Boat Power Tuning requires two very essential tools a good tachometer to read
engine speed and a manifold vacuum gauge which will be an indicator of engine load. The
vacuum gauge should read from O inches of Mercury to about 30 inches of Mercury. Propeller
selection is varied, but both pitch and diameter changes can be made to improve overall
speed and performance.
HOW ARE PROPELLERS SELECTED?
The boat manufacturer has a wide selection of propellers to choose from and does so
to optimize Fewer (ski boats) or speed (cruisers). The first basic to remember is that
propellers are listed by their diameter and pitch, in that order. So, a 16 X 18 propeller
is 16 inches in diameter and has a pitch of 18 inches which means the distance the
propeller would travel through a solid in one revolution. However, water is not a solid
and propeller slip which is roughly 30% will effectively lower the pitch. Propellers are
also right hand (clockwise rotation) or left hand (counter-clockwise rotation). More will
be said about propellers in the body of this presentation; but for now, your propeller
effectively transfers engine power to the water to move the boat.
I. SETTING BEST IDLE
Setting the best idle on your marine engine is a very essential step in power
tuning your boat. Proper idle speed is very important for transmission/outdrive life, as
well as dependable engine operation for docking maneuvers Check your manufacturer's
specification for idle speed recommendations and do not exceed these speeds. Engines
which are new have greater frictional losses. and, therefore, require higher
"green" engine speeds. These engines will require resetting of the idle speed as
the engine is broken in. Your boat will handle best with the slower of these two idle
Engine speed at idle depends on two factors: idle air adjustment (idle speed) and
idle fuel adjustment, with a good engine speed tachometer, an engine vacuum gauge, and
with the engine warmed up to proper operating temperature. Here is the procedure to
A. First, set engine speed (RPM on tach) to manufacturer's
Example: 600 RPM.
B. Then observe vacuum gauge with engine idling in neutral as
idle fuel is set richer and /or leaner. To assure an initial idle fuel balance, with
engine off, carefully turn each idle adjusting fuel screw in until it seats in
the carburetor casting before setting the speed of Part A. Do not use a
large screwdriver or excessive force since the seat may be damaged if the
screw is turned in too hard. After establishing that each needle is seated, back
out (counter clockwise) the screws one full turn each. (This will be the
initial set point at which the speed is set with the idle speed screw.)
C. Balance each idle fuel screw until the highest engine vacuum
is realized. It is highly likely that when this point is reached, the engine speed
will be higher than that which was set in Step A, The speed screw should be
reset until it reads true on the tach. and the idle fuel setting should again be
repeated for best idle mixture at the new speed setting.
This "best idle speed" will give you control during docking
maneuvers without adding undue stress to your transmission or outdrive. Depending on the
propeller you have selected, you may find, especially on ski boats with high pitch props,
that the boat will perform better while docking with an even slower than recommended idle
speed. In any case, select a best idle speed for your hull and prop, and set the best fuel
idle mixture to the procedure outlined above.
II. TUNING FOR BEST ACCELERATION
How your boat accelerates from idle (no wake speed) to cruise depends on a
great variety of factors. some of which are:
-boat weight and loading
-wetted hull condition (marine growth)
Assuming that the above list has been carefully addressed, the carburetor can be
custom tuned to deliver best performance during accelerations by:
A. Accelerator Pump Calibration
The function of the accelerator pump is to deliver fuel to complement the idle and main
system operation during transition operation. The start of fuel delivery from the pump is
as critical as the amount of fuel delivery from the pump. With the engine off, have
someone operate the helm throttle, while observing the pump discharge nozzles in the
primary venturi. The slightest movement on the throttle linkage should result in a
delivery of fuel from the discharge nozzle (pump "tip in"). If there is a lag,
accelerations will be compromised. By adjusting the pump override screw, the pump can be
set to deliver fuel as the throttle is opened. One caution here is to check the travel
left at the pump lever at wide open throttle, where there should still be some pump
diaphragm travel (approximately .020).
