I saw a reference to this site on Digg. Very interesting projects. It is impressive what you guys have achieved.
I know cooling is extremely important in overclocking, but I didn't see reference here to anything else. When you get up to 5,6, 7 GHz and beyond, you might be hitting limits that are not thermal, but related to signal integrity, timing, and propagation delay inside the CPU.
A 5GHz clock has a period of 200 picoseconds. Electrical signals in a chip travel at a rate of about 100 picoseconds per inch. So in that clock cycle, signals are traveling about 2 inches.
For the CPU to work correctly, internal clocks and other signals need to arrive at the right time, but they don't all travel the same path, or the same length of path, so you have
skew (difference in time between two signals arriving). Also, the other signals (address, data, and control signals) need to be stable some time before the clock (
setup time) and some time after the clock (
hold time). Since you need some time before and after each clock edge, that limits how short each clock cycle can be. (For example, if minimum setup is 100 picoseconds and minimum hold is 100 picoseconds, the clock cycle has to be at least 200 picoseconds, which is 5GHz). No matter how well you cool it, the chip just won't run any faster.
All chips are designed with some margin on timing. That's why overclocking works at all. But there is a limit. It varies slightly from one chip to the next, just due to manufacturing tolerances. So, even with perfect cooling, you might max out at 1.8X overclocking on one chip and get 2X on the next one of the same part number, from the same batch.
The guys who have achieved 8GHz certainly have extreme cooling, but they also "lucked out" and got a CPU with really good timing margins.
I didn't see any references here to these kinds of electrical issues, so I just thought I'd point it out. Apologies if it has been discussed before.
Just out of curiosity, has anyone tied just dunking the whole motherboard in liquid nitrogen? I don't think LN2 conducts electricity, so that should work.
Cheers, and good luck in all your efforts.
hwg