Overclocking is basically where you run your CPU (or other components such as the graphics card) past the speed that they are rated at to get the most performance for your money. An example is running a 1400 MHz CPU at 1600 MHz at or a 200 MHz CPU at 233 MHz.
Picture: Core2Duo E6850 @ 4000MHz on air
The following description isn’t exact, but it gives an idea. Most CPU companies create their CPUs and then test them at a certain speed. The CPU is among a batch of usually 30 of a kind, just one of these CPU’s from the 30 odd is tested at it’s rated speed, i.e. AMD test a batch of 30 XP 2700+ Barton CPU’s at 13 * 166, with 1.65Volts. If this CPU fails to run stable it is downgraded to a lower rated CPU, note that it’s not always the same downgrade, sometimes they’ll downgrade them all to XP1700+’s and change their Vcore to 1.5 which decreases temperatures but allows people to snap these CPU’s up for very cheap and overclock them to great speeds. The tests are usually very stringent so a CPU may be able to run at the higher speed quite reliably. In fact, the tests are often not used at all. For example, once a company has been producing a certain CPU for a while, they have gotten the process down well enough that all the CPUs they make will run reliably at the highest speed the CPU is designed for. To, just to fill the demand for slower CPU’s, they will mark some of them as the slower CPUs. It isn’t just the CPU that can be overclocked, graphics card’s can be too. Overclocking a graphics card requires a 3rd party Windows program such as Powerstrip for nVidia card’s like the GeForce 4 4200TI, peformance increases as the GPU core and memory speed is raised. Many people do this just to gain a few extra 3d marks and to show off with benchmarks.
Why Overclock a PC?
What reasons are there for overclocking a PC you may wonder!
You may be one of these people who don’t fully understand overclocking and think that it is just a sure fire way of killing components by frying them this simply isn’t true.
Overclocking is a way of getting the maximum performance out of your system without having to spend money on a faster processor or more system memory.
If you know what you are doing it is very unlikely you will fry any component. Components need to be of excellent quality if you want to overclock as it can put a strain on them.
You will need a good motherboard that has a BIOS that supports changing the FSB and multiplier and the ability to adjust various voltage settings too. Overclocking with a PSU that is at its limits is not advised as it can easily push it over the limit and cause it to break. You need a good quality motherboard, PSU, good RAM and decent cooling for a decent overclock to be achieved.
A good motherboard is required due to cheap all in one motherboards that tend to have slower chipsets than ones costing more being more difficult to overclock due to limitations within their BIOS.
The most important component required when overclocking by far other than the CPU is the heatsink/fan unit. This needs to be top quality and the heatsink needs to dissipate the heat away from the core of the CPU efficiently and then the fan needs to take the heat away from the heatsink. The bigger the fan on the heatsink the better basically, the more air it pushes the better it will cool the CPU.
Take a look at the screenshot above, it pictures my Intel Core2Duo E6850 overclock on air cooling, CPU-Z is a very useful program for monitoring clock speed and RAM timings. The clock speed is deciphered from the multiplier (9x) multiplied by the bus speed (FSB – 456MHz) giving a CPU core speed of approximately 4104MHz.
The overall clock speed your CPU runs at is determined by calculating the FSB speed multiplied by the multiplier
For example an AMD Athlon XP 2100+ runs at 1733MHz (133 x 13) this could be overclocked by either raising the FSB or raising the multiplier. Generally best system performance is got by running as fastest FSB speed as possible and keeping the memory (RAM) speed in sync with the FSB (1:1 ratio) to do this the multiplier will usually have to be lowered.
On this E6850 (socket 775) Intel I run it at 3700MHz from 3000MHz (default FSB/Multi – 333×9) stock clock by raising the FSB to 411MHz and keeping the multiplier at 9x. I run my RAM, rated for PC6400 (800MHz) just over 800MHz by keeping the RAM and FSB in sync for best performance.
Overclocking can be acheived by raising the FSB (Front Side Bus) and on some older CPU’s, particually Socket A Throughbred CPUs by changing the multiplier.