After adjusting the pump "tip in", take your boat out to an area where
accelerations can be conducted in a safe manner, and accelerator pump capacity testing can
be done. Pump Cam Kit Number 20-12 provides a wide range of accelerator pump cams which
have cam ramp angles to vary fuel delivery versus throttle angle. Using the chart provided
with the Kit, note which pump cam comes standard with your carburetor. With the help of a
stop watch and a "third eye" on the tach, run the boat from idle to 2,000 RPM
and check the elapsed time. Try cams with capacities above and below the standard cam and
record the resulting elapsed times. The best idle to 2,000 RPM speed times will indicate
the best cam for your vessel.
If, while running the above test, any bogs or hesitations occur, go no lower
(or higher) on the pump cam selection. Your boat has very simply told you that it needs
more (or less) fuel than last cam selected.
B. Correct Secondary Operation
Once you have tailored your accelerator pump to your vessel, the higher speed
accelerations are most affected by secondary throttle opening. A secondary diaphragm
"motor'' opens the secondary throttle according to a calibrated diaphragm spring The
vacuum opens the valve, and the spring closes it.
A variety of these springs are available in Kit Number 20-13. The explanations
contained in the spring kit rate the springs by "relative load". Simply put, the
lighter the load, the earlier the secondary throttle opens. If your boat runs at 4,400 RPM
at wide open throttle, a good rule of thumb would be to have your secondaries calibrated
to be wide open at about 3,800 RPM engine speed. Of course, the secondary throttle will
begin to open much sooner, about 2,900 RPM, and this opening will improve vessel
acceleration and will be audible with the engine hatch open. If the secondaries open too
soon (lower RPM), a bog or hesitation will result. If they open too late (higher RPM),
fuel distribution to the engine suffers. It has been our development experience that
springs heavier in load than the plain spring are not required in marine engines.
With stop watch in hand, and a safe course for acceleration, time your speeds from
2,000 RPM to wide open throttle for best secondary diaphragm spring selection.
Holley has developed a very handy Kit Number 20-59 which facilitates the removal of
the cover and the replacement of the diaphragm spring, without removing the choke
assembly. So, should conditions exist where continued change of that spring is indicated,
try one of these kits for a real time saver.
C. Main Metering Jet Selection
Your Holley carburetor has been developed to supply fuel for correct engine operation, and
should not require main metering jet changes for most acceleration calibrations. If,
however, operating conditions or engine displacement to hull length combinations require
fuel delivery changes, the changes should not result in main jet changes of more than four
sizes up or down from the standard main let supplied.
II Power Valve Timing and Power Results
By definition. a power valve is a valve which opens at a given manifold
vacuum and delivers fuel to match power requirements at or near maximum engine speeds.
This valve is closed at high manifold vacuum and opens at a set lower vacuum. Fuel
consumption when the valve is opened is from 25 to 40% higher than that at part throttle
(valve closed). It is very important that at all cruise speeds, the power valve be closed
for maximum fuel economy. Using the vacuum gauge from the idle set section, record the
manifold vacuums and engine speeds at cruise speeds. A typical small block Chevrolet might
cruise at 2,800 RPM at an 8" manifold vacuum. In this case. the power valve should be
selected below on 8" vacuum range. Example: 5.5" Hg opening.
During accelerations, the power valve should open according to the load of the engine. A
light throttle operation will be clean and economical without the power valve opening. The
acceleration would be made from 17" hg to 10" hg manifold vacuums. Deeper
throttle accelerations requiring more power from the engine will be made below 8"
manifold vacuum and, consequently, require the power valve to be open. Holley's single
stage power valves are available in l" hg vacuum increments from 2.5" hg to
10.5" hg ranges. Again, a good rule of thumb is to select a power valve number
(opening vacuum) which is at least 2" hg lower than your vessel's cruising manifold
III. TUNING FOR BEST CRUISE PERFORMANCE
(SECONDARY THROTTLES CLOSED)
A boat engine is much different than the engine in the family car. A vehicle
rolling down the highway at 55 miles per hour needs perhaps 15 to 18 horsepower to
maintain vehicle speed. A boat, on plane, cruising at 30 miles requires 10 times that
horsepower. The marine engine works much harder than the family car engine and therefore,
should be tuned for best performance at the
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