SoCal Jet Boats
Tech Talk => Jet Pumps => Topic started by: iloveitloud on May 19, 2013, 06:51:48 AM
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Just put in a new aluminum "A" impeller, wear ring, and seals/bearings. Boat had a "A" impeller before as well. Boat runs about the same top speed, with alot better launch, and way less cavitation. Only issue is it knocked my peak RPM's down by 1000. For the record...It runs 55@4000 RPM- it used to run that speed at 5000 RPM. Engine it a 468 chevy, nothing special, just domed pistons, shaved heads, and decked block. According to the BERKELY HP vs. IMPELLER chart, I am only making 200 HP! It was rated at 360 stock. Not sure about cam specs. Kind of dissapointed that my engine is not making the power to turn the new impeller any faster, but I am happy with the lower RPM, and less noise that goes with it. Boat is a heavy BAJA, that is used to take the kids along, tube, and mostly just run around shooting big rooster tails, so it does not really need to be any faster. Just wondering if there is usually that much of an RPM drop going from a "old worn out" pump, to a "new, tight" pump. Either way...boat performs better, at lower RPM's, so I guess I should be happy!
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The horsepower number is not as important as where the peak ends up in the RPM band. If it makes 300 at 5500, and 200 at 4000...your impeller is holding you down...the engine can't get past that point to actually use its peak HP. You might try cutting the impeller down to a B. I'd probably start with an A/B cut and take it from there.
Check the basics first...make sure the throttle is opening all the way, make sure timing is set right, etc...
Depending on how worn out your old pump was, what you've experienced is perfectly normal.
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How sure are you that you tachometer is correct?
Did you change anything else when you put the pump in?
A drop in rpm is pretty typical, that big of a drop is not the norm.
CHECK EVERYTHING!
Do not cut the impeller.
Check; wot timing, throttle linkage, carburetor, tachometer, timing marks to tdc, everything.
W hat is your manifold vacuum at wot?
Do you know what cam shaft?
Do you know how it is installed?
Your speed numbers seem reasonable, perhaps even a little too fast. So I think you are close, but something is amiss.
GT
Sent from my SGH-T989 using SoCalJetBoats mobile app
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Speed is confirmed by GPS...Not sure about TACH...-just know that RPM's are less than before. This was also the first run of the year for the boat after sitting since last September. Engine does not "sound" like it is reving up any too high though. Have not checked timing, but it has not changed since last season. Also have not taken air cleaner off and confirmed throttle is opening all the way, but again...nothing has been changed there either. I will probly pull all the plugs, and check the throttle cables. Also...the bucket is opening all the way. I modified the whole nozzle, bucket and steering to allow for the biggest roostertail possible, so I did check out all of that stuff to make sure it is working correctly.
Other than the basics... I probly have to much CAM for my 468, with a "new" A impeller. Before I get too exited about adding any power, I will see how the fuel economy is, and I can only guess that it will be better than it was by running the engine "slower" One thing that I also need to confirm it that the vaccum secondarys are opening at all...4000 RPM might not be doing it... It will be hard to confirm because I have a covered engine, and it would not be to good running around the lake at 55 mph with that up looking down the carb!
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Did you look at the old & new impellers side-by-side? Were the external dimensions very similar?
For a frame-of-reference: I had a mild 455 Olds that would turn a 9.25" impeller to 4300 rpm in a 22' day cruiser. Speed was 53-55 mph. So that's 3 sizes bigger impeller than yours turning 200 rpm faster, with about the same speed result.
Suspect you have a bad tach reading. Can't have a 9.25" impeller unless you're running an AT-309 pump.
And I'd be very impressed if the secondaries weren't opening and you were pulling 4000 rpm. :o
Is your rpm lower across-the-board for given speeds?
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With the old impeller...the engine would pull 4000 RPM on 2 barrels, and run exactly 40 mph at that RPM. I know this because I had the secondarys disconnected at one time. RPM's are lower than before at all speeds, with almost no cavitation on launch at all...-had some before. The impeller I put in is whatever aluminum impeller Berkeley gives you with a master rebuild kit...looks exactly like the one I took out...minus the damage.
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Got to run the boat again. Pulled 3 person tube. I was surrised...boat would run 25 mph @2800RPM pulling the 3 person tube. It would have been revved up alot more than that before, and cavitated a little starting off. Boat is doing everthing it used to do better with lower rpm at every speed