Semi peripheral fills the gap between full bridge port and full peripheral port. It will be a little more fussy on the street at low rpm compared to a bridge port, but has high hp potential. Works well with naturally aspirated and also turbo charged engines. Semi peripheral is used on the highest horsepower turbo rotaries in the world of drag racing.  This is a brand new OEM mazda housing machined and finished.  Epoxy will be required to seal the coolant jacket, or select our CNC machined dual o-ring plug available separately for a perfect leak proof seal.