Elevated cardiac stiffness presents a barrier to proper expansion of the left ventricle (LV), leading to diminished volume and diastolic dysfunction. LV expanders, designed as spring-like devices, aim to ameliorate diastolic function by facilitating mechanical outward expansion. Implantations in animals and humans have shown promising results, yet further evaluation is needed to assess a range of functions and the risk of use. In this computational study, the effectiveness and potential application of a generic LV expander were assessed using previously generated finite-element models of induced heart failure with preserved ejection fraction (HFpEF), representing different disease phenotypes. Following implantation, the treated models were compared with corresponding untreated and healthy pre-induction models. The influence of device orientation and material properties was also examined. The results indicate a reduction in LV pressure and an improvement in volume. Computed LV stresses revealed no gross irregularities. Stress elevation during diastole was observed with the device, while its effect during systole was minor, supporting a basic safety profile.