This study examines the effects of varying fuel injection pressures on the combustion characteristics of diesel (B0) and palm biodiesel (B100) using combustion pressure and heat release rate analysis. Experiments were carried out in a constant volume vessel system, simulating the combustion environment of a CI engine, prior to fuel injection, with precise control to maintain a 21% oxygen concentration and an ambient pressure of 43 bar. Findings demonstrated a 21% decrease in ignition delay for B0, while B100 displayed a noteworthy 33% reduction, matching the surge injection pressure from 400 to 1200 bar. Meanwhile, the peak combustion pressure speed rose from 1.69 to 4.02 bar/ms for B0 and from 1.53 to 3.52 bar/ms for B100, followed by a large integral heat release augmentation of 793 J for B0 and 779 J for B100. At injection pressures exceeding 800 bar, palm biodiesel showcased a shorter ignition delay than diesel, whereas an inverse pattern emerged at pressures of 400 and 600 bar. Moreover, the palm biodiesel exhibited lower integral heat release and maximum combustion pressure compared to diesel at all injection pressures.