X-bit labs - Articles - Intel Pentium D 820 CPU Review (page 5): "The actual overclocking of Pentium D 820 processor started quite unexpectedly. We managed to raise the FSB frequency to 250MHz in no time and without any special effort. This brought the CPU clock rate up to 3.5GHz.
The system started and booted-up normally at this speed and allowed running all sorts of small utilities (including single-threaded ones) without any problems. However, when we took a closer look at the system functioning in these conditions, we saw that it could not provide fully-fledged functionality in this case. The Zalman CNPS7700Cu cooler couldn’t cope with its task when the CPU got loaded heavily.
The temperature kept growing and the CPU would shut down. Since Zalman CNPS7700Cu is a pretty powerful air cooler, the conclusion we can draw here is not very optimistic. You need an extremely efficient cooling solution, a water-cooling system, for instance, if you intend to try overclocking your dual-core processor. We still decided it would be interesting to find out what is the maximum frequency our processor can work at stably with the air cooler, because this type of cooling is currently used in most computer systems. We reduced the FSB frequency to 240MHz and continued out stability tests. The CPU was working at 3360MHz in this case.
The results obtained in this case turned out even more interesting. The thing is that the system remained stable, the CPU temperature stayed within the acceptable range, and all major tests would run smoothly. However, when we started a benchmark supporting multi-threading, some weird things began happening. The results appeared suspiciously low in this case. When we launched two independent copies of a single-threaded application (we used WinRAR archiving utility), we managed to reveal a very interesting effect: Pentium D cores can get into thermal throttling independently. In other words, the first core continued working at its no"