Overclocking INTEL i7 920 processor: why not?

You should not overclock your CPU because:

• you void the warranty
• there is a risk to damage the CPU

But since I don't care about that and since I decided I needed some more juice to run the CPU-hungry Flight Simulator, I proceeded overclocking my wonderful CPU.

Basically, why should you overclock your CPU?

• first because it is fun
• second because modern CPUs have multiple cores but slow absolute CPU speed and very few applications take advantage of multiple cores
• third because you want to learn something more about your computers' internals

To achieve good results we need the following ingredients:

1. a CPU that is underpowered and therefore has a good margin of improvement. If you don't want to spend thousand of EUROs for a Intel Extreme edition, you can get a Intel i7 920 (Bloomfield), a great CPU at a fair price (~250Euros).
2. We need a good motherboard that will make it easy overclocking via the BIOS settings. My favourite is the ASUS P6T Deluxe v2 that costs roughly 300Euros. Expensive but worth the money.
3. Good DRAM modules (they make the difference in terms of stability). I got 6Gb (3x2Gb) of the Crucial Ballstix DDR3-1333Mhz 1.65v at 180Euros.
4. Cool cooler to keep your CPU temperature low: I got the ASUS Triton 88 for 50Euros.

Now that we have the hardware we can start thinking at the overclock: the default frequency of the Intel i7 920 cpu is 2.66Ghz; we want to bring that to at least 3.4Ghz gaining ~30% of CPU speed.

With an overclocked CPU it's crucial to keep its temperature under control: download the free Core Temp utility (google it) to monitor cpu temperature and set temperature warning limits. The CPU temperatre should be at around 65C as per Intel i7 920 specification.

The first test, before overclocking,  is to verify that the ASUS Triton 88 is doing a good job cooling the CPU: with the pc in idle (Windows XP loaded but no other program running) Core Temp reports 30C to 35C on each core. That is a great result, and is the confirmation that the CPU has room for improvement.

To overclock a processor few simple calculations are needed. We first calculate the BCLK (Base Clock) needed to achieve the desired speed. Since we would like a CPU speed of 3.4Ghz, the needed BCKL is:

BCKL = Target Speed / CPU Ratio = 3400 / 20 = 170.

The CPU Ratio is fixed at 20 in the i7 920.
We need to calculate the multiplier, that depends on the DRAM frequency. We got the 1333Mhz DRAM so the multiplier is:

Multiplier = DRAM Frequency / BCKL = 1333 / 170 = 7.8 =~ 8

We need to choose the closest selectable integer in the BIOS settings, in this case it is 8.
The new DRAM frequency is:

New DRAM Frequency = BCKL * Multiplier = 170 * 8 = 1360Mhz

The Uncore Frequency is:

UCKL = New DRAM Frequency * 2 = 1360 * 2 = 2720Mhz

Fianlly, the QPI Link Data Rate should be the lowest selecatble, in this case 6135MT/s; it can be also left to AUTO in the Bios settings.

Now we need to put these values into the BIOS to make it happen! Enter the BIOS and select the AI Tweaker tab and set:

• AI Overclocking Tuner   [Manual]
• CPU Ratio Setting          [20.0]
• BCKL Frequency          [170]
• PCIE Frequency            [100]
• DRAM Frequency         [DDR3-1363]
• UCKL Frequency          [2726Mhz]
• QPI Link Data Rate       [6135MT/s]
• leave everything else to [AUTO]

Reboot.
This should result in a stable 3.4Ghz CPU at 50/55C.

The i7 920 could be overclocked up to 4Ghz, and some overclockers report that they reached 6Ghz. To achieve these results you need to manually optimize the CPU Voltage finding the lowest possible voltage where 100% stability is achieved for at least a run overnight. That requires experience and there is a good chance to damage the CPU.

With a step by step approach, it was easy to achieve 4Ghz without any stability issue. Tip: I disabled the Hyper Threading functionality in the advanced BIOS settings to lower the temperature a bit.

Important: dear reader, this is not a tutorial on how to overclock the CPU. This is a note I took to keep track of what I did on my PC. If you follow this note you may damage your CPU. If you want, do it, but do it at your own risk: in other words don't blame me if something goes wrong!