I read Dan's link. Very interesting read.
I'll just add a few small "data points".
In Mopar small-block world, we resolved the following - factory AC cars had 6-blade impellers and non-AC had 8-blades (usually?). The difference probably wasn't due to better cooling, because no extra cost so why not give everyone the best? The difference was probably because the w.p. pulley diameter varied between AC and non-AC cars. But, I don't think the later is true in slants.
From general engineering, I learned that centrifugal pump/fans come with blades either straight, foward-curved, or backward-curved. We studied mostly fans (HVAC). The curvature affects the "pump curve", i.e. delta pressure vs rpm relation. Forward-facing are more sensitive to rpm, backward facing are more forgiving (less sensitivity to rpm), is my recollection but haven't worked with it since school. The water pumps in my other vehicles all have straight blades that aim at the center, best I recall.
One question of mine, is gasketing. I recently re-installed the w.p. on my slant (didn't notice the blade count). I put a paper gasket on each side of the sheet-metal barrier. Is that correct and/or OK, or even better? I used Permatex "gasket sealant" on both sides of each gasket (sticky rubber liquid).
