Lol good point...I just hear of people running a bronze impeller or maybe an "a" or "b" impeller..to me so far doing changes like this is like getting a cam for your engine..depends I guess on the overall setup..??
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Oh, so you are talking different cuts and materials.... Different story all together.
The impeller cut can be treated a lot like rear end gear ratio on a car/truck.
If you have the HP, the bigger the better. If you can't make the grunt below say about 5,500 RPM, then you may need to trim the impeller down to get to your peak performance.
As for different materials, this is not necessarily a performance thing, but more of a longevity deal.
Aluminum. Not too corrosion resistant as for brackish and/or salt water. Not really an issue for our rigs.However, these should really not be run at over 5,500 RPM and/or 500 HP. Risk of the impeller coming apart is the result.
Bronze/Mag bronze, this material is way more corrosion resistant, heavier, and a lot tougher than it's aluminum counterpart. Quite a bit more expensive than aluminum, but capable of holding together at over 800 HP and about 6,000 RPM+ They are still less expensive than a heat treated stainless steel impeller, the advantage of the stainless steel over bronze is open for debate, but lets just say that there is less material necessary in the construction of them which makes them ideal in many ways, toughness and of course the less material there is in the impeller the more room there is for moving water, which is the whole point, right?
There are limits to what a stainless steel impeller can handle, but unless you are running a helicopter turbine for power, I doubt you will ever find out.

GT