AIO GPU coolers cool only the core while VRMs are on air, and they release a lot of heat. 1x120mm will have no problems cooling the card, and it will have better results than air only coolers. More radiators => slower fans => quieter system.
The 1080Ti doesn't need water cooling to perform very well. That means with water cooling it'll perform on a variety of radiator setups without trouble. It seems to be an incredibly efficient design and a single 120mm radiator will be plenty. If it's an AIO, as Avet said it'll still need a fan to cool the VRM's. I don't like the design very much for a GPU like the 1080Ti as it's not thermally limited, but if that's what you want then you'll not be disappointed.
Yup it's not quite as bad as the Titan XP but the stock card does need better cooling. A 120 AIO is more than enough providing you are willing to put up with some fan noise (because a 120 rad gets bloody hot with a GPU on it !).
Obviously if you pick a nice thick rad (65mm+) the fan speed can be lower.
You should be able to keep the fans running at around 800-1000RPM under heavy load on a single 120mm radiator. That's not too loud, but it'll be a bit louder than what water cooling is capable of.
More radiators => slower fans => less noise. Which is kinda the point of water cooling, apart from the bling. I would recommend when doing the water cooling to go with the biggest radiator you can fit, if your budget allows.
More radiators => slower fans => less noise. Which is kinda the point of water cooling, apart from the bling. I would recommend when doing the water cooling to go with the biggest radiator you can fit, if your budget allows.
More radiators => slower fans => less noise. Which is kinda the point of water cooling, apart from the bling. I would recommend when doing the water cooling to go with the biggest radiator you can fit, if your budget allows.