A gas turbine engine (20; 150) comprising; a plurality of rotating components housed within a compressor section and a turbine section ; a tap connected to said compressor section , a heat exchanger (112; 164; 292; 312) connected downstream of said tap , and a cooling compressor (114; 162) connected downstream of said heat exchanger (112; 164; 292; 312), and said cooling compressor (114; 162) connected to deliver air to at least one of said rotating components; and a core housing having an outer peripheral surface and a fan housing defining an inner peripheral surface , at least one bifurcation duct (158, 160; 406, 408) extending between said outer peripheral surface to said inner peripheral surface , and said heat exchanger (112; 164; 292; 312) disposed within said at least one bifurcation duct (158, 160; 406, 408), wherein air is delivered into a heat exchanger first leg within said core housing , the first leg extending outwardly of said core housing and into said at least one bifurcation duct (158, 160; 406, 408), and to a second leg extending back toward a center axis (A) of the engine (20; 150), and back into said core housing to deliver the air to said cooling compressor (114; 162), and wherein there is an air inlet (176; 296; 314) for air to pass over said heat exchanger (112; 164; 292; 312) in said at least one bifurcation duct (158, 160; 406, 408) and an air outlet (180; 298; 320), characterized in that : there is a valve (300; 322) in at least one of said air inlet (176; 296; 314) and air outlet (180; 298; 320), and there are a plurality of first legs and a plurality of second legs , wherein said plurality of first legs and said plurality of second legs communicate with a common header to turn air from the first legs to the second legs. a plurality of rotating components housed within a compressor section and a turbine section ; a tap connected to said compressor section , a heat exchanger (112; 164; 292; 312) connected downstream of said tap , and a cooling compressor (114; 162) connected downstream of said heat exchanger (112; 164; 292; 312), and said cooling compressor (114; 162) connected to deliver air to at least one of said rotating components; and a core housing having an outer peripheral surface and a fan housing defining an inner peripheral surface , at least one bifurcation duct (158, 160; 406, 408) extending between said outer peripheral surface to said inner peripheral surface , and said heat exchanger (112; 164; 292; 312) disposed within said at least one bifurcation duct (158, 160; 406, 408), wherein air is delivered into a heat exchanger first leg within said core housing , the first leg extending outwardly of said core housing and into said at least one bifurcation duct (158, 160; 406, 408), and to a second leg extending back toward a center axis (A) of the engine (20; 150), and back into said core housing to deliver the air to said cooling compressor (114; 162), and wherein there is an air inlet (176; 296; 314) for air to pass over said heat exchanger (112; 164; 292; 312) in said at least one bifurcation duct (158, 160; 406, 408) and an air outlet (180; 298; 320), there is a valve (300; 322) in at least one of said air inlet (176; 296; 314) and air outlet (180; 298; 320), and there are a plurality of first legs and a plurality of second legs , wherein said plurality of first legs and said plurality of second legs communicate with a common header to turn air from the first legs to the second legs. <EOS>