Recent combustion research has focused on low temperature combustion to meet engine emission regulations and to advance the development of low temperature homogenous compression ignition engines. Autoignition studies in this temperature regime are primarily performed by Rapid Compression Machines (RCMs) which are sensitive to the heat transfer characteristics of the experimental device. RCMs are widely used to measure autoignition data such as ignition delay and species concentration. Measured ignition delays from RCMs are typically reported at an adiabatic condition; however, this assumption may produce a systematic error in ignition delay measurement as heat transfer is observed to reduce the pressure and temperature during the autoignition process, e.g., a longer ignition delay has a greater pressure and temperature drops. RCMs are custom built and have unique design characteristics that affect the heat transfer during the autoignition process. In addition, depending on the diluent composition (e.g., helium versus nitrogen or argon), different heat transfer characteristics are expected. As a result, autoignition results at similar conditions may vary from facility to facility or depending on the used diluent. The dependency of the measured data on the used facility or diluent may produce uncertainty in the data which impact the development of high-fidelity combustion mechanisms. In this work, a new method is developed and utilized to eliminate heat transfer from the ignition delay data. To evaluate the new method, the autoignition of n-pentane mixtures in the low temperature regime were investigated using an RCM. To vary the heat transfer, the compression ratio of the RCM was changed and the ignition delays were measured at similar pressure and temperature conditions. The tests were performed at an equivalence ratio of approximately one and nitrogen and argon as diluents. By applying the new method, the effect of heat transfer on the ignition delays were eliminated successfully and ignition delays at adiabatic condition were determined. A detailed kinetic model of n-pentane was used to simulate the measured adiabatic ignition delay, which agreed well with the experimental data.