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Advanced users

It is better to overclock both CPU and BUS.

Unfortunately many users can not increase the BUS setting beyond 66MHz.

We will look at the BUS (next Page).

First look at how to OC the CPU.


Yes! Overclocking can work, and can be very effective. However it definitely is only for very advanced users who are competent in such matters, and are not greedy.

All CPUs have a range of speed in which they can operate safely. Manufacturers, especially Intel, opt for a speed quite lower than the safe maximum, and mark their chips accordingly. Thus they ensure market confidence in their product.

Intel do not produce a chip of a specific rating. They manufacture batches of chips based on a certain wafer thickness, and THEN determine the optimum speed of a particular production batch - based on a high level of reliability. The speed of electrical transmissions within the chip is determined by the wafer thickness - the thinner the wafer, then the faster the chip (and also the more heat produced!).

As the manufacturing process proceeds, they 'tweak' the manufacturing process to produce chips (same wafer thickness) that will run faster while still maintaining a high reliability e.g. a Pentium 90MHz initially, then a 100, a 133, a 166, and finally a 200 - all of these are basically the same chip (or wafer thickness).

Having achieved the maximum (practical) speed, Intel now change to thinner wafers in their chip production, and we have the arrival of a substantially different type of chip - the Pentium II & III. Heat production by these chip is an even greater problem because of the relative thinness of the wafers used.

As these chip are substantially different, they require a different motherboard, voltage, etc. Likewise higher performing ram/peripherals are needed in order to avail of the performance benefit offered by the higher level Pentiums.

Again the same manufacturing 'tweaking' process is initiated, and we get progressively faster Pentium IIs, then IIIs - all based on a new wafer thickness.

[You may be lucky! Your 166, for instance, may in fact be a 200 which has been marked as a 166 simply because there was a heavy demand for the 166 when the 200 was in full production, and the 166 in low supply!]

Overclocking Gain

The CPU speed is very important, but is only one of many factors responsible for overall system speed. These other factors include main memory type and speed, motherboard chipset and BUS, Level 1 & 2 caches, video chipset, hard disk, and so on.

Increasing the CPU speed will obviously produce a performance gain. However that CPU gain will produce a lesser overall system gain. For example a 166MHz overclocked by 20% (to 200MHz) will usually produce an overall system speed gain of less than about 8%, depending on some of the other components.

In general terms. to gain a twofold gain over a classic Pentium 166MHz, you would need to upgrade to about a PentiumII 450MHz ! Even then you would probably also have to upgrade to superior peripherals to achieve such an improvement. You should only expect moderate (to the next level) performance improvement when overclocking.

CPU Speed

The CPU's speed is not controlled by the CPU itself - its speed is set in the motherboard. Basically the motherboard's BUS speed is multiplied by a factor, and the result becomes the speed at which that CPU will run, or attempt to run.

BUSspeed x Clockmultiplier = CPUspeed

This factor (commonly called the Multiplier or Clock) is selected by changing a Jumper (a Jumper is a movable physical link with multiple settings options; it connects two selected pins to create a circuit that will have the desired effect). Some motherboards are now Jumperless and the equivalent settings are made in the BIOS.

Example: 60MHz (the BUS) x 2 (the Clock) = 120MHz
(Each and every CPU put in a motherboard with these settings will try to run at 120MHz - it will do so even if that CPU was designed to run at 60MHz or at 200MHz! - so exercise care when making changes)

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Enter the Overclocker

So the chip manufacturer builds a CPU, which will run best, longest and safest at a certain speed, and he (especially Intel) recommends a speed which will be well below the maximum safety limit. A life of about 10 - 15 years should be expected and it is likely that some will purr away for twenty.

The vendor selects motherboard Jumper settings that will give the CPU the power/speed recommended by the manufacturer. The vendor will also set the Voltage Regulator Jumper to ensure the optimal voltage is used for that chip.

There is a normal electrical degradation of a chip giving it a guaranteed life expectancy of about ten years (and possibly twenty). So, in practice, there is room for increasing the CPU speed, and gaining an immediate speed benefit without financial cost. Few of us would expect to be using the same chip ten years from now!

