Ultimate Core i7 Overclocking Guide -- We Push Nehalem to its Limits

Here’s everything you need to know about overclocking Intel’s new CPU
Overclocking can kill your CPU. It can corrupt your OS, melt your motherboard, and cause you to lose a month’s work or more. Despite those dire orange-alert warnings, however, overclocking has moved on from the Nerd’s Only Club to become practically a mainstream hobby in the last few years.
So why overclock if the risks are so great? For some folks, it’s about bragging rights. Like drag-strip racers who burn up an engine just to set a quarter-mile record, there’s a small community who will overclock a CPU to the brink of destruction just to run a benchmark and take a screen shot of the result.
The bulk of overclockers, however, are more concerned with the cost dividends. If you can take a $300 CPU and make it as fast or faster than one that costs $1,000, the money you save can go toward other components in your system. For these folks, it’s like getting a free high-end videocard.
Whether you’re a cheapskate or a drag racer, you’ll find that Intel’s new Core i7 CPU is unlike any previous Intel CPU, and overclocking this beast requires more tinkering than you might expect. Follow along as we explore what it takes to push this chip hard.
A Brave New World -- The crumbling of the front-side-bus wall means a wholesale change in how you overclock Intel’s top parts
Veteran Intel overclockers know that little about the process changed since the beginning. Like the basics of flying (stick, rudder, ball), all you had to worry about with an Intel processor was the front-side bus, clock multiplier, and core voltage.
Not so with Core i7. With Intel retiring the front-side bus, you’ll need to brush up on your overclocking skills and concepts if you want to get the highest-performing overclock out of the Godzilla of CPUs.
The Base Clock
If you haven’t kept up on current events, you need to know that Intel eliminated the front-side-bus architecture that has connected the CPU to the core-logic chipset since 1978. The memory controller, which used to sit in the chipset, is now integrated directly into the CPU.
Go into the BIOS on a Core i7 and you won’t see any reference to the front-side bus. Instead, it’s now the base clock or bclock. Some BIOSes also refer to this as the host clock or reference clock. On the current i7 procs, the base clock is 133MHz. While it’s not a front-side bus, most overclocking methods will require that you tweak the base clock just as you did with older FSB-based Intel chips and push it beyond 133MHz. One important thing to remember about the base clock is that it is the main reference clock for other components in the CPU— goosing this one setting will also overclock the RAM as well as the “uncore” (i.e., the L3 cache, memory controller, and Quick Path Interconnect, or QPI). We’ll get to more on this shortly.
Doing the Math
One thing that has not changed at all is the multiplier. The Core i7-965 Extreme Edition features a multiplier of 24 and is unlocked so you can move it up or down. The budget Core i7-920 features a multiplier of 20 and is upwardly locked so it will not move past 20. Experienced overclockers can grab a hall pass and skip to the next section, while those who have never done this before will need to stay in the classroom. As it was with the Core 2 and Pentium 4, the overall clock speed of the CPU is derived by multiplying the base clock (formerly the FSB) by the multiplier. For the 965, take 24 and multiply it by 133 to get 3200MHz or 3.2GHz. For the 920, take your multiplier of 20 and multiply by 133 to get 2667MHz or 2.66GHz.
Turbo Talk
One wrinkle to the multiplier is the new Turbo Mode. This mode essentially automatically overclocks a single core of the CPU under certain loads. If you have overclocked a Core i7-920 to 3.66GHz and then you switch on Turbo Mode, the CPU will actually run at 4.03GHz in single-threaded apps. Is it worth it? Frankly, we’re not sure. We are getting to the point where it’s pretty rare to be running performance-intensive single-threaded applications, so the performance boost will be minimal. You do get a 1x multiplier boost in dual-threaded apps so you most games would run at 3.83GHz. Sounds good right?

Our engineering sample Core i7-920 gave us results in line with what others have achieved with engineering sample parts as well as retail parts.
Unfortunately you can’t set your individual Turbo Mode settings on the cheap chips. Intel limits fine-grain Turbo Mode control to the Core i7-965 Extreme Edition. The pedestrian Core i7-920 and Core i7-940 are limited to a single multiplier increase for single-threaded apps, which is of marginal usefulness. It’s also clear that not all motherboard vendors think Turbo Mode is worthwhile. We’ve tested two different Asus boards that don’t implement Turbo Mode the same way Intel does. Instead of letting the user set the individual Turbo Mode settings on an Extreme Edition chip, your only option is to overclock all cores simultaneously.
Turbo Mode is something that should be evaluated based on your needs and the specifics of your overclock. For example, our case study actually found that a moderate overclock with Turbo Mode gave us better benchmark results than a higher-speed overclock without Turbo Mode.
Uncore: Separate but Connected
The Core i7 is a modular design with two main areas, the “core” and the “uncore”. Inside the chip, the actual execution cores that do the heavy lifting are treated as the “core.” The other parts, such as the integrated memory controller, L3 cache, and the Quick Path Interconnect are treated as the “uncore.” Since they’re separate entities, you can overclock the execution cores without overclocking the uncore to the same degree, in theory. This should let you hit higher speeds, since you wouldn’t be running the QPI, memory controller, or L3 quite as hard. In reality, however, it doesn’t work that way. Intel’s non-Extreme Edition Core i7 CPUs offer limited control over the uncore multiplier, so a boost to the base clock boosts the uncore speeds as well.
You need to set your Uncore multiplier to at least twice the multiplier for the system RAM. Also pay attention to the memory voltage. We had to run 1.66 volts for stable performance even at low RAM clock speeds. QPI also needed to be nudged up to 1.3 volts.
One thing to remember as you fumble around the BIOS is that the uncore must run at twice the speed of the system RAM. Here’s where it gets a little confusing. The speed of the uncore is determined by multiplying the uncore multiplier by the base clock. On a Core i7-920 chip, for example, the uncore defaults to 16. The uncore thus is 16 times 133 for a total uncore speed of 2,133MHz or 2.1GHz.
To figure out the RAM speed, you have to take the memory multiplier and multiply it by the base clock. In the case of a Core i7-920 chip, the default memory multiplier is 8. So to determine the main memory speed, multiply 8 by 133 for 1,066MHz. Why aren’t higher DDR3 speeds available? The highest official memory speed of the Core i7 is DDR3/1066. You can overclock your RAM to higher speeds, but depending on the motherboard, the only way to accomplish a memory overclock will be to crank up the base clock for the CPU—unless you own an Extreme Edition CPU.
The take away here is to remember to keep the uncore speed at twice the speed the RAM runs. If you plan to run DDR3/1600, you’ll need to run the uncore at 3,200MHz. On a Core i7-965, you can run that speed without overclocking. On a Core i7-920, you’ll have to overclock the base clock to get the RAM at that speed.