Overclocking Intel chips

General information
Overclocking is the setting of CPU speed above its rated speed. The speed of a CPU is set by multiplying the system speed known as the front side bus (FSB), by a set value called the multiplier, to create the CPU speed. To overclock, either the multiplier or the bus speed can be raised. Raising the multiplier only increases the CPU speed, but raising the bus speed means that the other parts of the system will also be overclocked. The memory is the main concern, since the other buses (pci, agp, isa and usb) all have changeable dividers to keep them close to normal speed. Above certain values, depending on the motherboard, no more dividers are available so then the other buses may not take it. For instance, on my system, after a bus speed of 133, there is no more dividers, so I am limited by the pci bus to about 145 (I have a lot of pci cards). Read theBios guide to find out your current bus speed is and what your memory can stand. The settings for adjusting the bus speed and multiplier can either be found on the motherboard itself or in the bios on better motherboards - check your motherboard manual for details. If you do not have one, find out what motherboard you have and then look it up on the web as they normally have the manuals online. Raising these settings rarely causes problems unless taken too far for the chip, which may result in no booting or windows instability. This may lead to data corruption, but most of the time resetting the bios or dropping back the settings will result in normal operation again. Hardware problems after overclocking are very rare. The CPU may stop working if taken far too far for the setup due to heat, or after years of operation, which is usually long after most people upgrade. Another problem is data corruption due to the hard drive controllers not taking high bus speeds. Some computers cannot take a bus speed of 83 due to the big difference in the pci and agp bus speeds for instance.
The big bane of overclocking is heat. Raising the speed increases the heat given out by a CPU at each step. Increasing the voltage exponentially increases the heat as well. A powerful heatsink and fan is necessary before attempting overclocking. I will not suggest which to buy, look at the pages on my links page for information.
The other settings that some motherboards provide are voltage adjustment. This is the only setting you should be careful with - if you adjust it to far, you could blow your chip! You should be able to overclock without using these, Increasing them may result in the ability to overclock further, or better stability.

5 rules for overclocking

1) never overclock too far - take it in small steps, checking that the system is stable at all steps (run a 3d benchmark or game)
2) keep an eye on temperatures (use motherboard monitor). Above about 53C is too high - get a new heatsink and fan. If your case temp is above about 35C, invest in some case fans
3) be very careful when raising the voltage. most boards limit the voltage, and for a very good reason !
4) do not overclock too far. If the system does not seem to be stable, do not go higher. Try raising the voltage (if possible) to stabilise its current speed. I know i basically already said this, but it is the most important rule
5) all chips and systems are not equal. Just because your friend/brother/etc got the same chip to xxxxMHz doesn't mean yours will automatically make it

