Rad space per part
Rad space per part
You should allocate 140 rad space per GPU and CPU for this setup.
Let's imagine you're making a slight modification to your CPU. You'd want to examine a 240mm radiator, and if you add your 1080 Ti to the loop, you might consider extending it to 360mm or splitting it between two 240mm and a 120mm radiators. You could also go with a 280mm option, but I think this is the best approach.
Later on, if you add another 1080 Ti, just add another 120mm. Essentially, once you determine what you need for your CPU, you should add 120mm for each additional part in the loop.
My current setup runs an i7-980x at 4.2Ghz with a full cover motherboard and four GTX 670 graphics cards. I'm using a 480mm and a 360mm radiator all in one loop. Breaking it down, it looks like...
I'm using XSPC RX360 and RX480 radiators with Corsair Sp120 fans. It's not very efficient as I've configured it, but it functions. Both radiators are combined into one stack. Fans are ordered 360mm > 480mm > 11 fans in a push/pull setup. I'm using a DC motor control to drive the fans, so fan speeds aren't fixed. You can see the white Corsair sticker spinning on the fan easily. CPU performance is around 40-50% load with mid-40s response time, and GPUs are roughly 30 times that.