I managed to overclock a 3500+ Venice (socket 939) to 3300MHz in the BIOS (Stock speed – 2200MHz) and to 3280MHz on a £3.50 heatsink/fan within Windows XP to achieve the world record on air for an AMD 3500+ Venice processor, nearly a 50% overclock – now its second place however.
I Managed to overclock my AMD X2 Dual Core 3800+ (socket 939) to 3217MHz X2 on air from its stock speed of 2000MHz X2: not any kind of record on air but its a 1217MHz overclock. At the moment I have a Intel Core2Duo E6850 Conroe CPU (3000MHz stock speed) that is overclocked to 3700MHz (which I type this from) with 6GB DDR2 RAM (2GB ULL + 4GB OCZ Gold) and a 500GB Samsung SP hard disk and some other bits cased in a Lian Li black midi-tower, but it used to be case less – see below. I have had it running SuperPI at 4104MHz, on air cooling and 4010MHz for benchmarking SuperPI. (12.8s 1MB SuperPI) This was with the RAM timings @ 4-4-4-12 @ 2.1v.
Terminology
- Vcore – voltage going to the CPU – do not exceed 1.40v for a 45nm Intel or 1.60v for a 65nm Intel on air cooling as you can fry the CPU easily
- Vdimm- voltage going to the Memory – most DDR2 can run at 2.1v even though the manufacturers say 1.8v on the label.
- Mch voltage - gives the chipset ability to get to higher FSBs
- HTT / LDT multiplier – Hypertransport – this is the speed on A64 systems the FSB is running at, do not exceed 1000MHz HTT on most boards, some older 939 have a 800MHz HTT limit
- Chipset voltage – voltage going to the northbridge – this can usually be set +0.1/+0.2 but when you start reaching +0.3v it can get hot so only raise this if you have to
Stability Testing
When you have an overclock you are happy with and want to test it is stable download Orthos and run it for an hour to see if its initially stable and keep an eye on the CPU temperature during this time as it will get to the hottest it will probably see when running this program. Orthos blend test is a good variation of testing the RAM and CPU for stability and making sure there is enough voltage going through the CPU to make it stable and that no corruption to the installation of OS should occur. If the system remains stable for an hour it is initially sound stable but to run further tests to make sure it is 100% stable run Orthos for a long period, such as 12 hours to make sure of absolute stability. Even if the system is slightly unstable this can result in lock-ups, random reboots and other unwanted occurrences. The best CPU-Z verification I have gotten so far is 4104MHz.
The following description isn’t exact, but it gives an idea. Most CPU companies create their CPUs and then test them at a certain speed. The CPU is among a batch of usually 30 of a kind, just one of these CPU’s from the 30 odd is tested at it’s rated speed, i.e. AMD test a batch of 30 XP 2700+ Barton CPU’s at 13 * 166, with 1.65Volts. If this CPU fails to run stable it is downgraded to a lower rated CPU, note that it’s not always the same downgrade, sometimes they’ll downgrade them all to XP1700+’s and change their Vcore to 1.5 which decreases temperatures but allows people to snap these CPU’s up for very cheap and overclock them to great speeds. The tests are usually very stringent so a CPU may be able to run at the higher speed quite reliably. In fact, the tests are often not used at all. For example, once a company has been producing a certain CPU for a while, they have gotten the process down well enough that all the CPUs they make will run reliably at the highest speed the CPU is designed for. To, just to fill the demand for slower CPU’s, they will mark some of them as the slower CPUs. It isn’t just the CPU that can be overclocked, graphics card’s can be too. Overclocking a graphics card requires a 3rd party Windows program such as Powerstrip for nVidia card’s like the GeForce 4 4200TI, peformance increases as the GPU core and memory speed is raised. Many people do this just to gain a few extra 3d marks and to show off with benchmarks.
