Reply With Quote"I once cut a SS A down to a B and lost from 101mph down to 99mph on the IHBA clocks even tho I gained rpm"
Maybe you went over the top or your hp curve with that cut? I'm thinking Cave's not even close to your engine's power curve.
jer
396, yes, the way our pumps and boats are set up, we can change impellers in about 10 to 14 minutes.
Jer, "who's not out for all the performance they can get? " People like us that have a race boat and a lake boat both, don't necessarily want to spin their lake motor that hard all the time!
"I once cut a SS A down to a B and lost from 101mph down to 99mph on the IHBA clocks even tho I gained rpm"
Maybe you went over the top or your hp curve with that cut? I'm thinking Cave's not even close to your engine's power curve.
jer
Stock power plant here. LVJB, What would you suggest for a Jet boat cam? Since this impeller / cam deal all ties together. If I stay with the "A" impeller, Should I go with a Clay Smith? I would like to go alittle more radical on the cam but I dont want to change the pistons or run race gas at this time. I'm going to take my time with this project. I had a chance to clean up all my wiring that was just kinda everywhere. Now its all wrapped up and neat.
I think you are taking the idea to the extreme there Jer. No one said he should put a AA impeller in his pump. But if he drives long distances, he would be able to plane the boat out at a lower rpm with an A than a B impeller and thus, save fuel.
As for the big boat vs b impeller thing...its just been my experience that larger boats tend to have weak holeshots due to cavitation to begin with, so going to a smaller impeller will only worsen that condition. Cave doesnt have a large boat so obviously this isnt the case.
"Trying to pull a skier out of the water with a b-cut impeller is generally not a problem unless you are are in a larger, heavier 21-foot and up boat."
Cave runs an 18.5-foot Kachina I think. Not close to 21 foot.
But even at 21 foot, I'd wonder why that extra weight and drag with a b-cut won't pull up a skier. Assuming both pump and impeller in good condition. And trim control of course.
Just curious.
As for the, "lower rpms always good" mantra. Heard that before. I'm thinking depends on engine efficiency and impeller curve...as well as weight and drag.
Not so simple.
Although fuel economy seems to peak just after planning speed for one combo. Comparing fuel efficiency between two engines combined with two different impellers? I get well over 3 mpg cruise gas mileage with big hp engine coupled to a lightweight small "B" Berk (not Aggressor) impeller. Would I get better gas mileage if I ran an Aggressor AA and drug my engine rpms down maybe sacrificing engine efficiency? Not sure.
Are you sure?
jer
"Jer nobody said Lower RPM were always good."
Excuse me, plenty so-called nobodies as you said or "posted" that before. "Go bigger" they'd say, you'll no doubt get more torque, performance and more gas mileage? It's all about torque no matter right?
True or not you'll notice I didn't say lower or higher rpm's mean everything for all jet and impeller combos. I just said performance depends on engine power and impeller curves. Yes I think a Berk "A" works for cave's situation and may get more gas mileage. But how many here including Duane are actually concerned with gas mileage? You think?
Efficiency's important sure, but in the end...balls-to-the-wall performance who can not want that? With enough left over to pull that skier or two or a couple tubes or whatever, secondary goal. Yes a Berk B can do that in a 19' jet with a mild engine...I've done that...so I guess it works. Not to say it's the best match for Cave's jet, engine and pump and what he wants to do, just that generalities like a Berk "B" won't work for him or that an "A" or an an Aggressor "AA" or whatever is best for him maybe bogus.
Duane, I'd say your jet's not typical or even a good comparison to Cave's jet. What do you think?
On the thrust bearing...make sure the notch in the right direction. And bearing lubed. If the thrust bearing goes so can other things more expesive than just the impeller. Right Duane?
jer
i'm the first guy in line to sign up for all out performance. But I like to drive my boat a lot so saving fuel by cruising at a lower rpm is also on my mind. My current setup spins the impeller several hundred rpm below the peak power rpm of my motor. I could cut the impeller down to allow the engine to turn up to peak rpm, but the difference would only be about 40 hp. In my situation, I'd rather run the larger impeller unless I'm at the drag races where every little bit counts. In that situation, I'd rather have the smaller impeller.
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