The magnetic structure of the noncentrosymmetrical cubic ferromagnets with Dzyaloshinskii-Moriya interaction Mn1−yFeySi with y = 0, 0.06, 0.08, and 0.1 has been studied by means of the small angle neutron diffraction and magnetization measurements. These compounds are shown to be ordered into the helix structure below the critical temperature TC that decreases linearly with the Fe doping and approaches zero at y ≈ 0.13. We build the H − T (magnetic field – temperature) phase diagrams for each compound and interpret them taking into account the Bak-Jensen hierarchical model of the principal interactions. Following the concentration evolution of these principal interactions (such as the spin wave stiffness A, the Dzyaloshinskii constant D, and the gap Δ in spin wave spectrum) we show that A resembles behavior of TC approaching zero at y ≈ 0.15. Thus, it is the isotropic ferromagnetic exchange interaction that plays a dominating role in determination of the critical temperature TC of the compounds under study. T