"STi semi-closed deck engine block design.
Cylinder liner support at 4 locations ensures rigidity under severe stress.
Normal open deck designs can allow distortion or cylinder shifting when tuned for extremely high outputs.
This can affect head gasket sealing or lead to disasterous failures."
Notice this stock "open deck" SOHC 2.5L block as viewed from the side..
Notice how the cylinder tubes are only connected to the block (at the "outside" / non-crank ends at least ) at the front and back and to each other. The semi-closed deck adds supports at the top and bottom of each cylinder.
We wouldn't want those pretty cylinders "shifting around" now would we? :-?