[Question] A or AA? (here are my dyno charts)
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A or AA? (here are my dyno charts)

  1. #1
    Member John Thompson's Avatar
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    Default A or AA? (here are my dyno charts)

    This chart is my actual configuration (Q-Jet 750 and optimized timing). It is the most accurate data I have. "356 HP at 4700 RPM"
    Click image for larger version. 

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    This graph is an earlier dyno run and not as accurate as the chart. I am including it to show the wide band at which peak HP occurs:
    Click image for larger version. 

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    Here is the Berkeley chart with the actual configuration data plotted "356 HP at 4700 RPM"
    Click image for larger version. 

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    Here is the Berkeley discussion on an archived page:

    https://web.archive.org/web/20040220...s/impeller.htm

    I believe I need the AA impeller, since my HP/RPM plot is above the "A" line.
    Last edited by John Thompson; 11-18-2018 at 02:24 PM.

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    Be careful as far as which company you buy your impeller from vs those charts; They all have slightly different power curves. I went with a Legend A impeller which was slightly tighter than the Berkeley A.

    Also, the Legend Impeller had blades that are at a steeper angle; Theory- Your top end HP absorption is (Basically) set by the angle, while the LENGTH of the blades and their overlap sets your mid-range RPM......The idea here was to have longer blades at the steeper angle that still allowed the top RPM, while making the impeller more efficient at mid-range cruise. Basically, my 460/400HP still turns approx' 4800 rpm topped out, but the 30 mph cruise is now at 3000 rpm, rather than 3200 with the Berk A impeller. Kind of like an overdrive during cruise but with the same top end.

    Legend was bought up by American Turbine, and I believe their "High-Helix" impeller is basically the same idea. You can supposedly still buy Legend parts from them.

    And you can probably call them and ask about the right impeller size- having the exact HP curve on a dyno sheet makes it pretty accurate, so they can nail the proper size...

    (I have a CVX-20 Jet w/460....I had the engine rebuilt along GM LS engine standards (Tight quench, wide lobe separation on the cam) and then installed a Edelbrock Carb with it leaned out in the midrange...(You can't load a jet heavy at low RPM- it just moves up in RPM).....Bottom line; I get better MPG than the DD ski boats.....And my boat is faster, quicker (And of course, more fun!!!)

  4. #3
    Member John Thompson's Avatar
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    Anybody want to stick their neck out on my choice of AA?

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    It'll struggle to pull a A to peak

    Dare to be different, if it turns out great you can claim you planned it that way.

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    Member John Thompson's Avatar
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    According to Berkeley "A" will never fully load the engine, unless I am reading it wrong. My Plot is above the A line.

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    Senior Member Hass828's Avatar
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    Buy the AA. If you try it and would like to have 400+- rpms more, then get it cut to an A. You can always cut smaller-- cant put it back on!
    "if we keep doing it the same way we always do..we will always get the same results"
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    The scenario doesn't matter if only considering HP and impeller size. The larger unknown variable is: Is the boat and pump loading water?

    It's safer to go larger with the impeller and have the option to cut it smaller.

    To even consider an A impeller for a pump that is properly loaded at that HP is not even close.

    If you buy a AA and start getting the pump to load you will more than likely end up cutting it many times.

    Been there done that. I started with an A at 700 HP and when I worked on properly loading the pump ended up with a B/C

    When I got the educated feedback I didn't argue I followed the advice. I closed off the full race intake opening with a custom shoe and started all over again.

    Measure the pump intake pressures before making any decisions regarding impeller cut. If the pump is not properly loading water, Every effort is a waste of time.
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    LP-25.com Infomaniac's Avatar
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    Try this chart
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  11. #9
    Senior Member Hass828's Avatar
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    Quote Originally Posted by Infomaniac View Post
    Try this chart
    Problem is-- his engine is done well before it gets on that chart.
    Last edited by Hass828; 11-21-2018 at 06:58 PM.
    "if we keep doing it the same way we always do..we will always get the same results"
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  12. #10
    Senior Member Hass828's Avatar
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    OP, what engine is this that only makes-"356 HP at 4700 RPM" and how hard do you want to turn it? 4700? What is the intended use?
    Last edited by Hass828; 11-21-2018 at 06:59 PM.
    "if we keep doing it the same way we always do..we will always get the same results"
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    Quote Originally Posted by Hass828 View Post
    Problem is-- his engine is done before it gets on that chart.
    Sure it's an example. No where near A or AA range was the point.
    If For Some Reason I Do Something Worthy Of Recognition. God Provided The Ability And Deserves The Credit.


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  14. #12
    Senior Member Hass828's Avatar
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    Quote Originally Posted by Infomaniac View Post
    Sure it's an example. No where near A or AA range was the point.
    That all depends on how many rpm he is wanting from it. If you look at his chart above-- it is basically same as yours, just shows what he needs to see for his HP-RPM
    Both of the charts are the same at 5200rpms, his just reads lower down the rpm range.
    Last edited by Hass828; 11-21-2018 at 07:11 PM.
    "if we keep doing it the same way we always do..we will always get the same results"
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    Member John Thompson's Avatar
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    Guys look at the Berkeley chart with the red cross hairs. That's where my engine plots. It's a SBC 406 somewhat set up as a torque engine. It has a Offenhauser Dual Port intake manifold. Family jet, maybe skiing, no racing. I specifically wanted a lower RPM engine.

  16. #14
    Just Me snoc653's Avatar
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    Quote Originally Posted by John Thompson View Post
    Guys look at the Berkeley chart with the red cross hairs. That's where my engine plots. It's a SBC 406 somewhat set up as a torque engine. It has a Offenhauser Dual Port intake manifold. Family jet, maybe skiing, no racing. I specifically wanted a lower RPM engine.
    While your peak HP is above the line, does your HP stay above the A line curve the whole way up to peak? Once HP dips below the curve, it becomes very unlikely that you'll have enough slip in the pump to get the engine to accelerate and get to it's sweet spot on top of the curve. You could wind up stuck at a lower RPM and never make peak power. For example: if it takes 100 HP to move an object and you only have 98 HP, you'll never get it to move; even if it only takes 75 HP to keep it moving once it starts to move. Then look at the AA line and make the same comparison. Once you drop below the curve, how are you going to get back on top? Where you drop below is most likely very close to where you will stop accelerating.
    So many projects, so little time

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