ziphnor wrote:Even at stock, they are reporting 216W though?
I can't really help: set aside it's the entire system power draw at the wall (so the actual DC draw is a bit lower), the power measurements depends of the specific setup. Just for example,
Guru3D reported 185W at the wall, loading with Prime95 their test system with a 5820K at stock.
ziphnor wrote:I think you will find that this depends on what the OpenCL application is doing, if i re-implement the same computation that FurMark is doing in OpenCL, i would expect the same GPU load
Indeed: but GPU loads isn't the issue here. As said in my previous post, it
mainly depends of the overclocking and voltage figures, synthetic benchmarks like Prime or FurMark may worsen the things, but nonetheless it's almost all a matter of voltages.
ziphnor wrote:In general i dont want to have any set of applications that i can run that could theoretically overload the PSU. It has to be able to cope with the maximum stress possible, even though its unlikely to occur.
Whether you want to stick with original proposition of "not excessive voltage", there are virtually no chance you can reach the 700W DC mark, and IMO not even the 600W one: but whether you will put (to say) 1.4V on the CPU, the actual power draw will likely double, so YMMV.
ziphnor wrote:When comparing the HXi 750 vs 850 Corsair indicate that the first one is passive up to 300W while the latter is passive up to 350W, additionally the 850 doesnt ramp up the fan again until 550W, while the 750 does so at ~450W. So for this particular PSU, the bigger models really are more silent (assuming their specs are reliable).
As said to lodestar, there are general rules and there are exceptions, and both have to be taken with a grain of salt.
In particular, when you're dealing with a single card setup with an expected power draw nearby 300W, talking about an 850W unit instead of a 400-500W one is very often more counterproductive (money, noise, and efficiency wise), than when comparing a 850W and a 750W units for an around 500-550W expected power consumption.
Back to your thought, I haven't seen any serious comparison of both the HX750i and HX850i, so I can't do any educated guess: at first glance I think it's not unreasonable to trust what Corsair stated.
I can only say what I already said some days ago: that at 750W/850W those HXi have a sound pressure level advantage over the Seasonic Platinum, due to the more conservative Seasonic approach, cooling wise.
Even better, given the high idle power draw of an oc'ed X99/SLI setup, you can even go further, and look to the HX1000i, as it stays fanless at least up to 400W, and fan ramps up over 700W DC, or look to the AX1500i, which can stays fanless at least up to 600W DC and fan ramps up over 1200W DC.
It all depends of your goals (as well as in overclocking): in a not-rotated SLI setup at least one card will be noticeably heated, and anyway 350W of graphics cannot run under load at a relatively quiet level, so why bother of 50W, 100W or 300W more of fanless range for just the PSU, particularly when you will have to shell out money for their silence? It's up to your call. With reference to money, maybe a worthwhile alternative might be the already quoted eVGA SuperNOVA G2 750W: although the fan kicks in at an higher speed than the Corsair HXi, it has a comfortable fanless range, performance are top notch, build quality is as high as the Seasonic Platinum and the relevant price is substantially lower (not to mention it sports a 10-years warranty).
ziphnor wrote:Perhaps i should just give up on the R4 and get the R2 XL+Corsair HX850i and let the money saved from skipping the Seasonic pay for the larger case.
I'm not a fan of insulated enclosures for multi-GPU setups, but surely the Define XL might be a better option than the Define R4 (as well as the Fractal Design Arc XL).
ziphnor wrote:I am not planning SLI initially, but when the gtx 980 prices drop and if VR SLI has been shown to work, i might get another one.
IMVHO there are too much "if", and too much "conditional tense": I already gave you this advice, but as "repetita iuvant", I will repeat once more.
Whether I were you, I wouldn't bother about SLI, powerful PSUs and big enclosures, and I will shave off some substantial money (maybe for any future rebuild, or any more amusing task).
