Regarding the manifolds, it all depends on your engine/manifold
combination. I have twin 440 Chryslers that were originally set up
with the manifolds as part of the closed (freshwater) cooling system
and the risers as part of the raw water system. Chrysler designed
their system so that the could be set up this way or with the
manifolds as part of the raw water system.
Advantage to having the manifolds as part of the closed system: They
last nearly forever. Mine are 38 years old and as far as I know are
original ("CHRYSLER MARINE" on the original casting and the Chrysler
Marine tag under a bolt).
Disadvantage to having the manifolds as part of the closed system:
Any failure of the cooling system (plugged intake, broken raw water
pump impeller, broken raw water or fresh water pump belt, whatever)
and your engine is overheated RIGHT NOW. A minute or two inattention
to your water temp gauges and you may have a seized engine. This is
due to the amount of heat a fresh-water-cooled exhaust manifold dumps
into the coolant. There's a very very thin margin for the failure
mode. With raw water cooled manifolds you've got a couple more
minutes before temps go critical. My boat has a light-and-buzzer
overheat/low oil pressure alarm in addition to the gauges for that
reason.
Chrysler used two different plates between the manifold and riser:
one blocks water flow between the water jackets of the two, the other
permits it. The builder would use one or the other end plate on the
manifold depending on whether he wanted the manifold to be part of the
raw or fresh water systems. I expect most others are similar.
As far as having your transmission heat exchanger plumbed into the
freshwater side, bad idea. An earlier post noted that my transmission
manuals state that they're supposed to operate between 170-180 deg F.
In closed cooling systems, Chrysler specified a 165 deg. F
thermostat. What that would mean if you cooled the transmission heat
exhanger with engine coolant is the cooling medium (the engine
coolant) would only be a few degrees cooler than the operating temp of
the transmission oil.
With most systems, the raw water is pulled in and run through the
transmission cooler first. Even in the tropics, there's no place that
the ambient water temp is going to be more than 75-80 deg. F, so
there's a 100-degree difference between the cooling medium and the
transmission fluid. If you use engine coolant there's not enough
difference to pull the heat out of the transmission, and it's off to
Rebuild City.
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