Direct port injection nozzle location

[rotrex]

mounting the nozzles underneath leads to the spray mainly hitting the front of the inlet valves and with it uneven distribution within the cylinder.
The majority of the fluids incl. fuel enters the cylinder over the centre facing side of the intake valves and then flows in a tumbling motion filling the cylinder.
if you spray from the bottom, the majority of the water will more be found on one side of the cylinder. It needs to be evenly distributed like and together with the fuel, hence the top location.

This is same reason the fuel injectors sit on the top and not the bottom.

The spray needs to roughly follow the blue line. Air speed at full chat is so high, the spray from the bottom won’t make it to the top of the runner. The spray will hit the valve at a steep angle, partly scatter, partly evaporate and cooling metal like valve and cylinder wall, but not the gas.