Five commercial-off-the-shelf (COTS) engines ranging from 40 to 200 cc were tested as potential power plants for Group Two RPAs. The maximum brake power ranged from 1.8 to 5.0 kW. Of the five engines, two were designed for hobby aircraft (similar in size to Group Two RPAs) while the other three were designed for powered equipment applications. These five engines were run at WOT through their respective operating speed ranges to develop energy balances, specifically quantifying the split of energy between brake power, cooling load, exhaust enthalpy, and incomplete combustion. Using the energy balance data, general correlations for BMEP and fuel conversion efficiency as functions of displaced volume were generated to compare with previous two stroke engine results. Furthermore parametric studies capturing the effects of throttle setting, combustion phasing, cylinder head temperature, and equivalence ratio on the peak BMEP and fuel conversion efficiency were performed. The study data was used to develop normalized correlations to predict peak brake power, incomplete combustion, exhaust enthalpy, and cooling load as normalized deltas from the baseline condition. These correlations can be used to help engine designers understand the effect of these parameters for four stroke engines in the size class of 40–200 cc.