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What I have been cautioned about is that it is easy for the barrier between the exhaust manifold and the risers to leak and contaminate the closed system coolant with salt water. My first inclination was to go with a full system but have now heard from several sources that the difference in operating temp between coolant in the closed system and the raw water cooling water as it exits via the riser system means SS block off gasket may fail and you don't realize it happens until you disassemble the system at the end of the season.
I have looked at the so called dry joint exhaust systems that Mercury developed as an alternative but could replace the whole exhaust system at least once for the increase in cost over a generic system with a block off gasket.
Any opinions on best sources for a closed system either full or partial much appreciated. BTW I am taking your advice and going with a Davis Unified Ignition system.