Since I've seen a good many questions regarding overclocking on the Intel Core i7 CPU, I decided to make a guide to somewhat help better the understanding of how it all works. Now I didn't write the entire thing, just more or less pieced information together from various sources on the net.
DISCLAIMER: Neither I or Tech-forums.net can be held responsible for you borking up your computer...there certainly are risks, so just be aware of them
Considering The New Stuff
Ever since the i7 was first mentioned the rumour machine about overclocking it has been spinning at full speed. The first rumour was that overclocking would be impossible because of the removal of the good old FSB, leaving the overclockers with nothing to increase. After the first rush of panic people started to actually think about what would replace the FSB: base clock (BCLK). Increasing this would increase the overall clock, right? Right. After the lifting of the NDA (and on some dubious sites before) the overclocking results were released. An average core i7 920 turned out to be able to overclock nearly a full gHz.
But removal of the FSB and the introduction of the BCLK means a lot more chances. Everything is linked to the BCLK, which has confused quite a lot of the old school overclockers. Just to sort that out and set new overclockers on the right path, just keep reading ^_^.
Now the i7 is a very hot processor, it has a TJmax around 100c. So don't get too worried if you are seeing idle temps in the 40-50c range and loads of 70c+...although i would hesitate with anything above 80c
To overclock properly you should first inform yourself about the risks and advantages of overclocking. I will not be writing this all out, but this is the short version:
- Shortened life of components when giving more volts than specified
- Extremely shortened life of components when giving an extreme voltage
- System instability
- You could contract the Megahurtz fever
- Additional heat output
- Higher FPS in CPU based games
- More F@H ppd
- Higher benchmark scores
Theory Behind the i7
The system behind the i7 is pretty easy to understand: you have a base clock (BCLK), to which everything is linked using multipliers. This base clock can be adjusted from 100 to 250 in steps of 1. Stock is 133.
The following multipliers are available:
- CPU. With the 920 this is 12-20x (or 21-22x with turbo mode)
- QPI link (36, 48, slow). The QPI isn't really important for single cpu systems, but keep it below 7-8gHz as most seem to be bugging out around that range.
- Memory (6, 8, 10, 12, 14, 16x). Well quite obvious. Make sure your ram is running as fast as possible but don't overdo it.
When pushing hardware over its rated specifications it will most likely need more voltage to run properly. The following major voltages are available on the i7:
- vCore. This is the juice going through the actual core. Stock: 1.18 Max. safe: 1.35 Max. extreme: somewhere around 1.55
- VTT (asus names it different). This voltage is going through the memory multiplier which is now integrated into the processor unit. Stock: 1.1-1.2 Max. safe: 1.35 Max. extreme: somewhere around 1.55
- vDIMM is going through the memory. Stock: 1.5-1.65 Max safe: 1.65 Max extreme: 1.5*VTT or VTT+0.7
As all experienced clockers know, Intel has some features built into their CPU's that lets them use less power when idling. Overclockers however, are not interested in that power consumption at all and those features go at the cost of stability.
The following features are recommended to be turned off:
- C1E state
Revived from the old P4 days is hyperThreading (HT). HT basically means that a core is pretending to be two cores. In the i7 series Intel decided to have the cpu act like an octo core while being quad. This feature could be very useful in truely multithreaded applications but seems to make the cpu require 0.05 additional vCore. As long as you are not an extreme clocker or single/dualthreaded applications (games) user I'd recommend keeping it on. Personally I have it turned off for my F@H SMP client.
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?
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.
What this means for overclockers is that while in full manual mode you can achieve a max multiplier of 20x. While if you leave the multiplier on auto, and turn turbo mode on the system will default to 21-22x multiplier based on temperatures.
Uncore (like Siamese twins...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.
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.
Another new element to the Core i7 is the Quick Path Interconnect(or QPI) . This high-speed interface connects the processor (or processors, in a multi-proc machine) to the chipset. Intel currently has two QPI speed iterations. The Core i7-965 Extreme Edition runs at 6.4 gigatransfers per second and the non-Extreme chips, such as the Core i7-920 and Core-i7-940, run at 4.8GT/s.
The QPI is important to watch because ramping it up too far can kill your overclock. For example, pushing the base clock from its stock 133MHz to 200MHz on a Core i7-920 means that the QPI will default to 7.2GT/s. That’s quite bit more speed than the stock 4.8GT/s; however, we successfully tested a Core i7-920 running at 7.2GT/s and believe that’s still within the realm of viability.
The QPI speed of the Core i7-920 and Core i7-940 is derived by multiplying the base clock (133) by 36, which equals 4788 or 4.8GT/s. The Core i7-965 uses a default QPI multiplier of 48, but unlike the non-Extreme chips, the 965’s multiplier is not locked. If you believe your overclock is failing because you’ve cranked the QPI too far, you can try dropping the speed by changing the QPI multiplier. On some overclocking runs with a Core i7-965 Extreme Edition, we had to drop the QPI back from 7.68GT/s to 7.04GT/s to increase reliability. The bad news is that you can’t do this with the budget chips.
