The SPD info for the G.Skill F1-4000USU-2GBHZ also reminds Corsair’s TWINX2048-4000PT.
The latencies and frequency written in the G.Skill modules’ SPD exactly comply with their specification.
Since the G.Skill F1-4000USU-2GBHZ is in fact an analog of the Corsair TWINX2048-4000PT kit, we had expected it to perform on a similar level. However, G.Skill’s kit proved its superiority at 2.6V voltage:
At the default timings of 3-4-4-10 the G.Skill F1-4000USU-2GBHZ reached a frequency of 270MHz (540MHz DDR), which is somewhat higher than the Corsair TWINX2048-4000PT kit could manage. Otherwise, the results of these two memory kits are identical. The G.Skill modules can’t work at all at latencies better than 2.5-3-3-10 and don’t support 250MHz frequency with timings other than 3-4-4-10.
The G.Skill F1-4000USU-2GBHZ memory also reacts predictably to changes in the supply voltage. At 2.8V the modules behave exactly as at the default voltage, but at higher voltages their characteristics degenerate suddenly. This memory can’t work at all at 3.0V voltage, whatever the memory bus frequency may be. We suppose this problem is caused by overheating of the memory chips the ordinary aluminum heatsink can’t cope with.
So, the G.Skill F1-4000USU-2GBHZ kit can be viewed as average-level DDR500 SDRAM modules capable of working at 250MHz (500MHz DDR) frequency with poor timings only. On the other hand, this memory overclocks well, which may come in handy for overclockers who prefer to speed up the memory bus in sync with the clock generator on their Athlon 64 systems.