The effects of the high pressure die casting (HPDC) processes on porosity, microstructure, and mechanical properties of heat-treatment-free aluminum silicon (Al-Si) alloys have long been a focal point in automotive die-casting research. In this work, the combined effect of shot sleeve materials and slow shot speeds on porosity, microstructure and mechanical properties of a newly designed HPDC Al-Si alloy was investigated. Results show that employing a ceramic shot sleeve or increasing the slow shot speed significantly reduces both the average size and area fraction of externally solidified crystals (ESCs), as well as the average pore size and volume fraction. When the slow shot speed is increased from 0.05 m·s^-1 to 0.1 m·s^-1, the pore volume fraction decreases by 10.2% in steel-shot-sleeve samples, compared to a substantial 67.1% reduction in ceramic-shot-sleeve samples. At a slow shot speed of 0.1 m·s^-1, castings produced with a ceramic shot sleeve exhibit superior mechanical properties: 8.3% higher yield strength, 17.4% greater tensile strength, and an 81.4% improvement in elongation, relative to those from a steel shot sleeve. These findings provide valuable insights for minimizing porosity and coarse ESCs in die castings, offering promising potential for broader industrial applications.