late stage of compression stroke. A model for predicting the tumble speed is derived and verified by the in-cylinder flow measurement results by a LDA system.