A study has been made of the heat-transfer processes in liquid-cooled engines and an equation has been developed that relates the heat rejection to the coolant and the engine operating conditions. Tests of an Allison V-3420-11 engine have been made to check the accuracy of the equation and to establish the cooling characteristics of the engine. By determining the few constants of the equation, the heat rejection to the coolant may be predicted with good accuracy for any particular engine operating condition. The tests showed that the rate of heat dissipation to the coolant was only slightly affected by either the rate of coolant flow or the relative proportions of ethylene glycol and water composing the coolant mixture.