One would reason that a higher flow rate would mean a better cooling performance, with maybe a point of diminishing returns. In practice that's not the case however, and I've seen plenty of tests to back it up. There ends up being sort of a bell curve of performance, with the underrated pumps obviously performing worse, increasing in efficiency up to a certain optimum point, after which performance _decreases_, ostensibly from the radiator not having enough time to dissipate the heat into the ambient air, thus recirculating hot water. Different setups have different optimal flow rates based on fitting/pipe size, waterblock characteristics, etc. In most tests of typical computer watercooling systems, I've seen around 6gph realworld throughput as optimum. Note that this is NOT the same as the rating on the pump... in the real world, the pump works against viscosity, restrictions from bottlenecks (1/8" at any part of the system will make the pump work that hard effectively for the whole system), etc. So for optimum performance, I recommend using a relatively large capacity pump (personally, I would go with around 250gph, and yes they make magdrive units like this that aren't powerheads), and throttling the output with a check valve until you reach the optimal flow rate. There are scads of flow meters out there to aid you in this journey. Hope this was enlightening.
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