Increased cardiac stiffness hinders proper left ventricular (LV) expansion, resulting in decreased volume, delayed filling, and diastolic dysfunction. LV expanders are spring-like devices that aim to improve diastolic function by facilitating mechanical outward expansion. Implantations in animal models and in a few patients have shown promising results, yet further evaluation is needed to assess range of function and the risk of use. In this computational study, the effectiveness and potential use of a generic LV expander were assessed by utilising our finite element models of HFpEF-induced animal subjects. Following implantation, the treated models were compared to the corresponding untreated and healthy preinduction models. Influence of device orientation and its material properties was also examined. Our results demonstrated a reduction in LV pressure and a volumetric improvement. Computed LV stresses have shown no gross irregularities. The device contributed to stress elevation during diastole while having a minor effect during systole, supporting a basic safety profile. This is the first study to use numerical analysis to assess LV expanders’ performance on different HFpEF phenotypes. Improvement in heart function was demonstrated in both subjects, suggesting its potential use in various HFpEF manifestations, yet customisation and optimal deployment are essential to improve heart performance.