I may be a little late to the convo here but if your still comparing look at (read that- measure) where the trailing edge of the impeller shroud (large diameter) is set back from the suction housing bolting flange and the inside of the edge of the bowl bolt flange is where it flares out toward the vanes. Will the water upon exit of the impeller hit that straight/flat flange surface before it flows toward/into the bowl vanes or will it be beyond that I.D. flat flange area? You don't want the water to hit that flat "wall" upon exiting the imp. and have to tumble down it before flowing out into the large diameter before the vanes. I hope that makes cent$.
It depends on the how the pump is assembled and the combination of parts.
My pump, for example, is a Berkeley G with an American Turbine 17-4 shaft, AA impeller, and stainless non-shouldered wear ring. This combination of parts causes the water exiting the impeller at the shroud to crash into that wall, as you were saying, by about an 1/8".
I use this particular case to present a few characteristics that occur inside the pump. With an A or AA impeller, there is 1/8" of total clearance between the impeller and the Inside Diameter (ID) of the bowl flange or a 1/16" gap all the way around.
Should I have installed a shouldered wear ring, the transition of water flowing from the impeller into the bowl would be near perfect.
Should I change back to the B+ (or smaller) impeller that was in it when I bought it, the gap between the impeller and bowl flange would resemble a hot dog in a hallway. The mismatch becomes insignificant, however the added volume around the impeller becomes an area of turbulence.
Some people will cut the suction surface of the bowl to remedy that mismatch. This has the additional advantage of reducing the overall volume that has to be displaced before pressure can build, however it can cause a slew of problems. The bowl registers to the suction housing by its face and ID. This register is the foundation of the concentricity between the shaft and bearings and therefore the impeller and wear ring. Facing the bowl is not recommended because you can end up with interference between the impeller and wear ring or worse.
Jack McClure of MPD will radius the edge of the bowl flange just for the very reason you mentioned. This does not compromise the bowl location.
I have been contemplating a stuffer for this area along with a viable machining operation. That's low on my list of priorities.
Bottom line, this is all fascinating and I love to ponder jet pump efficiency, but it is all really much ado about nothing. There are some really quick jet boats where these mods were never performed and I doubt they would see any gain from having them done. We always tell customers that there are other things you should do that provide more bang for the buck. If you look at any jet boat as a system, there are always areas where efficiency can be improved. Start with the biggest gains first.
