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Motherboards could run only at bus speeds of 50, 60 and 66MHz; the speed selected depended on the CPU used with that board.

Now many boards use 75, 83, and 100MHz!. This allows for a safer method of 'overclocking' and again at no cost.

OC the BUS [1]

BUS Speed

If you increase the BUS speed you automatically increase the operating speed of some of the computer's components that are attached to the motherboard. Bus speed largely affects: system RAM, chipset, and PCI devices (especially the video card). With the BUS set to 75MHz the RAM is boosted by 12½% and video performance also improves.

However you must remember that the CPU speed comprised the BUS speed multiplied by the Clock setting. Therefore if you increase the BUS speed, you need to reduce the Clock (that is, if you wish to maintain the same CPU speed).

The speed of the PCI devices is set at half the BUS speed.

Remember: CPU speed = BUS Speed multiplied by Clock multiplier
Remember: PCI speed = BUS speed divided by 2


Again there are potential drawbacks. Your peripherals may not accept the higher bus speed (newer ones should - after all they are used in non-Intel PCs that normally have a bus speed of 75MHz!). However if you have some old components in there, then exercise caution - remove them, or don't overclock the BUS.

SDRAM is especially good at accepting a higher BUS speed. FPM and EDO RAM are not documented as accepting it, but experience shows they work fine in most cases. Tweaking the RAM settings is occasionally necessary (and worthwhile).

A cooling fan blowing directly over items such as the video card may be useful.

60MHz If your default BUS speed is set to 60MHz then you can easily change it to 66MHz and achieve a moderate speed gain from both the BUS and the CPU. Most CPUs with a default BUS of 60MHz will run without any problems at 66MHz - in some cases you will have to change the Voltage Regulator Jumper setting from Standard to VRE.

Effect on the CPU

Altering the BUS speed usually makes some change to the CPU speed inevitable. However a small CPU speed increase will make little difference to a CPU not already under stress.

A BUS speed increase may be the only method available to some users, especially those with some 133MHz and some newer PentiumIIs & IIIs

Should you wish to increase only the CPU, then try not to reduce the BUS. Doing so would reduce the operating speed of memory and some peripherals and be self-defeating.

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OC the BUS Example 1

Example 1 of Overclocking the BUS only

The original CPU and original motherboard are used.

This shows changing the BUS from 66 to 75MHz on a Pentium 166.
No risk of chip overheating - its speed has been reduced so there may be no gain!. If peripherals do not complain and do not overheat then it will probably be a faster system (depending on your peripherals).

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

Change the Bus to 75MHz, and change the Clock to 2,
creating an operating CPU speed of 150MHz (75 x 2=150)
It is difficult, in most cases, to keep the CPU speed exactly at its original speed.

OC the BUS Example 2

Example 2 of Overclocking the BUS only

This shows 166 to 166MHz! However the BUS is now at 83MHz
These changes can not damage your CPU which continues to operate at 166MHz.
These changes may damage some of your peripherals.

This would fly relative to a standard Pentium 166 if it works for you.

However some peripherals may complain, malfunction, or die. The system may freeze, you may need to tweak your memory, you may lose all your data. you may damage some peripherals (especially the video card - keep it cool).

It is possibly dangerous, but may work for you.

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

Change the Bus to 83MHz, and change the Clock to 2,
creating an operating CPU speed of 166MHz (83 x 2=166)

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Please remember that you alone are responsible for the consequences of any changes you make to your computer hardware or software.

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