Hey fhorst,
IMHO the old saying “you get what you pay for” generally holds true with PC water-cooling pumps.
You have to remember that the typical pond-pump is a culmination of design compromises. As has already been stated, most AC pond pumps use 2-pole AC synchronous motors (easy to water-proof => submersible, and inexpensive to manufacture) which will randomly rotate CW or CCW each time they start. Because of this, the pumps discharge port is always perpendicular to the axis of rotation (Eheim, Danner, Via, etc). The impeller in DC motor driven pumps and more expensive AC induction motor driven pumps only rotate in one direction. The discharge port on these pumps is mounted tangentially and captures both the centripetal AND inertial (rotating) forces of the moving water (Iwaki, Swiftech), which results in higher discharge head.
Pond-pumps are intentionally designed with loose clearances between the impeller and case so they can ingest debris and small pieces of gravel without jamming. (Looser tollerances also makes them less expensive to manufacture.) These large clearances lead to internal leakage - water flows from high-press discharge area back to low press suction area INSIDE the pump (commonly referred to as recirculation). This severely limits the pumps ability to generate much head. Apply a little resistance to the flow of water exiting the pump and the water will just quit flowing out, and instead will recirculate internally back to the eye of the impeller.
You mentioned in your initial post…
Quote: “As far as I can see, when a pump comes to its max head, the pressure is to high for the impeller to turn around… it first will "slip", missing a few pole changes and finally will stand still. When the pressure drops the impeller starts moving again…”
The impeller doesn’t actually stop turning – it may slip a little, but it doesn’t stop. The water just short-circuits inside the pump instead of flowing externally where it’s needed.
High-pressure pumps typically have tighter clearances between the impeller and casing to minimize internal leakage. This requires higher tolerances, which in turn costs more to manufacture.
High-pressure pumps may also incorporate a “closed impeller” like you noted on some of the Iwakis and the new Swifty. Enclosing the flow of water thru the impeller can add efficiency and helps minimize recirculation.
Depending on what kind of pump you decide to modify... one mod that has been successfully demontrated here at SC.c is close off a pond pump's axial discharge port and replace it with a tangential port...
(search "Volenti - Auto reversing pump?") This can significantly increase the pumps discharge head - like a cow pissin' on a flat rock - but ONLY when the impeller turns in the right direction!
One last thought... impellers and pump casings work together so if you modify one (like the impeller) then the other part should also be considered. Have fun and good luck...
