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INTRODUCTION TO JET DRIVES Impeller
to Engine Matching The following curve
shows the horsepower absorption characteristics for a Berkeley 12J size jet
drive.
- Berkeley Impeller Chart
- Engine to Berkeley Jet Drive Impeller Match
Click on the image for a larger view
Click on the image for a larger view In
factory installed installation the impeller selected will provide the best
average performance and is designed to keep the maximum engine RPM's possible
well within safe operating limits. You can
obtain small increases in performance or tune the jet drive to your type or
style of boating by making an impeller trim change. If
your engine is locked at 4200 RPM, let's say, and it will make power through
4800 RPM. then a smaller trim size for increased RPM should result in a higher
top speed, providing the power gain was sufficient. In this situation you
will need more RPM's at water ski and cruising speeds. You, however, may
not care much about absolute top end and be much more interested in economy,
quiet engines, strong low to mid range acceleration and the best water ski and
cruise conditions. You would want to use a larger trim impeller and limit
your top end RPM's. This set up will provide the most miles per gallon, a
quieter boat, less engine wear and tear and will be more comparable to a typical
stern drive setup. At 35 MPH the difference between an "AA"
impeller at 3200 RPM (125 SHP) and a "C" impeller at 4900 RPM (140
SHP) is easily seen and felt. These same
two impellers would probably end up being 4200 RPM (280 SHP) and 5200 RPM (320
SHP) on the top end. You should apply these rules:
- Load the engines for best low and mid range
performance.
- Unload the engines for higher top end RPM and
speed, provided you have adequate horsepower gain.
Impeller Matching (variations) for high
performance engines
Engine modification falls into two categories.
Changes which require higher RPM's in order to
reflect a power gain. A few of these changes are higher lift, larger
duration camshafts, higher compression, more or bigger carburetors. These
modifications will provide more power, but are usually accomplished by increased
RPM's and a torque curve which is moved 500 to 1000 RPM or more up the
scale. If the impeller is too big you may be disappointed and discover you
have worked hard and spent lots of cash to end up with little or no RPM gain or
even lost RPM's.
As a rule of thumb, you should always decrease
your impeller trim one size, i.e. "A" to "B" when going to a
hotter cam. The jet drive needs lots of torque on the top end a very
little below 3000 RPM so you must carefully match your new torque curve/HP curve
to the impeller SHP absorption curve. If you also add cubic inches then an
impeller size change may not be needed.
Engine modifications which are basically adding horsepower
at the same RPM's, i.e.. a blower, turbos, nitrous system, will either do well
with the same impeller as stock and turn a few hundred more RPM's or will
benefit from a larger trim impeller. Here are examples:
You turn 4600 stock and are at the peak of your
stock horsepower cure. Addition of a nitrous kit might give you an
additional 100 HP. Using the same impeller you will either gain 500 RPM
and be off the cam curve or you can switch to a larger trim impeller to absorb
the additional SHP at about the same RPM as before.
The alternative solution is to leave the
impeller the same, change the cam and valve train to be more effective at
higher RPM's and get a gross power gain which is the sum of the nitrous added
power and effect of the stronger cam.
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