During the operation of any marine engine -- gas or diesel -- exhaust gases are produced
and must be removed from the engine and conducted outside the boat.
While there are two basic marine exhaust systems, wet and dry. The wet system is the one most
often found in pleasure boats as well as many work boats. This system
cools the exhaust system outer surfaces then mixes the exhaust
gases with engine-cooling water and discharges them through the transom of the boat
via an exhaust line.
Attached to the engine is an exhaust manifold with its exhaust inlets matching the exhaust
outlet ports of the engine. This manifold is cooled by circulating cooling water through
the water jacket surrounding the exhaust gas chamber.
At the exhaust end of the manifold is an exhaust connector. This connector is an
intermediary between the manifold and an exhaust hose or pipe. This connector also mixes
the exhaust gases with water to further cool the gases, and it operates as a means for
directing the gas and water outflow so that a connection can be made between the manifold
and the exhaust tubing.
Particular attention must be paid to the relative height difference between the center
line of the exhaust end of the manifold and the center line of the transom overboard
exhaust fitting. When the manifold is appreciably above the overboard fitting, the exhaust
connector can be an exhaust dump or drop of a convenient angle.
In those instances where the manifold is not appreciably above or is
below the overboard fitting, an exhaust riser or elbow is used. By design, the riser acts
as a dam to keep both exhaust-cooling water and sea water from back-flooding into the
engine and destroying it.
From an engineering viewpoint, the most critical design requirement for exhaust systems is
to limit the back pressure on the engine to a minimum. Back pressure is the pressure that
builds up in the exhaust manifold and engine exhaust outlets. The lower this pressure, the
more efficient the operation of the engine. Low back pressure is achieved by designing
exhaust passages to minimize resistance to the flow of gases and to provide for a uniform
gas flow away from all cylinders.
Replacement Exhaust Systems
When the need arises to replace an OEM-installed manifold and/or exhaust connector, one of
three situations will exist: replacement of both manifold and connector, replacement of
the manifold only, or replacement of the exhaust connector only.
When replacing both manifold and connector, normally there are no problems in matching,
and connection to existing exhaust lines is usually very easy. When replacing just the
manifold or just the exhaust connector, problems can arise in matching the replacement
part to the existing part.
It is usually recommended to look at repair and replacement in terms of the complete
systems and how the system is designed and balanced.
Original OEM replacement parts reduce the cost, at times, because you would be assured of
a somewhat perfect match to the original system. Several other companies produce
almost exact duplicates to the OEM part and can be considered, if you know that the
performance of the part is identical to the original, which is often hard to verify. The
alternative at times is to replace the entire system which is desirable if a newer system
is known to give better operation and performance. Be careful here to understand that all
things new aren't always better.
V6 and V8 exhaust systems typically do not have a way to control how
much cooling water is diverted to one side of the engine exhaust cooling
or the other. So if one exhaust manifold becomes clogged; all the
cooling water will flow to the unclogged side. Which means one exhaust
manifold will run very hot and the other very cold.
If the reason for replacement of a single component is corrosion damage, you can figure
that corrosion has and/or will force replacement of the rest of the components. It's
better to replace as assemblies and not to piece-mill the engine back together.
-- Author unknown but, content based on the sales pitch from some parts manual from
a few years back. I have modified it a bit to keep it general in scope.