How An Impeller Works ?
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How An Impeller Works ?

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    Senior Member SkyHarborCowboy's Avatar
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    Default How An Impeller Works ?

    I have what seem to be easy questions but I am sure is pretty complex. I understand that the lower power you have the lower the letter of the impeller AA-C I think I have seen. My questions are what makes an AA and AA and a C a C (Diameter/Angle of the Individual Blades/Quantity of Blades)? How do they function differently from one another (Pressure/Flow)? What changes need to be made the to rest of the pump to use each different size of impeller?

    Joe

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    Hopefully this will help:



    As you trim off the back of the impeller the earlier it relieves the water thus freeing up the impeller to spin easier.



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    Quote Originally Posted by SkyHarborCowboy View Post
    My questions are what makes an AA and AA and a C a C (Diameter/Angle of the Individual Blades/Quantity of Blades)?
    Here's a pic that shows the differences there are between different sized implellers-

    Quote Originally Posted by SkyHarborCowboy View Post
    How do they function differently from one another (Pressure/Flow)?
    AA's flow more water which puts a bigger load on the engine as the engine rpms increase. For instance, an engine may spin at 4800 rpm with a AA but get to 5600 with a C.

    Quote Originally Posted by SkyHarborCowboy View Post
    What changes need to be made the to rest of the pump to use each different size of impeller?

    Joe
    Since the wear ring surface is unchanged, there is nothing to do to the pump for impeller changes. You might need to match a wear ring to each impeller though.

    here's a link to the Berk impeller info on the CP Performance website-
    http://cpperformance.com/bjd_impeller_info.aspx

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    E-7 Sheepdog (ret) SmokinLowriderSS's Avatar
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    Quote Originally Posted by SkyHarborCowboy View Post
    What changes need to be made the to rest of the pump to use each different size of impeller?

    Joe
    As noted, there SHOULD NOT be any changes required to change impellers.

    That said there COULD BE minor variation at the front face requiring a minor shimming on the shaft to reference properly to the wear ring.

    Also, since the Outside Diameter of the wear ring area is supposed to be machined to a std. size, there SHOULD NOT be any "replacement" of the wear ring forced by a simple impeller change,

    That said, if the impeller has ingested a good deal of grit and the wear surface is scored, it can be machined down, and a thicker wear ring installed (to a point). You might run accross a used imp. that has been turned down, if replaceing a turned one with new, the W.R. will have to be replaced, etc., etc.

    Also, many pumps are the same or so close as to be interchangable, parts-wise.
    Some are vastly different.
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    Senior Member bp298's Avatar
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    Quote Originally Posted by SmokinLowriderSS View Post
    As noted, there SHOULD NOT be any changes required to change impellers.

    That said there COULD BE minor variation at the front face requiring a minor shimming on the shaft to reference properly to the wear ring.

    Also, since the Outside Diameter of the wear ring area is supposed to be machined to a std. size, there SHOULD NOT be any "replacement" of the wear ring forced by a simple impeller change,
    that might be the way things should be. each impeller needs to be thoroughly inspected, and tolerances need to be checked anytime a new impeller is introduced into a pump. i have two impellers that are basically identical, except one is a B and the other a C. but one of them requires an 0.010 shim for tolerances to be right on while the other does not.

    Quote Originally Posted by SkyHarborCowboy

    I have what seem to be easy questions but I am sure is pretty complex. I understand that the lower power you have the lower the letter of the impeller AA-C I think I have seen. My questions are what makes an AA and AA and a C a C (Diameter/Angle of the Individual Blades/Quantity of Blades)? How do they function differently from one another (Pressure/Flow)? What changes need to be made the to rest of the pump to use each different size of impeller?

