It will effect the gearcase as the the water which use to flow through the midsection and through the gearcase now exits earlier through the poppet diverter (on a Mercury 3 liter, not sure on a 2.5). On very high engine heights, it will increase running temperature of the gearcase. On an optimax, this is even worse when considering how much hotter the optimax exhaust is compared to an efi.
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