Turns out I hit the exact same wall. I lowered the E6750's multiplier to 6X and was able to achieve a 490MHz bus speed with all-stock voltages. The system would not POST, though, with a 500MHz FSB. I tried raising the CPU, north bridge, and FSB termination voltages in the P5K Deluxe's BIOS, but nothing helped. This motherboardand perhaps the P35 north bridgeseems to have a ceiling of about 490MHz.
Those of you paying close attention may have realized that a CPU at a 6X multiplier on a 490MHz bus will be doing a very healthy 2.94GHzand as I said, our E6750 hit that speed at its stock voltage. That was only the beginning, though. After an epic, trial-and-error iterative process, I finally decided that the max stable clock speed for this E6750 is 3.64GHz, nearly a full gigahertz higher than stock. That's with a core voltage of 1.3875V and nothing fancier than regular old air cooling.
Throwing more voltage at the problem didn't produce any higher stable clock frequencies. I was able to get the CPU to POST and boot into Windows at up to 3.76GHz with 1.425V, but the system would crash pretty quickly after I kicked off any sort of CPU-intensive task. In fact, I had to switch from a stock Intel cooler to slightly beefier model in order to extract the last 40MHz from the E6750. With the stock cooler, the system wasn't 100% stable unless you dropped down to 3.6GHz.
But those are niggling details. We still got a near-1GHz overclock out of this thing, and we didn't have to resort to crazy-high voltages or heroic cooling measures in order to make it happen. That's a heckuva nice overclock from a relatively high speed grade CPU, and happily, its higher base FSB speed didn't get in the way; we still had a little bit of room left before hitting the motherboard's apparent bus speed ceiling.
Here's a quick look at performance with the E6750 clocked up to 3.64GHz.
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