    Joe
    i -think- your question asked how these things work??? berk/amt/dom/etc. pumps are single stage, axial flow centrifugal pumps. the impeller is keyed on a shaft that is directly connected to the engine. simply, look at jon's diaghram; inlet side of the impeller is at the bottom, exit is out the vanes at the top. essentially, water is accelerated centrifugally, in an outward direction through the impeller vanes. the amount of water accelerated for each revolution is dependent upon the length and design of volute chambers created by the vanes.
    by reducing the length of the vanes (going from aa to c), size of volutes are reduced and the amount of water accelerated per revolution is slightly reduced. the result is the impeller must be spun slightly faster to accelerate the same volume of water. thanks for not asking why people would do this

    ideally, no other changes would need to be made to a pump, simply from cutting down size. but if it's a different impeller, all tolerances should be checked because, even if they are supposed to be identical, they can be slightly different.

    hohoho...

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    The pitch on an impeller is small compared to a prop, that would use the same amount of HP. In theory, the Jet drive accelerates water threw a nozzle for thrust, this would be an Axial Flow jet pump. The Mixed flow Jet pump changes the direction of water flow threw the pump. Once the water ends up passing the front of the blades, the water is pushed threw to the back side, in a centrifugal manor, . As the pressure is building inside the bowl, the stator vanes are collecting the the heavy water, and directing it threw the vanes, to the back side of the bowl. While traveling threw the bowl, the vanes are getting smaller, causing the water to acelerate, resulting in a greater pressure, and flow.

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    Senior Member SkyHarborCowboy's Avatar
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    Thanks for everyone's explanations but the part I am not grasping is if an AA impeller flows more water at a given RPM how can making a pump flow less water at a given RPM faster. At 5000 RPM it was seem that an AA Impellered boat (all things equal) should be faster. At what point does RPM over rule the next impeller for speed/acceleration? It would seem to me that you should want to run the largest impeller your engine could handle but it seems like a lot of the built motors have smaller impellers to allow them to run at High RPM's but it seems to me that if you could build a motor with more torque you should be able to run a larger impeller at a lower RPM to maintain the same speed all things being equal. I guess it is a game of Pressure Vs. Flow and that is what I am trying to figure out.

    Merry Christmas,

    Joe

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    Some engines make max power at higher rpm's and may require the B or C to let it get there. If you have huge horsepower, you will have the power to turn a AA and still get to peak rpm and horsepower. Just my .02

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    Quote Originally Posted by SkyHarborCowboy View Post
    Thanks for everyone's explanations but the part I am not grasping is if an AA impeller flows more water at a given RPM how can making a pump flow less water at a given RPM faster. At 5000 RPM it was seem that an AA Impellered boat (all things equal) should be faster. At what point does RPM over rule the next impeller for speed/acceleration? It would seem to me that you should want to run the largest impeller your engine could handle but it seems like a lot of the built motors have smaller impellers to allow them to run at High RPM's but it seems to me that if you could build a motor with more torque you should be able to run a larger impeller at a lower RPM to maintain the same speed all things being equal. I guess it is a game of Pressure Vs. Flow and that is what I am trying to figure out.

    Merry Christmas,

    Joe
    Now that's the $ 64K question...

    Hey BP, do you feel like typing some more this morning????

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    Sit N' Spin Jetaholic's Avatar
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    Quote Originally Posted by SkyHarborCowboy View Post
    Thanks for everyone's explanations but the part I am not grasping is if an AA impeller flows more water at a given RPM how can making a pump flow less water at a given RPM faster. At 5000 RPM it was seem that an AA Impellered boat (all things equal) should be faster. At what point does RPM over rule the next impeller for speed/acceleration? It would seem to me that you should want to run the largest impeller your engine could handle but it seems like a lot of the built motors have smaller impellers to allow them to run at High RPM's but it seems to me that if you could build a motor with more torque you should be able to run a larger impeller at a lower RPM to maintain the same speed all things being equal. I guess it is a game of Pressure Vs. Flow and that is what I am trying to figure out.

    Merry Christmas,

    Joe

    Now I'm no expert by any means but...

