A LESSON ON CAST IRON MANIFOLD
AND RISERS
· Most Marine engines use a water- cooled exhaust manifold.
Each manifold will use a
riser or elbow to connect to the exhaust discharge. Yet very few people are
aware of the life expectancy and symptoms of manifold failure. We hope this
guide will briefly introduce you to exhaust manifolds and risers, failure
symptoms.
· What are Exhaust Manifolds and Risers·
Marine exhaust manifolds and
risers are normally made of cast iron. Depending on the marine engine, there
can be one or two sets of manifolds and risers. The manifold and risers are
usually painted to help prevent rust and are located on the side of the engine.
V-8 engines have two sets of manifolds and risers which are located alongside
each cylinder bank. Smaller 3 and 4 cylinder marine engines normally have one
manifold and riser locate near the top and along one-side. Risers, sometime
called elbows (depending on the location of the engine in concern to the
waterline), which look like an inverted horseshoe, are sometimes located at the
aft or on top near the middle of the manifold. The riser is then connected to
the exhaust hose.In order to combat the
heat of engine exhaust, manifolds and risers have two compartments, one inside
the other. The inner most compartment is for the engine exhaust, which is
surrounded by a water-filled compartment, called the water-jacket. The
water-jacket prevents the manifold and risers from over heating and cool to the
touch. The two compartments combine prior to the exhaust reaching the exhaust
hose to ensure that the engine exhaust is cool enough to not burn through the
exhaust hose. Although it is important to keep the engine exhaust cool it is
even more important to keep the water and exhaust gas separate until they exit
the riser. If the water-jacket starts to leak and water gets into the exhaust
compartment when the engine is not running, the water can end up in the
cylinders and seize the pistons with rust or create hydro-lock. Hydro-lock
occurs when pistons try to compress the water in the cylinder. Since water
cannot be compressed, the engine can suffer extreme damage. This usually
results in bent engine rods or extreme damage requiring an engine overhaul.
. Why Manifolds and Risers Fail·
The main cause of manifold
and riser failure is time. A common held belief in the marine repair industry
is that most manifolds have a useful life of 8 to 10 years. However this is
highly dependent on engine use. Manifolds and risers that see heavy use in
saltwater can have life expectancy as low as 3 years, while manifolds and riser
in engines rarely run and used in freshwater can last 15 to 20 years. No matter
how you use your engine, the older manifolds and riser get, the chance of
failure increases.· The environment
that manifolds and risers live in helps to limit the useful life of manifolds.
Internally, manifolds and riser are continually slammed with high velocity hot
exhaust and water. Externally, manifolds and riser live in an environment that
has perfect conditions for corrosion. Splash water, heated by the engine
sporadically ends up on the manifolds and the humidity in the engine
compartment, all take part in corrosion. The fact that manifolds and risers
violently vibrate during use and then sit for long periods of time allows water
and humidity to eat away at the manifolds and risers.
· Failure Symptoms·
One of the most common
symptoms that your manifolds have failed is difficulty in starting or your
engine will not start at all. One of two things may have happened: · You have a hydro-lock situation in your
cylinders (water does not compress). · Or
one or more of your pistons have rusted inside your cylinder(s). · These are definite signs that your manifolds
have leaked and water has entered into your cylinders. If your engine does
start but produces white smoke or runs roughly, you may have water in a
cylinder, which can foul the spark plugs.
· Another sign that your manifolds may be failing is your engine
overheating at high R.P.M.s.
This usually means that you
have a blocked water-jacket that is not cooling the engine exhaust and thus
causing the engine to overheat at high R.PM.s. Sometimes the blockage is not
reflected by the temp gauge and will simply cause one cylinder to run hotter
than the others. When you have a subsequent engine failure, the culprit may be
difficult to identify. · Manifold
Inspection· Probably the easiest way to
check if your manifolds are working correctly is to look at your exhaust and
see if your water output is lower than normal, if it is then you may have a
blocked water-jacket. A blocked water-jacket can lead to a water leak in the
manifolds. If you have two manifolds, you can compare the temperatures from
each manifold. You can use your hand, but it is better to use an infrared
pyrometer. If one of the manifolds is hotter than the other, you probably have
a clogged water-jacket. When inspecting your manifolds it is a must that you
visually inspect the manifold riser joint for corrosion. Corrosion at the
outside of the manifold-riser joint means that the manifold-riser gasket has
failed and water is at the very least seeping out. If the gasket has failed
there is a high chance that water may also be seeping along the inside of the
joint and possibly into the exhaust chamber. When visually inspecting the
risers you should also check the hose-riser connection. On some boats the
exhaust hose is connected to the riser with a stainless steel hose clamp that
does not have a stainless steel screw. Thus the screw can rust, causing the
clamp to fail and you can end up with your exhaust emptying into your engine
room. Note: Clamps with stainless screws usually have "All Stainless"
stamped on the screw housing of the clamp.· These
inspections can help flag a problem with your manifolds. You should
periodically remove the risers and inspect them, along with the manifolds. How
often you check your risers depends on where you live and how you use your
boat. A good rule of thumb is to check them every year before the start of the
boating season. When you remove the riser you need to look for corroded metal,
done best with a large flat screwdriver, scraping the surfaces of the water
jacket. You should also look for signs that water is entering the exhaust
chamber, signified by rust in the chamber. Look for pitting and or rust through
out the riser and on the coupling faces, any of which signifies a leak that can
lead to hydro-lock. It is common for your marine mechanic to suggest cleaning
the manifolds and pressure testing at inspection. This process of scouring
loose rusted metal from the manifolds can cause a number of new problems. (a)
the process may actually move particles within the manifold water jacket and
cause a blockage which will not be evident until you run your engine again, (b)
after cleaning, the thickness of the water-jacket is reduced and fails soon
after manifolds are replaced, or (c) the reduced thickness of the water-jacket
passes the pressure testing with cold water but fails in use due to high
temperature and porosity in the cast iron being exposed due to the cleaning
process.
· Our Stainless Steel - one piece manifolds were developed to
overcome much of the maintenance involved with marine exhaust systems.
We have most models available to replace
factory manifolds and risers both petrol and diesel engines – including taller
than standard risers. We can also produce custom fitments and for engines not
normally found in marine applications. By replacing your cast iron originals
with our manifolds you can expect increased horsepower and torque,
significantly reduced fuel consumption, massive weight savings, and the best
warranty in the market – 5 years.
Warning:
If the boat is in the water, whenever you disconnect your
exhaust hose from the risers, you must plug your exhaust hose or tie them
securely above the waterline to prevent flooding the engine compartment. If the
boat is out of the water, drain the manifolds before removing risers to prevent
water leaking into the exhaust ports.