Over the course of a weekend, we got all five CPUs tested, recording their top stable eight-core clock rate, single-core clock rate, maximum full-load temperature, peak single-core temp and the voltage required to get each sample stable for an hour of RealBench. Along the way, we took a number of interesting notes.
Let’s start with the big number—top clock rate with all eight cores fully utilized. In four out of five cases, we hit a ceiling at 4.5GHz. The fifth made it to 4.6GHz. That’s not to say those are absolute maximums, though. As Paul mentioned, a full load taxes the thermal solution’s ability to move heat away from the IHS. NZXT’s Kraken X41 is a great closed-loop cooler for around $110, but it’s not the largest or most aggressive of its kind. And even then, given the complexity of Haswell-E (a 2.6 billion-transistor die) compared to Haswell (at 1.6 billion, much of which is integrated graphics), 4.5GHz is a strong increase.
Working within the bounds of our cooler, we started tuning the other direction—find the top clock rate, flirting with voltages up to around 1.4V, and then back the voltage down to the edge of stability for a lower temperature.
Knowing that 80 degrees was where overclocking began to roll off, it became immediately apparent that 1.4V was too much for our cooling setup. So, most serious attempts started around 1.35V. From there, we’d back off in coarse .1V increments before adding voltage back in .025V steps at the first sign of instability. And although there’s no clear correlation between our top overclock and the voltage it took to get there, there is some consistency in the temperatures we saw.
Generally, the CPUs that were stable at 1.2V encountered the lowest temperatures. We’re reporting readings from the hottest core—meaning seven others ran cooler. Sample 5 looks like a sweet little piece of silicon. And although Sample 4 is the one that ran stably at 4.6GHz using a 1.3V Vcore setting, its hottest core is, well, five degrees hotter than what we see from any other processor.
Single-core overclocking was a little trickier, even though the Kraken had a lot less thermal energy to contend with. As you can see in the image above, Windows likes to schedule single-threaded workloads on different cores. So, your first five loops at 5GHz might be successful, only to crash on the sixth as a weaker core buckles under the load. Not surprisingly, then, it’s a real pain to loop something like our iTunes or LAME workload over and over looking for instability.
But after many hours of testing, we’re pretty confident that these chips could all take 4.8 and 4.9GHz across their eight cores, so long as only one was active at a time.
Depending on the core under load, temperatures were all over the place. We kept our eye out for the warmest-running one and made it representative of that CPU.