
03-44
SECTION 03 - ENGINE PREPARATION
Maximum efficiency is generally obtained by hav-
ing 1 pipe for each cylinder. But, this maximum
point may be across a narrow and/or high RPM
range, causing difficulties in driveability.
The 2 into 1 tuned pipes used on Sea-Doo water-
craft are very accurately tuned for the overall de-
sign of these particular engines.
NOTE:
Changing the tuned pipe for an aftermar-
ket unit will usually change RPM and torque of the
engine. An impeller change to compliment your
new tuned pipe will probably be necessary.
Water Injected Exhaust System
Several factors necessitate the use of a water
cooled, water injected, tuned exhaust system.
One major concern is overall bilge temperature,
which is kept low by keeping the part of the tuned
pipe closest to the engine jacketed with water
and the remainder of the exhaust system cool by
injecting water directly into the exhaust stream.
Injecting water into the exhaust system not only
helps keep the exhaust system cool, but also
changes the characteristics of the sonic wave in
the tuned pipe. The water injected exhaust sys-
tem reduces exhaust gas temperature, therefore
increasing exhaust gas density in the tuned pipe.
The sonic wave moves slower in this cooled,
denser air so it takes longer to reflect back to the
exhaust port. The net effect is that the tuned pipe
can be shorter yet achieving the same results as
a longer “dry” tuned pipe.
By changing the amount of water injected into the
exhaust stream, the characteristics of the engine
power band can be altered. It is important that
enough water is injected into the exhaust stream
to keep the rubber hoses and muffler from dam-
age due to excessive heat.
In Limited Class racing an aftermarket exhaust
pipe and manifold are allowed. The muffler how-
ever must remain stock with no modifications.
When purchasing an aftermarket pipe consult the
manufacturer of the pipe to determine if the pipe
is race legal by your race sanctioning body (IJSBA).
Some pipes will fit your craft, but may not be race
legal because of inadequate head pipe cooling or
mounting of the pipe requires relocation of the muf-
fler. Muffler relocation is illegal by several sanction-
ing bodies for Limited Class racing.
WATER FLOW REGULATOR
VALVE
All watercraft equipped with a 787 or 947 engines
use a water flow regulator valve located on top of
the muffler to control the amount of water inject-
ed into the exhaust system at the tuned pipe head
and the muffler. The purpose of this is two fold:
1. It effectively controls the exhaust wave speed
thereby creating a more useful or broader pow-
er curve.
2. It cools the exhaust system and hoses which
prevents heat damage.
NOTE:
The water injected in the muffler is not
regulated by the valve; it is controlled by a fitting
located at the bottom of the valve, which secure
also the valve to the muffler.
The water regulator is calibrated for use on water-
craft which have no engine modifications. The
regulators works quite well on the stock water-
craft and provides a noticeable increase in perfor-
mance over models not so equipped. If the
watercraft is not being raced but modifications
are made to the engine such as increased com-
pression, changes in port timing, etc. the water
regulator valve will not deliver the correct amount
of water for optimum performance. Recalibrating
the regulator water flow will require most users to
experiment with different size holes being drilled
into the regulator then threading and fitting the
holes with jets. Various size jets will then be tried
until the best performance is found.
The water regulator valve has been disallowed for
competition usage by the IJSBA. Furthermore any
valves used within the entire engine cooling sys-
tem must be of the fixed type. This rule was
placed into affect during the mid 1995 racing sea-
son. Due to this ruling the water regulator must
be removed and the exhaust system must be
“Hard Jetted” (fitted with jets of the fixed type).
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