the heat exchanger 10 of the present invention is utilized to remove heat from the engine coolant in an internal combustion engine the cooling air flowing across the exterior of the heat exchanger will be caused to flow in the long direction of the rectangular shape and perpendicular to the convex protrusions 33 this assures an optimum flow of air over the greatest heat exchanging surface and the convex protrusions 33 are disposed to maximize air turbulence the walls 21 of each corrugation 20 may also be provided with a plurality of concave protrusions 34 which extend into the interior of the corrugation the concave protrusions may be adapted to serve two separate and distinct purposes first of all the concave protrusions 34 enhance the heat exchanging surface area and provide interruptions which help create turbulence in the flow of fluid within the heat exchanger in addition concave protrusions extending inwardly from opposite wall sections 21 may be utilized to capture and hold in place the turbulator plate 27 as shown in fig2 the concave protrusions 34 may be positioned to bear upon the crests of the ribs 31 as a pair of wall sections 21 are brought together and sealed along the continuous outer seam 24 some separation must be maintained between the crests of the ribs and the inner surfaces of the wall sections 21 otherwise the flow of fluid therebetween would be restricted the concave protrusions 34 thus also provide the requisite spacing these inwardly extending protrusions.