A butterfly bypass valve, comprising: a housing defining a bypass flow passage therethrough; a throttle plate disposed in the bypass flow passage, the throttle plate being pivotable about a pivot axis oriented transverse to a flow direction through the bypass flow passage, an outer peripheral edge of the throttle plate being in substantially sealing engagement with a sealing portion of an inner surface of the housing when the throttle plate is in a closed position such that the throttle plate substantially restricts fluid flow through the bypass flow passage, the throttle plate being pivotable to an open position in which portions of the edge of the throttle plate are spaced from the inner surface to allow fluid flow through the bypass flow passage; and a port defined through the housing for allowing a portion of fluid passing through the bypass flow passage to be removed through the port, the edge of the throttle plate restricting fluid flow into the port when the throttle plate is in the closed position, the port being uncovered to allow fluid flow into the port when the throttle plate is pivoted to the open position; wherein said sealing portion of the inner surface of the housing in substantially sealing engagement with the edge of the throttle plate is configured to allow a predetermined amount of pivoting of the throttle plate toward the open position while maintaining the edge of the throttle plate in substantially sealing engagement with the sealing portion so as to restrict fluid flow through the bypass flow passage; and wherein the port is located with respect to the sealing portion such that as the throttle plate is pivoted from the closed position toward the open position, the throttle plate begins to progressively uncover a first part of the port to allow fluid flow therethrough while the edge of the throttle plate is still in substantially sealing engagement with the sealing portion restricting fluid flow through the bypass flow passage characterized in that: the port is located with respect to the sealing portion such that the throttle plate begins to allow flow through the bypass flow passage before the port is fully uncovered by the throttle plate. a housing defining a bypass flow passage therethrough; a throttle plate disposed in the bypass flow passage, the throttle plate being pivotable about a pivot axis oriented transverse to a flow direction through the bypass flow passage, an outer peripheral edge of the throttle plate being in substantially sealing engagement with a sealing portion of an inner surface of the housing when the throttle plate is in a closed position such that the throttle plate substantially restricts fluid flow through the bypass flow passage, the throttle plate being pivotable to an open position in which portions of the edge of the throttle plate are spaced from the inner surface to allow fluid flow through the bypass flow passage; and a port defined through the housing for allowing a portion of fluid passing through the bypass flow passage to be removed through the port, the edge of the throttle plate restricting fluid flow into the port when the throttle plate is in the closed position, the port being uncovered to allow fluid flow into the port when the throttle plate is pivoted to the open position; wherein said sealing portion of the inner surface of the housing in substantially sealing engagement with the edge of the throttle plate is configured to allow a predetermined amount of pivoting of the throttle plate toward the open position while maintaining the edge of the throttle plate in substantially sealing engagement with the sealing portion so as to restrict fluid flow through the bypass flow passage; and wherein the port is located with respect to the sealing portion such that as the throttle plate is pivoted from the closed position toward the open position, the throttle plate begins to progressively uncover a first part of the port to allow fluid flow therethrough while the edge of the throttle plate is still in substantially sealing engagement with the sealing portion restricting fluid flow through the bypass flow passage the port is located with respect to the sealing portion such that the throttle plate begins to allow flow through the bypass flow passage before the port is fully uncovered by the throttle plate.