Primary diesel and gasoline reference fuels, along with secondary reference diesel fuels across a very broad cetane range were tested in an Ignition Quality Tester (IQT) rig. Additionally, numerous pure component fuels across a range of hydrocarbon size and structure were evaluated. Various methods were used to determine the Ignition Delay (IGD) of the various fuels. All of this testing followed the ASTM D6890 protocol. The reference fuels’ IGD followed expected trends, however, the diesel PRF fuels in the low cetane range produced DCNs (Derived Cetane Numbers) that were moderately higher than their cetane reference values. From the perspective of fuel size, IGD shows a significant ‘shortening’ – faster nature with increased fuel carbon number. For a given carbon number fuel molecule, normal alkanes were provided the ‘fastest’ IGD, with alkenes and branched aromatics leading to moderately longer IGDs. Cyclo-paraffins show the ‘slowest’ – longest IGDs. The pressure recovery start of combustion IQT criterion is quite comparable to the 5% energy release criterion often used in diesel engine combustion analysis. Some companion modeling was pursued in order to further understand these experimental results.