I don't think he's trying to be thick, the S5 simply cant handle much more than 12V, unless you maybe had some serious cooling on it.
We'll have to agree to disagree on that.
If you run the S5 in near-zero environment its possible to get 1.3-1.4TH from it at clockrates of 400MHz-425MHz and ~700W consumption
If you gave it 12.5V and sub-zero temperatures you might be able to get 1.4-1.55TH from clockrates of 400-437.5MHz and ~800W consumption but at that level the PCB and heatsinks might not keep up.
I am not sure what you mean by
near-zero environment but I already can squeeze 1350 GH/s @ 412.5 by simply keeping it in my shed, with 1 S3 fan running at full pelt in a pull configuration.
tl'dr - dont go above 12V. its not really necessary as overclocking generally brings it to the thermal limit already, and increasing the voltage would radically increase w/gh
And here-in lies the real question. Would I be able to achieve a better hash-rate (and efficiency) if I over-volted and under-clocked? e.g 13.5v @ 375
there's no point in doing that. Your hashrate might stay about the same but your efficiency would go from 0.5w/gh to ~0.6w/gh, and likely exceed the current amount of cooling you provide. I personally was able to get a stock unit with 5C intake to max out around 412.5MHz (going to 425MHz resulted in hashrate loss). The best ive seen is guys in below-zero (celcius) garages/sheds getting to 425-437.5MHz (1.4TH) before anything higher is a hashrate dropoff. in both cases actual chip temperatures dont seem to be the limiting factor as long as its below 65C
so if you can get 412.5MHz at the most right now, supplying 13V might let you run at the same frequency and push out ~5% more hashrate at the expense of ~10% more power draw, which seems like a lot of work to squeeze out 70w/50gh more by running it outside of specs, not to mention that power efficiency on a 13V PSU is typically lower than a 12V-gold psu.