There are potential dangers. Increasing the speed to a worthwhile level is likely to immediately overheat (possibly fry), and degrade the chip with resulting permanent damage. At a minimum, the life expectancy of the chip is likely to be reduced, and immediate death is a possibility.

A prerequisite is the use of a comprehensive cooling method using an extra cooling fan and a large heat sink stuck to the CPU with a heat conducting material. Even then it is advisable to go for just a moderate increase, up to about 20%.

As there are many other factor involved in a system's speed, a 20% CPU increase will often give only about 7 - 10% gain in system performance (the same is true of ordinary chip upgrades). This would be useful, if quite moderate, on many systems.

Most overclocking is associated with a CPU speed increase of 10 - 20%.

Suitability for Overclocking

The Pentium appears to be very stable and tough, and is most suitable for overclocking.
Pentium133 (marked 'SY022' and 'SU073') are poor for overclocking.

Many of the Celeron range are highly overclockable.

Many of the newer PentiumIIs & IIIs have been disabled to prevent overclocking.

Cyrix 6x86's are known to be more easily fried by overclocking.

The official clock speeds of Cyrix, IBM and AMD CPUs are fairly close to their maximum clock speed. All these CPUs run very hot, much hotter than Pentiums. It is tough enough to cool the CPU properly at its original clock speed, much less than at overclocked speed. There are now new and 'cooler' 6x86 and K5 - K7 chips available.

If you are in any doubt, then you could just try it.

Those Jumpers

Remember: Three sets of Jumpers can be adjusted :

  the Bus, with settings of 50, 60, 66, (75, 83, 100) MHz
  the Clock (or Multiplier), with settings of x1 to x 3½ in steps of ½

  and sometimes
  the Voltage Regulator, usually with two settings; VRE or Standard, for Pentiums
    (be even more cautious if altering the VR beyond those two)

The Jumper settings for all three should be shown in your motherboard manual, or will be available at your motherboard manufacturer's Web site.
On jumperless boards the changes are made in the BIOS

Newer boards have 75, 83, or 100MHz BUS settings, and greater than x3 clock option.

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Examples of NORMAL motherboard set-up procedure

(A). A vendor wishing to insert a 133MHz CPU will make changes to the new motherboard Jumpers (note that 132 is read as 133 - the figures are often in fractions)

Set the Bus at 66MHz and set the clock at 2,
creating an operating CPU speed of 133MHz (66 x 2=133)

(B). This is an example of what the vendor will do when asked to upgrade a 133 to a 200MHz. He makes the following Bus and Multiplier changes on the original motherboard, and then inserts the new chip (a genuine 200MHz chip).
Original settings: Bus at 66MHz, and Clock at 2 (66 x 2=133, as above)

Leave the Bus at 66MHz, and change the Clock to 3,
creating an operating CPU speed of 200MHz (66 x 3=200), the manufacturer's rating

Its that simple - even if he charges you x$.

EXAMPLE of Overclocking the CPU

Overclocking ONLY the CPU speed:

Use the original CPU and original motherboard.
This shows converting a 166 to a 200MHz.
It gives a 20% CPU speed increase, but overall system improvement is 7-10%.
It carries some risk of chip damage - depends on each individual chip.
Additional CPU cooling is recommended for safety.

Original settings: Bus at 66MHz, and Clock at 2½
(66 x 2½=165, reported as 166MHz)

creating an operating CPU speed of 200MHz
(66 x 3=198, reported as 200MHz)

Note: PentiumII systems require dual voltage.

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HOW NOT TO increase only the CPU speed:


Sure, it runs (or briefly tries to run) your 166 as a 233MHz chip, but you are likely to FRY YOUR CPU. A 40% increase is far too dangerous.
Additional cooling is unlikely to save you.

Original settings: Bus at 66MHz, and Clock at 2½ (66 x 2½=166)

Leave the Bus at 66MHz, and change the Clock to 3½,
creating an operating CPU speed of 233MHz (66 x 3½=233) - and a fried chip!

It is vital to be sensible, cautious, and not greedy, when overclocking

Note: PentiumII systems require dual voltage.

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