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A Good Specimen
The Core 2 Duo is an excellent processor for overclocking. Overall, the Core 2 Duo architecture is highly efficient in terms of energy consumption. This is important, because a processor that consumes more energy and uses it less efficiently generally produces more excess heat. That heat will have to be dissipated through the processor heatsink and by your case fans, but your cooling can only do so much. If there is less heat being generated by the processor in the first place, then overclocking is simpler.
Overclocking involves running parts out of manufacturer specification, and by definition two identically specced parts won't necessarily overclock identically. You can, however, hope that your product will be comparable to what others acheive with the same hardware. Let's lay down what you can expect from a Core 2 Duo. With a stock cooling solution, overclocks of 10% are easily possible. That will get a 3Ghz processor up to 3.3Ghz. If a nice, large cooler is added, then overclocks of 15-25% are obtainable. That puts our 3Ghz processor between 3.5Ghz and 3.8Ghz. Add an effective air cooling solution, including well laid out case fans and a massive tower cooler, or a water cooling set-up, and overclocks of 30% and beyond or obtainable. This means 3.9Ghz or more.
These are impressive results. A 10% overclock is enough to provide a noted performance increase in anything that requires a great deal of effort from the processor, while an overclock of 25% puts your processor into an entirely different category of performance. Best of all, this performance increase can be obtained with relative ease.
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What You Need
There is only one thing you absolutely must have if you want to overclock your Core 2 Duo - a motherboard with a BIOS that allows you to modify the speed of the Front Side Bus and the speed of your RAM. This is because the vast majority of processors have locked multipliers, which means that the only way of overclocking is to adjust the Front Side Bus.
The stock speed of the Front Side Bus will depend on the motherboard and the processor you are using, as not all Core 2 Duos use the same speed. However, the final speed of the processor generally depends on Mhz of the FBS times the Multiplier of your processor. For example, I have a Core 2 Duo E8400. On my Gigabyte board, the stock speed listed in BIOS is 333Mhz. The Multiplier on the processor is 9x. Therefor, the result is 2997 Mhz, which is rounded to 3Ghz.
Being able to adjust the speed of your RAM is also important, because the Front Side Bus speed also affects the speed of your RAM. Because of this, increasing the FSB speed without decreasing the speed of your RAM will likely demand it to operate at speeds it cannot sustain.
Besides having a Core 2 Duo processor (obviously) there are no other must-have requirements. Better fans and heatsinks will increase the maximum overclock obtainable, but are not required to overclock.
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Learning how to overclock a Core 2 Duo is easier than you'd think. In fact, provided you have the proper motherboard, overclocking can be done extremely quickly and with little prior knowledge of computer hardware. There are still risks involves, including the possibility of damaging your processor. But if you're willing to take a bit of a step on the wild side, overclocking your Core 2 Duo can result in an easy and free performance gain. Core 2 Duo, Core 2 Duo Overclocking, CPU Overclocking, BIOS
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Performing Your Overclock
Boot your computer into the BIOS. This can be accomplished by holding down a key (usually DEL) as the commputer boots. If your motherboard supports adjustments of the FSB, RAM, CPU, and etc, there should be a specific category in your BIOS which lists these options. Different motherboard companies use different names, so you'll need to consult your motherboard's manual, or simply explore your BIOS until you find the proper area.
Once you have come to the part of your BIOS that allows for overclocking, you should reduce the speed of your RAM, relative to the FSB. The easiest way to do this is, if your BIOS has this option, is to run the memory unlinked from the FSB. If your BIOS dosen't provide for this, you'll have to turn down the System Memory Multiplier. It is possible that your motherboard will list the same function differently; check your manual for details.
If you can run your RAM unlinked, you'll want to set the RAM to the speed at which you usually run it (if you don't know and can't find out from documentation it is usually listed on a sticker on the RAM itself). If you have to keep a multiplier, set it as low as possible. In my case, I had RAM that typically runs at 800Mhz. I set it down to 667Mhz, the lowest my motherboard would allow. As your increase the speed of your FSB, the RAM's speed will also begin to increase. Setting the memory multiplier low will help ensure that you're only worrying about overclocking your CPU, and not dragging your RAM along for the ride.
Having isolated the RAM, you are now going to change the speed of your FSB. Most likely, it is set to "Auto", or there is a frequency control option which is set as "Disabled". This is because your motherboard has been automatically determining how quick the FSB needs to be. Turn off the automatic control, and you should find a field with a number in it followed by Mhz. This will be titled CPU Host Frequency or similar.
Selecting this field will allow you to input the Mhz you want the FSB to run at. Remember, the frequency of the processor is determined also by a multiplier. If the multiplier is 9x, then increasing the FSB frequency by 10Mhz will increase your processor's frequency by 90Mhz. You want to start small. Increase the FSB by 5mhz. Save the settings in your BIOS, and reboot. Allow the computer to boot to your operating system.
Congratulations. You've performed your first overclock.
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Pushing The Overclock
Deciding how far to push your overclock depends mostly on how much effort you're willing to put in.
If you simply want to squeeze some more performance from your computer, the best way to proceed is usually to continue increasing the FSB in small amounts until you hit a reasonable target. A reasonable target would be 10% for stock cooling, 20% for an improved heatsink, and 30% for a water-cooled system. The fact that these are "reasonable" targets does not mean you'll be able to reach them, however. Each chip is unique in terms of its limits, and sometimes you simply find yourself with a chip that doesn't overclock well.
