A heat exchanger includes first fluid passages that each have a first inlet that communicates into a first core passage and then a first outlet. The first inlet has a first inlet cross-sectional perimeter. The first core passage has a first core cross-sectional perimeter. Second fluid passages are interleaved with the first fluid passages. Each of the second passages have a second inlet that communicates into a second core passage and then a second outlet. The second inlet has a second inlet cross-sectional perimeter. The second core passage has a second core cross-sectional perimeter. The first and second core cross-sectional perimeters are larger than their respective first and second inlet cross-sectional perimeters. The first and second core passages are undivided from their respective first and second inlets to their respective first and second outlets.