For more information on CPUs, check out my links page, since this guide is purely about overclocking.
Intel specific information
All Intel chips have been locked since the early days of Pentium. Unlike AMD chips, it is not possible to unlock them again. Intel chips across a series are all made in the same way, and then sorted later by how high they can stand. For instance, if a PIII chip can't make a GHz stable, they may market it as a 933. The Intel testing procedure is very harsh and intensive, and most CPU's will never encounter anything that bad again. As a result, it is very possible that that 933 could run at a GHz stable on a normal desktop. Celeron chips are made from PIIIs that have faults - they simply cut out the bad bit and remark it as a Celeron. As a result, that chip may go a lot higher. PIIIs are comparable to Classic Athlons, and just below Thunderbirds. Celeron chips are not comparable to Durons however, due to their lower bus speed.
The older chip: the Pentium II
The PII only came in one format: the Slot 1 format, which was very different to all previous CPUs because it sat up from the board. Quite surprisingly, the old PII is actually a very good overclocker, particularly at low clockspeeds like 233-300MHz. It is quite possible on a good system with good cooling to get them up to a 133 bus. However, be careful with voltage adjustments as these old chips cannot take too much, and older boards often did not have a low enough limit. Performance increases from taking the chips from 66 or 100 to 133 are quite high due to the high system speed. However, these old boards do not have the dividers to keep the PCI and AGP buses in spec above 100, so you may find on some systems 133 is not possible.
The first overclocker friendly chip: the Celeron arrives
Celeron chips came out towards the end of the PII series. They were basically PIIs that could not stand the 100 bus, on the same slot 1 format or later the socket 370. The first Celerons came out with no l2 cache, and it was very easy to tell them to run on 100bus, giving them a massive increase. However, due to low performance, Intel released them with l2 cache intact. Due to the intensive testing that Intel employed, even these made it fine to 100 bus. The 300a clocked to 450 9 times out of 10 on most systems, and it even seemed to beat the PII at this speed. Since the PII cost about twice the price, many people chose to do this. This really started up the overclocking community. Due to being at 100bus, most systems ran fine at this overclock with no problems whatsoever. The later Celerons (400MHz or more) on the socket 370 interface (or slot one with adapter cards) were not as good for overclocking due to the limitation of the PII core of about 450-550MHz, meaning that the PCI and AGP buses had to run out of spec.
The PIII: Intels update for the PII
So that the clock speed could keep on rising, Intel released a chip called the PIII. Running cooler and at a lower voltage, It goes from 450MHz all the way to 1GHz, and was the first to reach it. It comes in two formats: the slot 1 on a 100bus, and the new socket 370 Fcpga, which is 100 or 133. The slot 1 chips were always a bit harder to overclock due to their higher voltage, but the real stars were the new fcpga chips. The best for overclocking are the newest core revision (check this site to see how to find which is which) of the 100 bus ones. Many 700-750 chips could easily make the 133 bus and nearly a GHz, plus they were a lot cheaper than the real GHz. You need a good cooling system to make it though. Most lesser clocked chips make about 200 MHz increase. The 133 bus chips are harder to overclock, because the pci and agp buses will have to be out of spec, and because of their lower multiplier the speed increase is a lot less. Increasing voltage is usually possible, and destroying the chip hard due to most boards being limited in their adjustments to avoid this.
The Celeron returns - 'Celeron2' in the fcpga format
As before with the PII, Celerons based on the PIII core were produced. Starting at 533MHz (beware: there is a 533 old Celeron too that does not overclock at all) and going all the way to 733MHz on a 66MHz bus, they overclock just as well as the older Celerons - from 66 to 100MHz bus. However, with these new Celerons, this means a much bigger increase in clock speed due to their larger multiplier. A 600MHz chip, for instance, will often go from 600 all the way to 900MHz ! Chips above 600MHz will probably not make the 100bus, so try and get chips below 600MHz if you intend to overclock them. The chips with the newest core revison (cc0 rather than cb0, check the intel site to see how to tell the difference) overclock easier and at lower voltage settings. There are also 800 and above Celerons on a 100bus - these only overclock as well as the PIIIs, and because of their halved cache and high price it is not really worth it. As with the PIIIs, increasing the voltage can be done, but all boards will not allow you to destroy your chip by it, unless your cooling is severely inadequate.
The new chip: the Pentium 4
The Pentium 4 is the latest chip from Intel. Like the AMD Athlon Thunderbird chips, it runs very hot so a very powerful cooling setup is needed to even use it, let alone overclock it. It basically runs on a 100bus, so overclocking the bus speed should not result in overclocking agp or pci buses too much. Internally this bus speed is quad speed, so the chip is still fast. From information off the web, it looks like a 200MHz increase should be possible on all chips, with 400MHz not too unlikely. With chips (1.3 to 2GHz) on the old style interface, overclocking is harder than on the new northwood cores (1.6-2.2Ghz) which run cooler. These northwood cores can get double the overclock of the older chips. Basically, the lower the speed, the more you can expect to get out of it. A few hundred MHz should be possible without adjusting the voltage, but after you adjust it you can expect to get more. Motherboard support for overclocking is improving now with the new DDR chipsets - the rambus based ones do not overclock well and lack many features neccesary for overclocking. It seems as long as you have all the adjusments (bus speed and voltage), a low speed chip (below 1.8), memory rated to work at 133 and a powerful heatsink and fan, you should be able to clock it to 133 bus and hence get a lot more MHz for you money.
Improvements to the PIII core
The change in the Pentium 4 core has also been applied to the PIII core, and therefore also the celeron. Overclocking these cores has not really been tested due to the high price an low availability.
Intel have also released the Celeron on this core. This has been tested a lot more, and some have got 50% overclocks on air cooling. testing is still inconclusive though, but do not expect to get much above 1.5GHz.
Last modified: 01/06/2005