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JRPM/e-PerformanceMarine
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I have only seen speed and accelleration differances, but I never really checked down to the 50rpm - 500rpm. In circle racing too many variables.
 

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Retired Air Force
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940 Posts
Discussion Starter #5
Ran the boat with a new impellor yesterday and RPMs were lower than expected. Have the smallest insert in there now and was wondering if a larger one (red) would do that for me. I suppose I will have to have the impellor cut down a little.
 

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If you run to small of a nozzle it could cause a over load condition on the pump depending on the loader used causing RPM changes because of overloading.( Water in/Water out )The nozzle dia should be in tune with the loader and the attitude of the boat.
 

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I thought that physics would say a restiction in orifice size would lead to an increase of pressure. To pump more pressure takes more power? My POS wouldn't keep up. I think I'd lose RPM. But what would it do to my foward progress and at what cost/Benifit?
 

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Retired Air Force
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Discussion Starter #8
I had the teal (smallest) one in with a A+ impellor and 850hp. Too small maybe? I will try the red next time out
 

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I had the teal (smallest) one in with a A+ impellor and 850hp. Too small maybe? I will try the red next time out
going from a 3.0625 (teal) to a 3.125 (red), you won't see any difference in rpm. A+ is awfully big, especially if it's blueprinted. at what rpm does it make 850, and what is the rpm you are seeing with this impeller. at what rpm does peak torque occur???

with what you have, the 0625 will make the boat a little quicker than it will be with the 125.
 

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I thought that physics would say a restiction in orifice size would lead to an increase of pressure. To pump more pressure takes more power? My POS wouldn't keep up. I think I'd lose RPM. But what would it do to my foward progress and at what cost/Benifit?
true, but the physics depends on the application. these are single stage axial flow centrifugal pumps. and generally speaking, because it's a single stage, accepted centrifugal pump laws don't apply. when the orifice size is changed, mass flow rate and velocity are altered. if one value increases, the other decreases. it's solely fine tuning.

example. running a 3.125 insert with a good 45psi suction pressure at top speed. that 45 is based on biting edge depth, loader a little bit, and insert diameter.
if the insert is changed to a 3.00 with no other changes, you'll see a slight increase in bowl pressure, and a significant increase in suction pressure.

in a lower hp deal, the pressure changes won't be nearly as dramatic as they will be with higher hp. because not that much pressure is being generated in the first place...
 

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Retired Air Force
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Discussion Starter #12
The engine peaks at 853hp at 5900 rpm and it was maxing at 5500 yesterday (810hp). I have to resolve some tuning E85 issues with the engine. Once I know the engine is running right I will send impellor back if needed.
 

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"Need For Speed"
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Once I know the engine is running right I will send impellor back if needed.
Keep in mind the altitude corrected weather conditions. The air in my part of the country was killer good yesterday. My dyno was actually correcting down from the observed or measured power. Here the engine will make five to eight percent less power in the summer which will affect the cut on your impeller if you are trying to get it spot on with your power curve.
 
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