GPU water blocks are huge and despite their wattage being 2 -3 times that of the CPU, that allows them always run cooler in a mixed loop. The best way to approach this is to split the flow and run the GPU water blocks in parallel, with each seeing half the flow that the CPU sees. I'm running a 35X2 dual pump which feeds a 280mm rad, then splits into 2 paths, thru the GPUS, then the split recombines into a single flow path to the 420 rad, MoBo Block, CPU, reservoir and back to pump. Under stress testing the CPU hits the mid 70s with fans at 1200 rpm ... GPU temps are 39C. Letting the fans operate between 325 and 850 rpm, (they shut off at idle) and peak loads, they CPU sees hi 50s to mid 60s and the GPUs are at 42C.
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I have not seen the Alphacool AIO pumps tested but I don't expect themto do any better than the Corsair units which I have seen tested and the ones with 240/280 rads are pushing out just 0.11 gpm.
I don't have the dualie 35x2 to push the fluid thru the loop, it's basically if one pump fails, the other still there. But at 50% PWM, Im pushing about 1.5 gpm. See 50% and 100% graphs here:
https://martinsliquidlab.wordpress.com/2012/01/29/swiftech-mcp35x2-pump/6/
Things to consider:
1. I have never seen an AIO w/ flows even approaching 0.25 gpm .... this really is inadequate. I would go with a Swiftech variable speed MCP655 or if ya wanna go all out geek a 35X2 w/ stand and heat sink
https://martinsliquidlab.wordpress.com/2012/12/25/swiftech-mcp-655-pwm-drive-pump-review/7/
http://www.frozencpu.com/products/1...Pump_w_G14_Thread_Ports_Perfectly_Tapped.html
http://www.frozencpu.com/products/1...C_Industrial_PWM_Pump_-_Black_MCP35X2-BK.html
http://www.frozencpu.com/products/1...ch_MCP35x2_Dual_Pump_Heatsink_MCP35X2-HS.html
2. Split the loop and run the GFX card in parallel .. reduces backpressure to 27% of serial routing. See pic above.
3. Let's do the math and see if we can salvage the rads
CPU say 225 watts OCd
Vega 64 say 350 watts OC'd
I assume no MoBo or other water blocks.
Start here...
https://www.overclock.net/forum/61-water-cooling/1457426-radiator-size-estimator.html
You have 925 watts of "theoretical load" ... for dual GFX cards, we can assume that 40% of that is not loading the rads because not everything is at peak and much is being radiated off blocks, tubing, rad shrouds and other surfaces ... so you have 925 x 60% for the rads to deal with or 5r5 watts.
Your rads are very thin but unlike most AIOs, at least they are copper. Im not sure of the thickness but they look to be almost the same thickness as the fans or say 30mm .. unfortunately no test data for that thickness in the charts to we'll uses 45mm.
If ya download the alphacool data spreadsheet from above link, a 45 x 420 radiator from alphacool is good for about 245 watts at 1250rpm. Two rads would get you 490. With fans in push / pull, that would be 296 x 2 or 592 watts.
with 1800 rpm fans, you'd get 686 watts of cooling.... with 2200 rm fans, 990 watts of cooling in push / pull.
To stop fans from spinning up and spinning down making whiring sounds as loads change Id suggest:
a. Increase the thermal mass with a large reservoir
b. Use a decent MoBo with fan control utility that allows you to ramp down rpm over 90 seconds
3. Grab a 2nd Byisiki water block and as big a res as you can fit in the system