    It's all dependant on where your motor is making max horsepower at. If your engine makes max horsepower at 5000 RPM, it makes sense to use an impeller cut that will allow the engine to spin to 5000 RPM.

    If the engine makes its max power at 5500 or 6000 RPM, then you would wanna use a smaller cut. This is due to the fact that when you increase the RPM at which max horsepower is being made, you lose torque at the lower RPMs.

    Different engines make their max horsepower at different RPMs. This is why you have different cut sizes.

    Also, some guys like to build engines that make their max power at a much higher RPM because it's easier to do. Try making 700 horsepower at 5000 RPM. Impossible to do on a naturally aspirated engine...you'd need a blower or nitrous for that. But 700 horsepower at 6000 or 7000 RPM, less torque will be needed to make that happen. Again the drawback is that you would have to use a smaller cut impeller to allow the motor to get up to that RPM due to the motor having less lower RPM torque in exchange for higher RPM horsepower.

    Another way of putting it is...if you run an A impeller and the A impeller allows the engine to swing up to say 4800 RPM. Let's say for example sake that your engine only makes 480 horse at 4800 RPM.

    Now cut that impeller down to a C cut. And for example's sake let's say the C cut lets you spin up to 5500 RPM. If your engine is making more horsepower at 5500 RPM with the C cut, the C will move more water at WOT than the A would at WOT. This is because more work is being done (more horsepower made).

    That is the theory behind it anyway.
    Last edited by Jetaholic; 12-25-2008 at 11:42 AM.



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    Joe take a look at the chart below, for sake of arguement, just look at the line that read's "observed hp" those are the dyno #'s from my 565" Ford graphed out on the Berkley HP chart:



    Your question from above is a valid question, about 15 yrs ago our boat had an AA American Turbine 9.25" impeller ( it would be like a AAA on this graph)
    The engine was strong enough ( 780 [email protected] 6,200 565") it pulled that impeller to 5,200-5,300 off the Nos and ran 92-93 mph all day long. On the Nos it pulled 6,200 and ran 102-103 at 6,200 (approx 1,050 hp) That style impeller is no longer available ( thin blades pealed off after a couple of yrs)

    That engine was strong enough to pull a big impeller at the lower rpm's, but most engines that make big HP do so at a higher RPM so the impeller is trimmed to allow the engine to run on the cam where it makes power. Our new combination is a compromise between the old engine and letting the engine rev high off the Nos. It's still a two-stage engine; 6,200 off the Nos (99 mph) and 6,800-6,900 on the Nos (108 mph) the current impeller pulls like an A+ on this chart. Next yr I'll be doing some testing with another impeller (different style) but it will run untouched and then be trimmed to allow the engine to rev 6,500, so will see.

    If I recall correctly the current impeller when it was first installed allowed the engine to rev 5,800 and ran 93 we trimmed it to pull 6,200 (820 HP on the chart above observered #'s) and it now runs 98-99 mph, but the cruising speed and rpm's are basically unchanged.

    Hope that helps.

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    Last edited by Sleeper CP; 12-25-2008 at 12:05 PM.

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    Quote Originally Posted by Jetaholic View Post
    Now I'm no expert by any means but...

    Try making 700 horsepower at 5000 RPM. Impossible to do on a naturally aspirated engine...you'd need a blower or nitrous for that. But 700 horsepower at 6000 or 7000 RPM, less torque will be needed to make that happen. Again the drawback is that you would have to use a smaller cut impeller to allow the motor to get up to that RPM due to the motor having less lower RPM torque in exchange for higher RPM horsepower.

    .
    Well, I almost got there......we'll see in April



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    Last edited by Sleeper CP; 12-25-2008 at 12:42 PM.

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    Quote Originally Posted by Jetaholic View Post

    Also, some guys like to build engines that make their max power at a much higher RPM because it's easier to do. Try making 700 horsepower at 5000 RPM. Impossible to do on a naturally aspirated engine....
    bs... half the guys on this forum make 700 at 5k. even fords in ski boats...

