[Long Haired Git]

My approximator works on pressure, but with the PQ graphs unadjusted for two pumps. I trust this is right.
I do not have any PQ information for the HL fusion GPU blocks. I have assumed they are similar to the LRR block, as they employee the same technique for cooling (restrictive fins with impingement over core).

Two LRR's in series each on 70W (I assume you're cooling high end GPUs?)
MCR220QP radiator with 2 fans on 7v
Approx 1.5m of 1/2" tubing

Flow rate = 6.1 LPM
Water temp at rad exit = 7.1°Caa (degrees celsius above ambient)
GPU 1 = 19.7°Caa
GPU 2 = 19.9°Caa

Add in a second MCP655 @ 5:
Flow rate = 8.1 LPM
Water temp at rad exit = 7.7°Caa (degrees celsius above ambient)
GPU 1 = 19.9°Caa
GPU 2 = 20.1°Caa

So, I get it as being worse. This, in my ever humble opinion, is because the MCP655 is already working at a high flow rate where it dumps the most heat into the coolant. As we know, pressure head is related to the square of the flow rate, and so the tiny extra flow rate results in much more head, hence only an extra 2 LPM.

Swapping to an Iwaki 20RT gives the best temps according to the figures I have (heat dump for the Iwaki is an estimate as none available) being 7.2LPM and GPU temps of 19.3°Caa and 19.5°Caa.

To give you an idea of the restriction:flow-rate curve and how little flow rate matter with modern blocks and radiators, a single MCP350 pump gives "only" 3.8LPM but the GPU temps are 20.1°Caa and 20.3°Caa

If it were me, I'd take the single MCP655 and be darn happy.

BTW, with the single MCP655 @ 5 I have the following for the LRR block. Anyone who can confirm/deny and compare/contrast to the Fusion would be great to hear from!

Flow = 6.1078 LPM
Head from one block incl Y joiner and two 5cm sections of 1/2" tubing: 1.2315m of H20