Dangers from Overclocking
If you know what your doing it’s unlikely that any components will be damaged unless you’re very unlucky. The most likely way the CPU will get fried is if you put too much voltage through the chip (more than 2.0 V for a T’bred XP isn’t adviced, even with a Vapochill/Watercooling) these kind of voltages shouldn’t be needed if you want a mild overclock. The thing to remember is not to go for a huge overclock to start with, gradually increasing the FSB with the stock multiplier is adviced, not one big leap at once, this could put too much strain on the CPU and cause it to fail. The latest nForce 2 motherboards have an option which allows you to lock the PCI/AGP bus to 33.3/66.6 MHz, which means none of the PCI/AGP or any devices on the IDE controllers will run out of spec. With a mobo that doesn’t allow the PCI/AGP bus speed to be locked, another element of difficulty is added to overclocking so you have to aim to get the FSB to a speed that keeps the PCI/AGP at it’s rated speed. Then comes dividers. These work on boards with no PCI/AGP lock. For example a 1/4 divider would be used when using a 133.3 MHz FSB as 133.3/4 = 33.3 and a 1/5 divider would be used when running a 166 MHz FSB. Most PCI devices can run at upto 38 MHz but there is a great risk or coruption to the HDD, which means if your unlucky with the overclock you’ll need to reinstall Windows.
How to Overclock
You work with the FSB and multiplier to aim either to:
Increase outright speed of the CPU, by keeping multiplier at what it’s meant to be or higher AND raising the FSB higher than stock
Increase memory bandwidth greatly by using a high FSB and low multiplier, this works well if you have a poor clocking CPU and good RAM. The clock speed of a CPU is calculated by multiplying the multiplier by the FSB, for example my XP2100 runs on a multiplier of 13 and an FSB of 133.3 which is double pumped but that adds complications which gives a clock speed of 1733 MHz. It gets the XP2100 name because AMD reckon it would take an original Athlon T-bird to run at 2100 MHz in order to compare performance wise to the XP2100 @ 1733. Many people believe that AMD are trying to compare it to the P4 in their naming but this isn’t true. Most CPU’s in the past have been multiplier locked, which means to change the multiplier you need to physically unlock the CPU by joining the L1 bridges together for Durons & T-birds. Note that any Intel from the PII upto the PIV have a locked multiplier and there is no known way to unlock them. If your motherboard has any options to overclock they will be in the BIOS, some cheap board’s like mine don’t allow any overclocking, and some older board’s worked mainly with jumpers to overclock. I’ll add more to this soon.
Overclocking AMD 64 CPUs
I’ll start with the principal of overclocking these processors and the motherboards and RAM that go with them. The thing you have to watch when overclocking Socket 939 CPU’s is the HT Link Speed which always has to be kept under 1000MHz…once this goes above 1000MHz it can cause instabilities throughout. The speed is worked out by multiplying the HTT (used to be known as the FSB, front side bus) by the HT Link Multiplier (both of which are in the BIOS, under the tweaking section although this is different for different makes of motherboards) keep the ht link multiplier on 4x until you reach a 250MHz HTT – 250 x 4 = 1000..once over 250MHz HTT bring it down to 3x to keep the system stable. Now onto the RAM, unless you have good RAM (i.e BH5/TCCD Samsung) or other RAM made for running at faster than the default FSB of these CPU’s (200MHz / DDR400) then you will need to be using a RAM divider whilst overclocking the processor. If you set the RAM divider to 166MHz and you’re RAM is rated at 200MHz and you don’t wish it to run over 200MHz you can goto to a max HTT of roughly 230MHz before the RAM creeps over the 200MHz mark – it shouldn’t matter if the RAM is slightly overclocked anyway by a few MHz, if you get instabilities simply up the VDIMM (Memory voltage a knotch, or two if one isn’t enough) 2.9v is safe for all modules really and their default is usually 2.5v/2.6v. Say you have an AMD Athlon 64 X2 3800+ Toledo processor that you wish to overclock on a motherboard such as the DFI Lanparty Ultra D, then try setting the HTT to 210MHz to start with, leaving the vcore at default, then check for stability using SuperPI – it is very likely to be stable with such a low overclock providing you are using the 166 RAM divider…then try 220MHz HTT (making sure the HTT link multi is set to 4x because we’re over 200MHz HTT here, right. I ran my x2 3800+ at 2750MHz on stock voltage (CPU voltage = 1.374) and it was stable, I used the 166 RAM divider because my Value PC3200 could do 229.3MHz 24/7 with no issues with the RAM set to 2.8v in the BIOS. All other voltages such as the chipset voltage were left at default and the cooling was an Artic cooling freezer 64 pro with a 80mm fan screwed onto it. Once you are happy with overclocking in the BIOS or if you want to overclock within the operating system, namely Windows as Linux/OSX doesn’t have support for the ClockGen program yet or for that matter, any other overclocking software that uses the PLL to change the frequencies. Here’s a link to ClockGen.
