OVERHAUL
SERVICE MANUAL:
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.
