In this paper, the electrical-to-thermal energy transfer efficiency of the transistor coil ignition system for spark-ignition engines is investigated using both electrical and calorimetry measurements. The gap voltage and discharge current are measured to determine the electrical energy supplied to the spark gap. A pressure-rise calorimeter is used to estimate the thermal energy transferred from the plasma channel to the gas. Firstly, this paper studies the influences of spark gap size, electrode geometry and background pressure on the energy transfer efficiency. To further investigate the effectiveness of increasing breakdown energy on the energy transfer process, a direct-capacitor is paralleled to the spark gap to redistribute the spark energy in both breakdown and glow phases. The varying of the capacitance enables the investigation of the energy transfer efficiency under different breakdown energy level. Results show that the electrical-to-thermal energy transfer efficiency is strongly dependent on gap size, electrode geometry and background pressure. Increasing the breakdown discharge energy is beneficial for the electrical to thermal energy transfer process.