    Quote Originally Posted by Jetaholic View Post
    Another way of putting it is...if you run an A impeller and the A impeller allows the engine to swing up to say 4800 RPM. Let's say for example sake that your engine only makes 480 horse at 4800 RPM.

    Now cut that impeller down to a C cut. And for example's sake let's say the C cut lets you spin up to 5500 RPM. If your engine is making more horsepower at 5500 RPM with the C cut, the C will move more water at WOT than the A would at WOT. This is because more work is being done (more horsepower made).

    That is the theory behind it anyway.
    sorta right.
    Quote Originally Posted by SkyHarborCowboy
    the part I am not grasping is if an AA impeller flows more water at a given RPM how can making a pump flow less water at a given RPM faster. At 5000 RPM it seems that an AA Impellered boat (all things equal) should be faster.
    true. if horsepower curves were flat lines instead of curves. the pump is a horsepower absorber. if horsepower was a flat line, going down one or two sizes would only result in higher rpm without any change in performance (or horsepower absorption) because the pump will only absorb the same flatlined horsepower. but, horsepower curves are not flat; horsepower continues to increase as rpm increases, until you reach "peak" hp. so, while going down one size allows engine rpm to increase a few hundred rpm, the pump would be asborbing more hp, which results in higher performance.

    Quote Originally Posted by SkyHarborCowboy
    It would seem to me that you should want to run the largest impeller your engine could handle but it seems like a lot of the built motors have smaller impellers to allow them to run at High RPM's.
    true again. you want to run the largest impeller you can, that will absorb the most horsepower available, to a point. it all depends on the hp numbers, and where it peaks.

    Quote Originally Posted by SkyHarborCowboy
    At what point does RPM over rule the next impeller for speed/acceleration?
    that's a great question. it's not about rpm, but about what the horsepower/rpm curve looks like. it's desirable to extract as much hp as possible, but at some point, the hp curve begins to flatten before peak, where you might see a 10hp gain over the last 500rpm. the question each individual needs to resolve for themselves is whether or not that last 10 hp is worth chasing, since the added rpm will have negligable gains considering the additional wear and tear on the engine at the elevated rpm.
    from 5000 to 6000 rpm, my engine hp increases by 130hp. from 6000 to 7000rpm, my engine hp increases about 65hp. i.e., the higher the rpm, the less the gain. so, my decision is to size the impeller to absorb hp available in the 6000-6500 range. it doesn't matter what the letter designation is; how much hp the is the pump absorbing, and how much more is left on the table?

    Quote Originally Posted by SkyHarborCowboy
    but it seems to me that if you could build a motor with more torque you should be able to run a larger impeller at a lower RPM to maintain the same speed all things being equal. I guess it is a game of Pressure Vs. Flow and that is what I am trying to figure out..
    building more torque means, ultimately, more hp. it's not about torque numbers per se, it's about horsepower numbers, which is related to work performed.

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    Great Explanations but I still have questions. I can see where a Big Block making 500 HP at 5000RPM might run a AA (numbers and letters for example only) and go 50 MPH and a Small Block making 500 HP at 7500 RPM would use a C to go 50 MPH in order for the boat(s) to go 50 mph EVERYTHING being equal (Weight, drag etc.) the pumps would have to flow approximately the same amount of Pressures and flow rates at the outlet of the pump nozzle.

    Maybe I have been asking this the wrong way but what are the flow rates and pump pressures for the normal impeller designations at a given RPM with a stock nozzle outlet. That is what I have been trying to figure out. I understood how the pump works and absorbs HP but I dont know HOW the different impeller sizes impact the output of water at the outlet. To go faster you have to either increase the Velocity or the water, the Quantity of the water or do both in Varying Ratios to achieve the higher boat speed of increasing the HP available to the pump.

    Joe
    Last edited by SkyHarborCowboy; 12-26-2008 at 11:49 AM. Reason: Clarification

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