A DDC3.2 with top is quite a bit stronger than a CPX-pro.
In the end of all this, it probably depends on the waterblock/rad setup. Naeku said he saw good gains with the RD-30 at around 21V.
A DDC3.2 with XSPC res top is pretty close in performance to an RD-30 at 15V, where dual DDC 3.2s with XSPC tops is closer to an RD-30 at 21V.
From the little bit of discussion I have heard on the RD-30, 24V is almost always worse, and back in the day most people were running them around 18V. Somewhere in the 25-21V range is probably optimal.
Think of it like an air/fuel mixture ratio on your motorcyle, too much air and you can lean out your motor and fry a piston, too much and you'll foul plugs....neither of which produces the best horsepower
That's similar here, there is not any bad pump or perfect pump, it's all about tuning....the only way to know whats perfect is to test and tune your own specific setup....there may not be any sort of rule of thumb to follow..
All we know at this point is:
-There are thermal gains to flow rate, more is always better but that gain is quickly flattening beyond the 1-1.5 GPM range
-Pump heat dump plays a significant role and will quickly cross the lines of the thermal gain at some point
-Open channel blocks from several years ago had nearly 4X the thermal gains from flow rate in the 1-2GPM range than some of todays blocks
-Pumps of today such as the D5, DDC, CPX-Pro, etc...have significantly higher pressure capabilities that pond style pumps from several years ago.
The best pump and setting for your system will be one that gives you the most delta between flow rate thermal gain and heat pump heat dump loss. It's likely not going to be any set number and will heavily depend on the radiator fan combination, the waterblock used, and the amount of heat generated by the pump.
I never said an RD-30 was a bad pump, all I'm saying is heat is important and you need to spend the time to balance flow rate gains from pump heat dump losses if you truely want the "perfect" setup. We can't generalize like that, we need the patience and will to test our own setups if you really want the best possible performance possible. An RD-30 at 24V is likely well over optimal for any system, but it is an awesome pump and can run clear down to 12V if needed, so it's tunable and an excellent pump. If we don't want to bother with the work, we need to accept it's probably not perfect. For most people that don't care about a degree, which may mean the larger audience, I wouldn't worry about it...pick something and go with it.
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