Helically shaped intake port of an internal combustion engine

A helically-shaped intake port of an internal combustion engine, which comprises:

• a helical portion having a outlet open end and an intake valve arranged at said outlet open end, said helical portion being defined by an upper wall and a peripheral side wall which extends circumferentially about an axis of said intake valve, and an inlet passage portion tangentially connected to said helical portion and having an inlet open end, said inlet passage portion being defined by an upper wall, a bottom wall, a first side wall arranged at a position near the axis of said intake valve, and a second side wall arranged at a position remote from the axis of said intake valve and connected to the peripheral side wall of said helical portion, wherein the first side wall of said inlet passage portion has an inclined wall portion which is arranged to be directed downwards, a projection which extends downwards and is arranged to enclose a valve stem of said intake valve being formed on the upper wall of said helical portion, said projection having a circumferential wall and a maximum thickness portion at a position near a cross point of said circumferential wall and a straight line which passes through the axis of said intake valve, and extends in parallel with a plane containing the inlet open end of said inlet passage portion therein, an upper half of the inclined wall portion of said first side wall being tangentially connected to the circumferential wall of said maximum thickness portion, wherein the width of the upper wall of said inlet passage portion is gradually reduced towards said maximum thickness portion, and wherein the width of the upper wall of said helical portion is temporarily increased towards a flow direction of a sucked air from said maximum thickness portion and, then, gradually reduced towards the flow direction of the sucked air.