Alternatively, you can try and push your overclock as far as possible. If this is your desire, continue increasing your FSB in small amounts until your computer is not stable during its boot sequence. This may sound dangerous, and it can be - there is always a small risk related with pushing your processor beyond its manufacturer recommended limits. But for the most part, a computer that shuts itself down during a boot sequence due to processor overclocking will simply reboot into safe-mode settings. If it does not, and refuses to even boot to BIOS, then you can simply reset the CMOS jumper, allowing the BIOS to reset itself to normal settings.
Once you've found the limit of your overclock, you should determine if your RAM or your Voltage is holding you back. Finding RAM red-handed is easy. If your computer's RAM shows in BIOS as running beyond its stock speeds, and your computer is unstable, then it is very likely the RAM. The only exception to this is the case of RAM specifically built to be overclocked. Even then, RAM doesn't overclock to anywhere near the extent a CPU can. If the RAM does appear to be taxed beyond its limits, then your only real option is to buy RAM which is made to run more quickly, or a motherboard that can run the RAM and FSB clocks independently, provided that you followed the earlier instructions to set your RAM to as low a multiplier as possible.
If you RAM appears to be operating below its maximum frequency, however, then it is time to mess with your Voltage , often termed Vcore in BIOS. The voltage is essentially the amount of power being pumped into the processor. Often times, the limit on an overclock occurs when the speed at which the processor is running demands more power than the motherboard's BIOS is set up to provide. Increasing voltage increases that power, and can thus increase the ceiling of your overclock. If you want the absolute most out of an overclock, then increasing the voltage is often the way to go.
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Overclocking is not without its risks, even with a relatively easy to overclock processor like the Core 2 Duo. Whenever you take a piece of computer hardware past its recommended limitations, there is the possibility of damaging it. That said, overclocking in small steps can avoid most problems, and when you do run into an issue, resetting the BIOS often solves the problem. Core 2 Duo, Core 2 Duo Overclocking, CPU Overclocking, stability
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Danger! High Voltage
That said, increases in voltage must be performed with extreme caution. Processors can tolerate temperatures well beyond stock and still function. However, a processor which is over-volted will almost certainly have a reduced life span, or could even be fried instantly. If you're overclocking simply to increase performance, I would not suggest even messing with voltage.
However, if you simply want to overclock for the sake of it, for the thrill of seeing how far you can push a piece of hardware, then begin to increase the voltage. Increase the voltage in small steps, generally .025V or less. With each increase, reboot your PC and see if you can make it to your operating system without issue. If you can, then try and adjust the processor frequency to a higher amount. Once you begin crashing again, start raising the voltage. Continue to do this until you find that increasing the voltage does not allow you to increase the processor frequency.
Note, the limit to what a Core 2 Duo processor voltage can reasonably be increased depends on the kind of processor in question. Older 65nm Core 2 Duos aren't recommended to go higher than 1.5V, while newer, lower-power 45nm processors aren't recommended to go above 1.3625V. These are specifications set down by Intel. These limits are not a hard wall, and you could certainly increase the voltage much higher. However, the higher you raise the voltage, the more likely you are to run across reliability issues. Raise it high enough, and you'll likely damage your processor.
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When It All Goes Wrong
The most common problem you're going to run into when over-clocking is a hard limit in your processor or your motherboard which will result in an almost instantaneous shut-down of the computer. This most likely will result from an inability of the RAM to function at an overclocked speed or a lack of sufficient voltage. This can be a terrifying experience, at first, because your computer may not even stay on long enough to boot into BIOS. But never fear.
Open your motherboard manual and refer to the page about CMOS jumpers. A jumper is simply a physically movable connection on the motherboard. There will be a jumper location listed in your manual which will reset the BIOS. Place the jumper in the correct position for a reset, then wait about a minute. Then place the jumper back in the normal position. This will reset the BIOS.
Alternatively, if you can't find the jumper or don't know how to use it, you can simply remove the system battery from the motherboard. However, you'll have to keep the battery out for about 30 minutes before the BIOS resets.
Resetting BIOS will solve 95% of problems you experience. If, however, the system still does not boot, then there may be hardware damage. This is unfortunate, but can happen. Reset your BIOS, find alternative pieces of hardware, and place them into your system. If your system boots normally with a different processor, then the processor you were overclocking is probably fried.
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Long-Term Stability Check
The final step in testing your overclock is gauging the long-term stability of the overclock. This is generally done by using a program which is keeps all processor cores running at nearly 100% capacity for a long period of time. This sort of use will test the endurance of your computer at its new settings. My personal favorite program is OCCT.
Ideally, you'll want to set your computer to run the utility overnight. If your computer can make it through the night without crashing, than you have yourself a nice, stable overclock. In fact, you may even what to go higher, if doing so doesn't cause your computer to fail to boot to its operating system. If your computer crashes, however, then you'll need to back off. Back off in the same way you increased the overclock - in small amounts. Once you've found an overclock which allows you to run OCCT overnight without crashing, you've found your maximum overclock. Don't forget to see if you can increase your memory multiplier while keeping the RAM at a stable setting. Again, if the FSB and RAM clocks can be unlinked, then you don't have to worry about this.
Once you've found that maximum overclock, congratulations are again in order. You have successfully milked the most out of your Core 2 Duo, and in doing so you've officially become a computer geek. Don't worry - you need not change your behavior. Instead, rest well in the knowledge that you've made your computer faster without spending a dime.