In a marine conversion of a “Duramax” V8 diesel engine, each bank of cylinders has a jacketed exhaust manifold including a solid elongated casting including coolant galleries and a central exhaust duct. Recirculating coolant cools each cylinder then enters the exhaust manifold through separate apertures aligned with openings made by removal of a frost plug. Each manifold coolant aperture has a controlled diameter, ensuring most of the coolant passes along the length of the engine then along the manifold yet enough coolant cools each cylinder. The coolant then traverses and cools a manifold extension and a turbocharger.