Autoignition chemistries of several paraffins, olefins, and aromatics were examined in a motored engine at different engine conditions. Paraffin chemistry was dominated by “negative-temperature coefficient” (NTC) behavior which became more pronounced at lower pressures, higher temperatures, and shorter reaction times. In contrast, olefin and aromatic chemistries did not exhibit NTC behavior.Measured pressures and calculated temperatures at fired octane rating conditions showed slightly lower pressures, higher temperatures, and lower reaction times at Motor octane rating conditions when compared to Research conditions. Therefore, paraffins would have a more pronounced NTC behavior under Motor rating conditions than under Research conditions. Since a more pronounced NTC behavior requires a higher compression ratio to produce autoignition and higher compression ratios equate to higher octane qualities, paraffins (generally low sensitivity fuels) have enhanced octane qualities under Motor conditions. Olefins and aromatics do not exhibit NTC behavior, and therefore, do not enjoy enhanced Motor octane qualities which accounts for their high octane sensitivities.