Water brake dyno Capacity constant

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New Member
Nov 4, 2022
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South Africa
Does anyone know how to calculate the capacity constant for a water brake dynamometer. I am using the article "The basic theory of Hydraulic Dynamometers and Retarders" : The Basic Theory of Hydraulic Dynamometers and Retarders on JSTOR. I'm really struggling to determine the K value to then determine the nominal diameter. Does anyone have experience designing a dyno using this as a guide?

T = KN^2 D^5 (T = torque, K = capacity constant, D = diameter)

Is the blade angle used the angle at which the water exits the torus pocket?

r = RI/RO with (R=Radius, I=Inner, O=Outer) but this doesn't seem to have anything to do with the torus inner and outer radius but rather the radius of the design path which is defined as rd=fn(theta) with fn being the coefficient of friction.

Any alternative methods for determining torus shape and impeller dimensions are appreciated as well.

Am I also overdesigning this. The Engine to be tested is a Honda GXH50 which only has 2.1 hp. Would it be better to just design a straight vane impeller as the rotor with some vanes or pockets on the stator to help with the transfer of momentum. Can then just use the general centrifugal pump calculations to determine the diameter required for the impeller.


Well-Known Member
HMEM Supporting Member
Jul 8, 2009
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On YouTube there's a guy that built an electric brake dyno for his 50cc "racing moped", now THERE's an oxymoron. I can't find the link now, and I'm not sure if it would be any help to you or not. But this guy is making his parts in his home machine shop. Casting cylinders and heads, whatever he needs. I'll check tonight and see if I can find the link and post it. He may not be succeeding in getting to his 20Hp goal, but he sure is trying.

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