What type of performance loss can I expect using a fixed grate vs the hydro surge?
Little to none, depending upon which grate you choose. Here's a repost from some analysis I did several years ago.
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I took some measurements of the aluminum and stainless steel grates just to see what is really going on. Here are the results. Lengths are not considered because all comparable lengths are essentially equal.
The unobstructed intake opening is approximately 7.188 inches wide. The aluminum grate's tines taper from 0.384 to 0.310 inches wide, yielding an average width of 0.347. With six such tines, the total obstructed width is thus 2.08 inches, or 29% of the unobstructed width. The stainless steel grate's tines are 0.235 wide for most of their length, and with 11 such tines the total obstructed width is 2.59 inches, or 36% of the unobstructed width.
While I strongly prefer the stainless steel grate due to its better-profiled tines (less turbulence) and increased strength (vs. aluminum), there is no question that the aluminum grate permits more potential flow - approximately 24% more than the stainless steel grate. Whether the rest of the pump system can take advantage of that increase to yield more performance is unknown (to me).
24% is enough of a difference that, as a wintertime project, I'm considering removing six of the 11 tines on my stainless steel grate. Starting with the outermost tines and removing every other one would leave five equally spaced tines with a total obstructed width of 1.175 inches, or just over 16% of the unobstructed area. The spacing between the tines would only be slightly larger than the aluminum's spacing, but much stronger due to the use of stainless steel. Best of all, this would represent a potential water volume increase of 31% over the standard stainless grate and 18% over the aluminum grate. A 20-30% increase in potential volume is definitely worth exploring.
I've heard of at least one SportJet owner that removed the two outermost tines from his stainless steel grate. The resulting 9 tine grate would have an obstructed width of 2.12, almost identical to the aluminum grate's 2.08 while retaining the stainless steel's substantial strength advantage. Such a modification is pretty simple to do with common tools since access to the outermost tines is very straightforward, so this might represent a good best-case compromise for we garage tinkerers [grin].
Here's a table of the effective intake widths. Percentages indicate how much of the original unobstructed width is still open.
7.188 (100%): No grate
5.108 (71%): Aluminum
4.598 (64%): Standard Stainless 11
5.073 (71%): Modified Stainless 9
6.013 (84%): Proposed Stainless 5
The 71% stainless figure is easy to achieve. But that 84% figure at the end is pretty tempting....
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Later, I actually did create the "Stainless 5" grate. I've been running it for several years now:
It works, er, "grate"! {grin}
