You won't see the difference in size of the impeller unless you open it up. It is the base diameter that is the difference. The top of the impeller is the same which is why they are interchangable. The inside allows for a larger diameter then the small Sci trim. I am not sure about the size difference but I think it is 3" for Sci and 3.25" for Si. Quick search can figure that out but I am just being lazy right now.. lol
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