How to overclock socket 775 Processors – Exxxx / Qxxxx – dual and quad core
Much of the same applies as with the AMD Athlon XP text, keep the FSB and RAM in sync while you can, it helps performance very much on this platform but running the FSB:RAM async on the AMD 64 platform didn’t hurt performance much. The main thing different is all of the features in the BIOS that you are greeted with that are not required if overclocking and sometimes having these enabled can cause the overclock to go belly up so disable all of them including C1E (enhanced halt state), EIST etc because these settings cause the CPU to use a lower multiplier and lower voltage when the processor isn’t Memory timings with DDR2 doesn’t seem to make much difference to memory bandwidth or benchmarks so it is very hard to notice much difference from using slack timings. Running the memory faster than the FSB doesn’t even hurt the performance, although it barely makes the computer benchmark better in my experience with LGA775 CPUs and DDR2 RAM.
How to overclock socket AM2/AM3 Processors – Athlon II, Phenom, dual,tri,quad core processors
Much the same as overclocking socket 939 CPUs, keeping the RAM in sync with the FSB on a 1:1 ration is best for overall system performance as it creates more memory bandwidth. With certain dual core AM2 Phenom CPUs, there is a chance you can unlock cores to make your dual core processor into a quad core one! to do this you need a motherboard with a BIOS that is capable of unlocking the cores as well as the right CPU. I won’t say much more about making a dual core into a quad core because it depends on many factors and what I write now will probably be wrong in a few weeks but I recommend reading or posting on forums such as overclockers UK, PC Perspective or Tom’s Hardware for the best combination of CPU and motherboard for unlocking the extra cores.
How to overclock socket 1155 i5 series processors
These processors were introduced in January 2011 and there are type types of i5 CPU; k and non k with the latter not being able to overclock much if at all.
Overclocking the i5 ‘k’ series processor involves using it’s unlocked multiplier to manipulate the clock speed. You can also change the blck (baseclock – fsb) but most socket 1155 motherboards will not do over just 107MHz, and overclocking the baseclock on i5′s can also deteriorate the life of your processor.
The best way to overclock the ‘k’ series i5 is to change the multiplier from 33x to 40x to start with to give yourself a 4000MHz clock speed. Most of these CPUs will do this on the default vcore so leave that alone for now.
Run Prime95 or Orthos for a few hours to test for stability then try upping the multiplier 1x at a time (leaving the baseclock at 100MHz) and again test for stability.
I find my i5 2500k requires the vcore upping to 1.35v to achieve a stable 4500MHz, this can be done with a moderate to good air cooler and still get you reasonable temperatures under CPU load.
Maximum clock from the i5 2500k varies greatly depending on your luck but the vast majority will clock to 4500 – 4700MHz on air cooling with 1.35v.
I don’t advice using more than 1.4v on air as your processor will not live as along as well as the temperatures getting too hot. The maximum temperature these CPU’s can run at is 98ºC but I don’t recommend letting them go above 80ºC.
The maximum clock speed I have completed a 1MB run of SuperPI at is 5154MHz using a thermalright ultra 120 extreme (1156 bolt thru kit) but this was not stable for everyday use and even 5GHz requires too much voltage to keep the temperatures down for 24/7 use so I keep mine at a mind overclock of 4500MHz.
* Disclaimer: I take no responsibility for fried processors or damaged computer components from people using this guide to overclock. Do so at your own risk.
