We study the ion-acoustic instability driven by a drift between Maxwellian protons and electrons in a nonmagnetized plasma using a Vlasov simulation with the realistic proton to electron mass ratio. Simulation results for similar electron and proton temperatures are in good agreement with predictions. Namely, during the linear and saturation phases the effective collision frequency observed in the simulation is in quantitative agreement with the quasi-linear predictions. However, previous estimates <Galeev and Sagdeev, 1984; Labelle and Treumann, 1988> give the effective collision frequency less than one tenth the simulated values. The theoretical and simulation results are in a partial agreement with the simulation work by Watt et al. <2002> who used a non-realistic mass ratio. After the saturation, the effective collision frequency increases owing to the existence of backward-propagating ion-acoustic waves. These waves result from induced scattering on protons and contribute to the anomalous transport of electrons.