Impeller sizing and detailing
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Impeller sizing and detailing

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    Senior Member Alter Ego's Avatar
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    Default Impeller sizing and detailing

    TNYoungblood brought up a forum in the WFL section about impellers and it got me thinking.

    Most say that they see rpm drops after detailing their impellers. Say you detailed a B impeller and it gave you the rpm's of an A or AB, would you not be better off simply buying an A or AB cut impeller? If you were buying a new impeller, it seems you could get the same effects going bigger rather than buying smaller and detailing it leading to the same outcome.

    Someone please educate me. Because the only areas that I would see that need detailing if you simply went with a larger size impeller would be the hub area and casting flaws or rough spots.
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    a few variables to account for.

    The MPD "B" impeller in my pump bites like an "A+" on the charts :


    the 565 made an observed 820 hp at 6,200 ( was 902hp corrected) if you look at the chart at 820hp it should be able to spin a "B" impeller 6,700 rpm. The MPD "B" bites like and "A+" and holds the engine back, but the pump is more efficient for the rpm it is turning.

    there is more to detailing an impeller than just trimming off the back of the blades to gain rpm. The MPD detailing includes enough work on the "face" of the impeller to increase it's bite. The increase bite adds to the impeller's efficiency.


    Hope that helps .....

    S CP
    Last edited by Sleeper CP; 09-13-2013 at 11:48 AM.

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    Senior Member HBInHeat's Avatar
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    Detailing an imperial lowers ur rpm because it is no longer cavitating and is now moving more water through ur pump. It slips less. U will see mph and quickness gains by doing this. It has nothing to do w just buying a bigger impeller as u may need to cut ur impeller after detailing it to get back to peak hp range that ur motor runs at. Some people may detail the impeller as they went a little to big on the cut but that's not the reason most detail their impellers and is only a side effect

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    Senior Member Alter Ego's Avatar
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    So are you saying that all impellers have some sort of cavitation out of the box, that is why folks resort to going to larger impellers? If so, you would think that someone would have employed the very best casting methods (assuming current castings are not) using a detailed impeller for the pattern or plug, if you will.

    What I haven't wrapped my head around is that going to the larger cut impeller is a means of moving more water. It may have the cavitation that you are talking about, but the larger cut makes it as efficient as simply detailing a smaller cut.

    I can see the cavitation from deflection at the hub, but my undetailed AT a cut is petty nice and smooth out of the box (looking at the blades) the hub deflection would be more of the issue causing cavitation. I thought that was the entire purpose of the HCM inducer over the DPS, smaller hub=less deflection/cavitation.
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    Quote Originally Posted by Alter Ego View Post

    What I haven't wrapped my head around is that going to the larger cut impeller is a means of moving more water. It may have the cavitation that you are talking about, but the larger cut makes it as efficient as simply detailing a smaller cut.
    .
    a smaller cut moving as much water as a larger cut, the smaller impeller is the more efficient of the two.

    One of the reasons the AT impeller became so popular is that they came from the factory with a pretty damn nice detailing on them. It's like buying a blue printed impeller out of the box. But they can still be improved on.

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    Senior Member Sweet Pickle's Avatar
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    I think it boils down to moving the most amount of water with the least amount of drag=effeciency. An A cut might move the same amount of water as a B cut detailed but has more drag or ineffeciency which equals wasted HP and slower Boat. I',m sure you will hear from a pro on this one.

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    Senior Member HBInHeat's Avatar
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    Impellers are sand cast so the surface and defects are what they are off the shelf. U may think u have a nice impeller till u hold a really good detailed impeller next to it and makes a stock deal look like complete junk. A detailed impeller is more efficient then a non detailed of the same size and because u go larger doesn't mean the boat will be as quick or fast as a detailed deal just means the load is enough to hold the motor at the same rpm as the smaller detailed unit.

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    Senior Member Alter Ego's Avatar
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    Quote Originally Posted by HBInHeat View Post
    Impellers are sand cast so the surface and defects are what they are off the shelf. U may think u have a nice impeller till u hold a really good detailed impeller next to it and makes a stock deal look like complete junk. A detailed impeller is more efficient then a non detailed of the same size and because u go larger doesn't mean the boat will be as quick or fast as a detailed deal just means the load is enough to hold the motor at the same rpm as the smaller detailed unit.
    Ok, your last sentence made a lot of sense to me. It raises another question to me though. If the motor made X amount of power, the pump acts like a water brake dyno, would you not be moving the same amount of water with the larger impeller if the smaller detailed cut is making the motor turn the same rpm, where I get confused is that it seems the larger cut is making up the difference. Too bad we can't measure the actual gallon per second/minute difference between a standard A impeller and a detailed A for comparative purposes.

    so to your point, detail a smaller cut, because I am gathering that I am most likely goin to have to cut mine down if I detaile it. Then do you have to worry about the clearance of the impeller/bowl interface? I can see that you will have less metal on the impeller to impede /restrict flow, and possibly a tiny amount of decreases rotating weight if it is noticeable at all.
    Last edited by Alter Ego; 09-13-2013 at 01:18 PM. Reason: Dumbass auto spell check
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    Senior Member Lildave's Avatar
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    I'm thinking of it like this (and correct me if I'm wrong pros)...

    The slicker something is, the more it grips water. That's why you don't want show finish freshly waxed paint on the bottom back end of your boat. It creates drag.

    Thinking like that.... Maybe a detailed impeller (super slick and polished) grabs the water harder and sends it out with more force?

    IF that's true.....the detailed B would send out less volume, but with more force.


    Maybe?

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    Quote Originally Posted by Alter Ego View Post
    so to your point, detail a smaller cut, because I am gathering that I am most likely goin to have to cut mine down if I detaile it. Then do you have to worry about the clearance of the impeller/bowl interface? I can see that you will have less metal on the impeller to impede /restrict flow, and possibly a tiny amount of decreases rotating weight if it is noticeable at all.

    who is going to do the work on it ?

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    Quote Originally Posted by HBInHeat View Post
    Impellers are sand cast so the surface and defects are what they are off the shelf. U may think u have a nice impeller till u hold a really good detailed impeller next to it and makes a stock deal look like complete junk. A detailed impeller is more efficient then a non detailed of the same size and because u go larger doesn't mean the boat will be as quick or fast as a detailed deal just means the load is enough to hold the motor at the same rpm as the smaller detailed unit.
    the last AT impeller I got was pretty damn well detailed out of the box, certainly would have made the old Berk stuff look like junk.

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    Senior Member Alter Ego's Avatar
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    Quote Originally Posted by Sleeper CP View Post
    who is going to do the work on it ?
    Would it matter? If so, explain please.
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    Senior Member Alter Ego's Avatar
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    Quote Originally Posted by Sleeper CP View Post
    the last AT impeller I got was pretty damn well detailed out of the box, certainly would have made the old Berk stuff look like junk.
    thats another thing I was thinking. The AT impeller I have is fairly nice and smooth compared to the older ones I have laying around the shop.
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    Quote Originally Posted by Alter Ego View Post
    Would it matter? If so, explain please.

    Quote Originally Posted by Alter Ego View Post
    I am gathering that I am most likely goin to have to cut mine down if I detaile it. Then do you have to worry about the clearance of the impeller/bowl interface?

    Who is going to do the work on it ?

    S CP

    "Dark Sarcasm"
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