Exhaust pressure modulation valve

An exhaust pressure modulation valve comprising a body having an inlet, an outlet, and a passageway between said inlet and outlet, a butterfly pivotable about a spindle pivot axis in the body to close communication between said inlet and outlet, said butterfly having a spindle and said spindle pivot axis being offset from an axis of symmetry of said passageway such that a resultant torque generated in response to increasing pressure at said inlet tends to open the butterfly, said valve further comprising operating means for closing said butterfly and adapted to apply to said spindle pivot axis a closing torque of the same magnitude as said resultant torque generated at a pre-determined pressure at said inlet, and a bleed means from one side of said butterfly to the other, said bleed means being sized to impose a significant back pressure when said butterfly is in use closed and the flow of gas through said valve is significantly less than the maximum flow of gas though said valve, wherein said bleed means comprises an aperture in the butterfly.