The concentration dependence of the second harmonic generation efficiency has been analyzed in binary mixtures containing a ferroelectric liquid crystal material (W313) with high spontaneous polarization and an achiral host. It has been found that the effective nonlinear coefficient is not, in general, proportional to the mixture mole fraction, although it is linearly correlated to the spontaneous polarization. Far from the transition to the chiral smectic C phase, both quantities display the same temperature dependence.