The paper shows that the Sraffian Supermultiplier model provides an alternative closure for the analysis of the relationships between economic growth, income distribution, capacity utilization and effective demand in heterodox growth models. The new closure comes from the variability of the ratio of the average to the marginal propensity to save (herein called fraction), which is entailed by the assumption of the existence of (an independently growing) autonomous expenditures that do not generate capacity for the private sector. This variability allows the marginal propensity to invest to determine the saving ratio without the need of changes in income distribution. If it is also assumed that changes in the marginal propensity to invest are induced by the competitive need to gradually adjust capacity to demand, this adjustment by means of endogenous changes in the fraction provides a closure that allows us to reconcile demand led growth, exogenous distribution and a tendency towards normal capacity utilization, even across steady states. A comparative analysis points out the distinctive features of this new clousure by contrasting its main results to the ones obtained with the closures associated to both the Cambridge and the Kaleckian growth models.