What is the purpose of marine
manifolds and how do they work?
A marine petrol engine is basically a car
engine that has had certain components added to it to make it
suitable for work on a boat. Mostly this includes the use of a marine
gearbox, a shaft connected to a propeller and a modified cooling
system replacing the wind blown through a car's engine bay.
A manifold takes the exhaust gasses from the
engine head and leads them to the exhaust pipe. In a car they are
kept cool by air forced around them when the car is in forward
motion. Most engines in a a boat are enclosed. A car manifold is
unsuitable for most boats as it can not be sufficiently cooled. The
solution is a water jacketed manifold where cooling water is pumped
around the outside of the exhaust chambers. This water is provided by
an engine water pump and is expelled from the boat through the
A riser is used where there is a
risk of sea water being pushed up the exhaust pipe and into the
engine's head- which would be damaged. The riser also increases the height of
the exhaust above the waterline. A riser can sometimes add more
torque (pulling or pushing power) to an engine by slightly increasing
the back pressure to the engine head.
Why do manifolds fail?
Marine manifolds, especially those in continuous salt water
environments, can fail for any of the following reasons:
Cast iron, Aluminium or stainless
In Australia, it appears that cast iron
after-market manifolds are imported chiefly from the USA. With our
low A$ exchange rate this makes them quite expensive. Original
manufacturer manifolds are extremely expensive. Cast iron manifolds
and risers are also very heavy and can add over 50 kilos in weight.
Like all manifolds, those made from cast iron will corrode
over time if left permanently in a salt water environment. The cast
iron manifolds we market are very well made by a reputable manufacturer. These
manifolds will give a good service life and in most cases carry a manufacturer's
Stainless steel manifolds are a new product
in the Australian marketplace. They can be made for just about any
engine as they are generally custom built, whereas iron and Aluminium manifolds
are made from moulds. They can be expensive, but if properly made will provide a
good service life.
There are generally two types of Aluminium
manifolds available- either die-cast or sand-cast.
Sand-casting reduces gas
build up (and hence bubbles) as the casting cools. This reduces the
porosity and chemical impurity in the casting which leading to a
longer service life, especially in salt water conditions. The cost of
making sand-cast aluminium manifolds is far more expensive, but
results in a more robust product.
Aluminium manifolds are starting to be installed on
some marine engines, but generally require a pencil anode to stop
Die-cast manifolds not made of marine grade
aluminium generally do not last as long as sand-cast manifolds as the
pouring and cooling process can create air pockets. Further the
aluminium used is not suited to the marine environment.
What about cost and service
As you are undoubtedly aware, marine
manifolds have never been a cheap boat accessory. Every so often you
are faced with the problem that they need to be replaced- either
because they are getting old or they have begun to leak. A manifold
which allows salt water to enter the cylinder head and hence the
cylinder may result in hydraulic lock (engine is seized), corroded
pistons, rings, cylinder walls or valves. Irreversible damage can be
caused In just a few days by salt water leaking from the manifold or
The cost of owning manifolds can be
Stainless steel manifolds can have an
inbuilt riser incorporated into their design.
A set of very good American after market
manifolds and risers for a Mercruiser V8 engine might cost