You probably learned as a three-year-old not to mess with electricity after that incident with the wall socket. The dangers of electricity haven’t changed, but to wring the greatest clock speed out of your CPU, you’re going to need to overvolt the suckah. Actually, it won’t be just the CPU—our highest reliable overclock was only achieved by upping voltage to the CPU and parts of the chipset.
How much is too much? The default core voltage of the three current Core i7’s is 1.2 volts. We were able to push voltage to 1.5 on a budget Core i7-920 and successfully make it through some torture testing, but in our opinion, that’s probably too much juice (at least with air cooling). Sure, it ran our stress tests for a reasonable amount of time, but we don’t think the chip will live for long. A more reasonable voltage is probably 1.4 volts (just slightly more than the maximum allowable of 1.375), but take that with a grain of salt, too. Because your CPU, motherboard, cooling, and PSU will be different, the amount of voltage you can add will likely vary from our scenario.
Another area you’ll have to overvolt is the QPI interface. You’ll likely have to increase the voltage from its stock 1.1 to 1.3 to get a reliable overclock. Some folks recommend running QPI voltage at or above the CPU’s core voltage, but we didn’t have to on our budget chip as it was happy with 1.3 volts. Our Extreme Edition, however, needed 1.5 volts to the QPI, which was equal to the CPU voltage. Finally, you’ll have to add voltage to your RAM to get it to higher speeds. There has been chatter that a RAM voltage exceeding 1.65 can destroy a CPU— fortunately, most of the high-performance DDR3 binned for Core i7 doesn’t seem to need more than 1.66 volts to run, which is certainly within reasonably safe limits.
Now as stated earlier, the i7 is a really hot processor. If you really want to acheive any good overclocks you'll need at least a 120mm HSF for air (get around 3.8ghz max on a 920). Or a good water cooling loop (think 4.0ghz stable and suicides of 4.5-6)
If your head isn’t spinning by now, you’re in good shape, but most people, even somewhat seasoned overclockers, will want a stiff drink after trying to absorb this information. So let’s review.
You buy a new 2.66GHz Core i7-920—you just couldn’t splurge on the Core i7-965 Extreme Edition. You’ve got a reasonably chunky air cooler, a good-quality PSU, and you want to overclock that proc. What should you do? First, you need to start goosing up the base clock until you get a clock speed that seems reasonable for the CPU. So, let’s say you want to aim for a nice conservative 3.5GHz. Start by setting your base clock to a speed that will get your CPU in the region. Take it from 133MHz to, say, 160MHz. The target clock speed you’ll want is actually 175MHz for 3.5GHz, but we’ll start with 160MHz.
We’ve read posts on the Internet of some people being able to reach a high clock speed without the need for additional voltage to the CPU, so you’ll want to see if your CPU is capable of it. If you want a good real-world test, run your favorite multithreaded encoder, such as HandBrake. Nero’s Recode is also multithreaded and will put a reasonable load on the CPU. If you want to really torture it, download Prime95 from GIMPS Home
. Unzip it and run Prime95.exe. Select In-place FFTs and make sure the number of threads is set to eight. By default, it should be eight for a Core i7. If it runs for, say, half an hour, you can aim higher. Add 5MHz to your overclock and try again. Go until it fails.
Now it’s time to add voltage. A good starting point is 1.35 or 1.375 volts. Since 1.375 is the maximum allowable voltage under spec, you’re actually still playing it very safe. While you’re there, you may want to add voltage to the QPI. So nudge it up from 1.1 volts to 1.3 volts. Some say that the QPI voltage should at least equal the core voltage, but that’s up to you. You should also add voltage to the RAM to get it to spec. If you’re using RAM rated for higher speeds, give the RAM the amount the maker suggests. The safe limit seems to be 1.66 volts.
Intel says it locked the QPI and memory multipliers on its budget chips but some motherboards appear to circumvent this. If your board does this or you’re running an Extreme Edition, which has unlocked QPI and memory multipliers, you can try backing down your uncore and RAM multiplier. Most BIOSes should correct the ratios for you, but remember that the uncore multiplier should be twice that of the RAM. So if the RAM multiplier is set to 10, set the uncore multiplier to 20. If you back the RAM multiplier down to six, you can set your uncore multiplier to 12.
Now reboot and rerun your stress tests. Keep repeating the steps we’ve outlined until you hit your target or hit a wall. Once you’re at your target, do a longer stress test with Prime95 to see if it is truly stable. You may have to add cooling or voltage to get it to run for several hours.
Remember, cooling and quality power matter. If you think you can do all this with a cheap power supply and straw heatsink, think again.