A simple two degrees of freedom classical model of the nonlinear resonance interaction between symmetric bond bending and torsional motion in linearly bonded tetra-atomic molecules is investigated. The rate and mechanism of intramolecular vibrational energy redistribution at different relative vibrational frequencies is evaluated, and comparison is made with quantum calculations. The rate of torsional isomerization is compared to the prediction of transition stale theory, and related to the observed intramolecular vibrational energy redistribution.