help- with impeller
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help- with impeller

  1. #1
    Junior Member Elimin8R's Avatar
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    Default help- with impeller

    I'm confused about selecting the right impeller. I have a 12jc with what I was told anyway, a "c" impeller machined to an "A". I run a new 455 olds in an 18 ft. Eliminator and pretty much top out at 5000 rpm. I want more top speed if possible. Will an impeller change grant me my wish?
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    Senior Member lbhsbz's Avatar
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    Quote Originally Posted by Elimin8R View Post
    , a "c" impeller machined to an "A". ?
    unpossible.

    You can machine an "A" to a "C" but not the other way around.

    If you're spinning an "A" to 5000, I'd say thats not bad for a stock olds.

    If you're spinning a "C" to 5000....I'd say get a new olds....


    Here's a link to the Berk. impeller charts


    http://www.berkeleyjet.com/impeller_manual.aspx

    You can pull the bowl and measure your's up and see what you've got.

  4. #3
    Sit N' Spin Jetaholic's Avatar
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    Quote Originally Posted by Elimin8R View Post
    I'm confused about selecting the right impeller. I have a 12jc with what I was told anyway, a "c" impeller machined to an "A". I run a new 455 olds in an 18 ft. Eliminator and pretty much top out at 5000 rpm. I want more top speed if possible. Will an impeller change grant me my wish?
    That depends...what's your wish?

    The way impeller sizing works is...

    The higher up in the alphabet the impeller cut size letter is, the less horsepower per RPM it absorbs. Example, it takes more horsepower to turn an A impeller to 5000 RPM than it takes to turn a C impeller to 5000 RPM.

    And X2 on lbhsbz's post...if you have an A impeller, you can get it cut down to a smaller cut like a B or a C cut instead of getting a whole new impeller, but you can't go the other way around.
    Last edited by Jetaholic; 08-07-2009 at 06:13 PM.



    Quote Originally Posted by HammerDown
    Kendall L-427 Super Blu...extreme-pressure (ep) lithium complex keeps my thrusting balls happy
    Quote Originally Posted by back2k5 View Post
    Well I am putting in a nice stereo system in the Carrera...maybe Ill put on some rap and turn the scoop sideways

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    Senior Member jetboatperformance's Avatar
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    Considering the inherent Olds "issues" with oiling ,lower end longevity etc I can't imagine why some one would have used a "C" cut impeller Depending on where this motor makes power you would likey be better served with a "A" or even maybe a "A2" Tom

  7. #5
    About 500' AGL MudPumper's Avatar
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    Quote Originally Posted by lbhsbz View Post
    unpossible.

    ???

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    Senior Member lbhsbz's Avatar
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    Quote Originally Posted by MudPumper View Post
    ???
    just like it sounds

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    Junior Member Elimin8R's Avatar
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    The boat had a stock 425 when I bought it. The new 455 is .030 over has comp cam, flat tops, roller rockers. The old motor struggled to reach 4500 rpm. Does an a impeller provide more top end than a c?

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    Sit N' Spin Jetaholic's Avatar
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    Quote Originally Posted by Elimin8R View Post
    The boat had a stock 425 when I bought it. The new 455 is .030 over has comp cam, flat tops, roller rockers. The old motor struggled to reach 4500 rpm. Does an a impeller provide more top end than a c?
    As far as the engine is concerned, impellers don't provide anything other than a load. However if you're asking if an A will let the engine rev higher than a C, no...it's the other way around (an A impeller provides a bigger load than a C impeller and won't let it rev as high).

    However, higher rev doesn't always mean faster boat. If you cut the impeller down to a B and it lets the engine rev up past the RPM where peak horsepower is being made, you'll drop in speed because less work is being done past the RPM at which peak power is being made. In other words, impeller sizing isn't about cutting it down to rev the highest you can...it's about sizing it to a size that will let the engine top out at the RPM where peak horsepower is being made and no faster than that.

    Also, as Tom mentioned earlier, most people don't rev an Olds past 5000 RPM due to the known oiling issues that they have unless they have pretty extensive oiling mods done to them.
    Last edited by Jetaholic; 08-07-2009 at 07:09 PM.



    Quote Originally Posted by HammerDown
    Kendall L-427 Super Blu...extreme-pressure (ep) lithium complex keeps my thrusting balls happy
    Quote Originally Posted by back2k5 View Post
    Well I am putting in a nice stereo system in the Carrera...maybe Ill put on some rap and turn the scoop sideways

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    Elimin8R
    I have to ask, how is that droop attached to the "C" bowl?

    I am guessing that it is welded on there?

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    Junior Member Elimin8R's Avatar
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    Quote Originally Posted by Jetaholic View Post
    As far as the engine is concerned, impellers don't provide anything other than a load. However if you're asking if an A will let the engine rev higher than a C, no...it's the other way around (an A impeller provides a bigger load than a C impeller and won't let it rev as high).

    However, higher rev doesn't always mean faster boat. If you cut the impeller down to a B and it lets the engine rev up past the RPM where peak horsepower is being made, you'll drop in speed because less work is being done past the RPM at which peak power is being made. In other words, impeller sizing isn't about cutting it down to rev the highest you can...it's about sizing it to a size that will let the engine top out at the RPM where peak horsepower is being made and no faster than that.

    Also, as Tom mentioned earlier, most people don't rev an Olds past 5000 RPM due to the known oiling issues that they have unless they have pretty extensive oiling mods done to them.
    Well I'm not really looking to rev the motor higher I just want more top speed!

  13. #11
    Junior Member Elimin8R's Avatar
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    Quote Originally Posted by Real McCoy View Post
    Elimin8R
    I have to ask, how is that droop attached to the "C" bowl?

    I am guessing that it is welded on there?
    No it's not welded. Pins were driven in and ground off. Weird huh!

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    About 500' AGL MudPumper's Avatar
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    Quote Originally Posted by lbhsbz View Post
    just like it sounds

    If you say so....

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    Senior Member chevy's Avatar
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    Dump the logs, that might be enough to make you happy

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    Sit N' Spin Jetaholic's Avatar
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    Therein lies another question -

    Has the pump ever been rebuilt?

    There are many variables present in the infamous "MPH equation". Engine power is just one of the many, with condition of the pump being the 2nd most important of the many variables. Do all the other tricks you want...if the engine and pump are not operating at optimum efficiency (BOTH have to be right...not just one or the other) you'll end up chasing your tail. If the pump is worn you'll notice a night and day difference by having it rebuilt.

    Pull the handhole cleanout cover off and see what the front of the impeller looks like. If it looks pretty worn and hammered, you can pretty much bet it's never been rebuilt. It would be money well spent to rebuild it if thats the case.

    Have you GPS'ed it yet? If not, you should run it and get a baseline of where you're at MPH'wise first. Let's find out what you got to work with first and then we can go from there.
    Last edited by Jetaholic; 08-07-2009 at 11:39 PM.



    Quote Originally Posted by HammerDown
    Kendall L-427 Super Blu...extreme-pressure (ep) lithium complex keeps my thrusting balls happy
    Quote Originally Posted by back2k5 View Post
    Well I am putting in a nice stereo system in the Carrera...maybe Ill put on some rap and turn the scoop sideways

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