A wastegate valve assembly (34, 80) for a turbocharger having an exhaust gas passage, a wastegate passage, and a wastegate port opening between said wastegate passage and said exhaust gas passage to divert a portion of exhaust gas flow to said wastegate passage, said wastegate valve assembly (34, 80) comprising: a valve housing (40,81) which is mountable proximate a wastegate port and comprises a cylindrical valve cup (41,82), said valve cup (41,82) including a valve member (35,83), which is spherical and movable within said valve cup (41,82) for selectively opening and closing said wastegate port, said valve member (35,83) being biased toward said wastegate port by a valve biasing member (52,90), which generates a valve closing force, and being displaceable axially away from said wastegate port by exhaust gas pressure within said exhaust gas passage which exceeds the closing force generated by said valve biasing member (52,90), said valve member (35,83) being engageable with a valve seat to selectively open and close said wastegate passage, wherein when exhaust gas pressure generates an opening force which exceeds said valve closing force, said valve member (35,83) moves axially out of said valve seat (37,92) to define a valve passage therebetween, the size of said valve passage depending upon the exhaust gas pressure wherein said biasing member (52,90) allows the size of said valve passage to increase as the exhaust gas pressure increases and decrease as the exhaust gas pressure decreases; and a spring-biased piston, which generates a piston closing force and normally biases said valve member to the closed position, wherein a terminal end of said valve cup (41,82) has an interior diameter smaller than an outer diameter of said valve member (35,83) to capture said valve member (35,83) within an interior chamber (51,93) of said valve cup (41,82), wherein the valve seat is defined by an annular valve insert and wherein the terminal end of the valve cup is aligned with the wastegate port in close, but axially spaced from the annular valve insert. a valve housing (40,81) which is mountable proximate a wastegate port and comprises a cylindrical valve cup (41,82), said valve cup (41,82) including a valve member (35,83), which is spherical and movable within said valve cup (41,82) for selectively opening and closing said wastegate port, said valve member (35,83) being biased toward said wastegate port by a valve biasing member (52,90), which generates a valve closing force, and being displaceable axially away from said wastegate port by exhaust gas pressure within said exhaust gas passage which exceeds the closing force generated by said valve biasing member (52,90), said valve member (35,83) being engageable with a valve seat to selectively open and close said wastegate passage, wherein when exhaust gas pressure generates an opening force which exceeds said valve closing force, said valve member (35,83) moves axially out of said valve seat (37,92) to define a valve passage therebetween, the size of said valve passage depending upon the exhaust gas pressure wherein said biasing member (52,90) allows the size of said valve passage to increase as the exhaust gas pressure increases and decrease as the exhaust gas pressure decreases; and a spring-biased piston, which generates a piston closing force and normally biases said valve member to the closed position, wherein a terminal end of said valve cup (41,82) has an interior diameter smaller than an outer diameter of said valve member (35,83) to capture said valve member (35,83) within an interior chamber (51,93) of said valve cup (41,82), wherein the valve seat is defined by an annular valve insert and wherein the terminal end of the valve cup is aligned with the wastegate port in close, but axially spaced from the annular valve insert.