​This study utilizes homogeneous charge compression ignition (HCCI) engine experiments to characterize fuel effects on Low Temperature Heat Release (LTHR) and Intermediate Temperature Heat Release (ITHR) of different primary reference fuel (PRF) mixtures. Experiments varied intake pressure from naturally aspirated to 1.8 bar boosted conditions, equivalence ratio from ϕ=0.3 to 0.4, and a variety of different fuel mixtures (PRF 85 to PRF 100). The engine experiments were used to guide single-zone HCCI simulations, using detailed chemical kinetic mechanisms comprising PRF mixtures. The experiments revealed important trends related to pre-ignition reactions in blends of iso-octane and n-heptane. As previous research has found, the pre-ignition reactions seen in these mixtures have a high sensitivity to pressure. Intake pressures below 1.4 bar showed no LTHR and minimal ITHR for all of the fuels tested, while pressures of 1.4 bar and above showed varying onsets of LTHR based on mixture composition. It was also found that significant increases in ITHR were coupled with increases in LTHR. Simulated heat release profiles of different PRF mixtures were examined and compared with the experimental results. The simulations results showed good agreement with the experiments for different fuels at low pressures. However, at higher pressures, the expected LTHR and ITHR profiles at the given experimental conditions were not observed. Sensitivity analysis was conducted to identity the fuel mixtures and pressures needed for to exhibit similar heat release profiles as the experiments. The simulation results were used to identify the dominant reaction pathways contributing to ITHR under various conditions.