With jet boats, there exists a
number of problems with the factory cooling system
design that have come to light over the years.
First, most jet boat engines run too cool for optimum
performance. Secondly, some set ups are prone to
excessive water pressure problems in the engine block as
high pressure is taken from the jet drive and routed
into the engine cooling system. Third, jet boats
with water injected headers tend to have the problem of
using too much water which ends up contaminating the oil
in the crank case. To address these problems you
need to understand how most jet boats were plumbed from
the factory and how these factory set ups were supposed
aluminum bypass thermostat will not fit on the Chevrolet
Offenhouser Tunnel Ram.
PERFORMANCE JET used the thermostat pressure relief
system in this case.
I will start with the standard jet
boat set up with a through transom exhaust system
utilizing log exhaust manifolds and risers. In
these systems water is pumped from the jet drive through
a gate valve to a tee fitting under the front of the
engine. The water is split and directed to the
front of the engine where the water is pumped into the
exhaust logs on each side. Once in the exhaust
logs, water is preheated and routed to the front of the
block where the water pump would normally be located.
Water runs through the block, on each side, up into the
heads and exits out the engine through two fittings on
the thermostat housing. From here, the water is
carried to the risers and dumped overboard out the
exhaust. This set up works pretty well except for
the fact that the cold water runs through the engine so
quickly that the engine temperature usually runs too
cold. In a performance engine, I like to see a
temperature range of 140 to 180 degrees. You may
disagree but I feel that all jet boat engines are
To deal with this cold water
temperature issue, the factory folks installed a gate
valve close to where the water exits the jet (for
cooling). The idea was to gate down the water flow
to the engine until a desirable temperature was reached
and leave the valve set at that position.
The problem with this set up is
that by closing the coolant valve, you reduce the amount
of water flow to the engine and if you go a little bit
too far, you end up with a condition where the engine
block is running at a good temperature but the cylinder
heads run too hot due to the lack of enough water volume.
A lean condition can result in your engine, causing
blown head gaskets, cracked heads, tuliped valves, or
worse, a blown engine. This problem is especially
acute with the 455 Oldsmobile engine. The improper
adjustment may also cause extreme swings in engine water
temperature from idle (very hot) to wide open throttle
(very cold), which is not a good thing. The
solution to this system is to use a bypass thermostat
kit on the engine to regulate engine temperature.
Leave the water valve full open or remove it altogether.
water feed and control valve
water feed & control valve
with by-pass thermostat kit
There are two thermostat kits on
the market and PERFORMANCE JET sells both of
these items. The cheaper of the two is made of
aluminum and will fit the big Chevy and Olds engines, on
most manifolds. A more expensive unit is
made by Hardin Marine and is made of stainless steel.
This unit is supposed to fit the Chevy, Olds, and Ford
big block. It works well on the Chevy and Olds but
does not fit well on most Ford applications without
cutting or modifying the base of the unit. There
is a better way to go on the Ford engine application, as
I will explain later.
Once the engine temperature issue
has been addressed, the next step is to deal with the
possibility of high coolant pressures in the block.
If you are running a blue printed jet drive in the
higher RPM ranges at higher speeds, with a lot of power,
then you are likely to have excessive pressure in the
block. The average jet drive can build pressures
in excess of 150psi and the engine cooling system is
designed for no more than a maximum of 22-25psi.
Excessive water jacket pressures can cause head gasket
and intake manifold gasket problems and water in the
oil. Once again, Oldsmobiles are especially
vulnerable to this problem. Mondello likes to see
12-14psi in the cooling system of the Olds engine.
Over the years there has been a lot of heated debate on
how to deal with this issue of high coolant pressure,
but to me, the solution is really very simple. You
must dump at least as much water out of the engine as
you put into it. If you still have too much
pressure then install a pressure relief system that
reduces the pressure before water enters the engine.
In the first system example I just discussed, there are
two water inlets and two water outlets. Excessive
water pressure is usually not an issue with these stock
set ups. Water temperature and volume are usually
the two primary problems that I see with the stock
exhaust log/riser combo.
In most cases I see, the issue of
high water pressure in the engine block really becomes a
problem when the jet boat is set up with water injected
headers. On these systems the water comes from the
jet to the front of the motor, splits at the tee and
enters the block where the water pump would normally be.
So far so good. The problem arises with the
factory design to get the water back out of the engine.
