Combustion behaviour in gasoline direct injection engines is strongly influenced by the interaction between start of injection timing and injector spray guidance, because these parameters jointly control spray–air interaction, mixture stratification, ignition stability, combustion phasing, and pollutant formation. This study applies three-dimensional moving-mesh computational fluid dynamics to evaluate the coupled effects of injection timing and injector guidance in a single-cylinder, four-stroke, gasoline direct injection engine with a 86 mm bore, 86 mm stroke, a compression ratio of 9.2, and an engine speed of 1000 rpm. Three injection timings were examined relative to intake-valve operation: after intake valve opening, at maximum intake valve lift, and after intake valve closing. For each timing, air-guided, wall-guided, and spray-guided injector orientations were assessed under identical operating conditions. The spray process was represented using detailed breakup, evaporation, and wall-interaction sub-models at an injection pressure of 50 bar, with 4.398 × 10⁻⁵ kg of fuel injected per cycle and 300,000 computational parcels. Combustion was simulated using the SAGE detailed-chemistry solver, while nitrogen oxide formation was represented using the extended Zeldovich mechanism. The results show that injection at maximum intake valve lift provides the most favourable interaction between spray momentum and intake-driven turbulence, improving mixture preparation and producing compact heat release. Among the investigated cases, wall-guided injection at maximum intake valve lift delivered the strongest pressure and temperature development, the sharpest heat-release profile, the lowest carbon monoxide and unburned hydrocarbon levels, and the highest carbon dioxide formation, indicating more complete oxidation. Late injection after intake valve closing produced poor mixture preparation, weak heat release, and increased incomplete-combustion products, particularly for the spray-guided case. The study demonstrates that injection timing and injector guidance must be optimised jointly rather than independently to achieve precision combustion with improved efficiency and controlled emissions in modern gasoline direct injection engines.