In most factory set ups, one water
line exits the thermostat housing and feeds the header
tee valve. The other water line on the thermostat
housing routs to the transom of the boat, through a gate
valve, then dumps overboard through a transom dump
fitting. The gate valve is supposed to help adjust
water pressure to the headers. Closing the valve
forces more water to the header tee because it can't
dump overboard. Opening the dump valve reduces the
flow of water to the headers by dumping more water
overboard. This set up does not work well in
reality. The problem is that this set up puts far
too much water to the header tee valve and excessive
water in the headers puts water in the oil.
Secondly, there is only one full size line that is
dumping water from the engine and it is not enough to
keep the engine block water pressure from climbing.
Problems usually start when there is too much water
reaching the headers. To control excessive header
water the jet boater will install a header water control
valve somewhere in front of the header tee. When
the water is reduced to the headers so they don't milk
up the oil, you are cutting down even further the amount
of water that can get out of the engine and the engine
block pressure goes up even more as a result.
Throw in the fact that half of the jet boaters out there
don't have a clue as to how the system is supposed to
work and you have a recipe for real problems. I've
seen a lot of customers turn valves, just to turn valves
and they really don't know why. Turns out that a
lot of these people are just guessing.
one for bilge pump, one for pressure relief system, 2
The solution to high water
pressure in the engine block is still simple, but you
have to change some of the factory plumbing.
First, you take the water line that feeds the headers
off of the thermostat housing and put it aside for a
minute. You add a new dump line (5/8"ID) hose
and put it on the fitting on the thermostat housing that
fed the headers. Run this line back to the transom
and connect it to a new dump fitting. So, you
should now have two dumps coming off the
thermostat housing and going overboard at the transom.
On the original dump line that had the gate valve, get
rid of the valve and put it aside. Use 5/8"ID
hose. Note* Always use 5/8"ID hose or -10
hose when you plumb the water system on your engine,
don't go smaller. I also like to use 3/4"ID
hose or -12 hose from the jet drive to the water tee at
the front of the engine, especially if you run an oil
cooler. The header feed line can be smaller,
1/2"ID or -08 is good. Now we have two
inlets and two dumps on the system. The engine
block pressures should be greatly reduced.
You now need to find a water
supply source for the headers. No problem.
On your intake manifold water crossover (under the
thermostat housing) you should have a pipe plug, or two,
that block off water in the manifold water jacket.
Take one of these plugs out and install a 90 degree hose
fitting and connect your header line to this fitting.
Take that gate valve that you put aside from the
original dump line and install it between the 90 degree
fitting and the header tee. Now you have water
control to your header tee without restricting the
amount of water exiting your engine. Really, you
can take water from any other source to feed your
headers. Legend and Aggressor jet
drives have two water fittings on their jets and you can
even use one of these to feed your headers. If you
are using the electronic water control valve, you will
need a cold water source for the headers. You
could even tee off of the water supply line somewhere
before the engine inlet to feed your headers.
Note* Headers do not need a lot of
water. Just crack the valve open very slightly so
that you have a fine mist or steam exiting the headers
off of idle. Header water should shut off
completely below 1500RPM if the header control tee is
working properly. With very large cams the water
should shut down to the headers at 2000RPM. The
factory header tee valve just does not do the job in
this case and the electric water control system is best.
PERFORMANCE JET Sells the Banderlog header
control valve for this application.
PRESSURE RELIEF SETUP
Add a bypass thermostat to this
set up and you have solved the problem of engine
temperature, coolant pressure and proper header
function. In some high output applications you may
still have too much coolant pressure in the block.
If this is your situation, then install a pressure
relief valve system on your jet drive water feed in
addition to what I have just discussed in the set up
above. PERFORMANCE JET sells our own
pressure relief kit for $110. I set them to open
at 12PSI so that extra pressure is dumped overboard before
it reaches the engine. (and you still maintain
full volume.) They work great.
Now, for you Ford people that
can't use the bypass thermostat kit because it won't fit
your manifold, this is what I do, and it works just
fine. I take off the stock thermostat housing and
mill out the divider on the inside just enough to
install a 160 degree marine thermostat. I drill
two 3/16" holes on each side of the thermostat body
so that water will still exit the engine when closed.
Re-install the thermostat and install a pressure relief
kit in front of the engine (off of the jet). Set
up your water lines as previously discussed. NOTE*
You must use the pressure relief kit or you will have
too much water pressure in the engine when the
thermostat is closed. In my opinion, these are the
basic solutions for most engine cooling problems for the
average jet boat. PERFORMANCE JET can set
you up with everything that you need, or you do it
yourself, but remember, don't skimp. Buy good
parts and do the job right the first time. Remember
your jet boat is supposed to be a pleasure boat.
So keep it that way.