The best method for improving system performance through overclocking is to increase
the bus speed. If your motherboard doesn't support higher bus speeds or your RAM or any of
your PCI devices can't handle it, you can change the multiplier instead. Be aware however,
what you gain in speed you will lose in performance if you increase the multiplier but you
decrease the bus speed. For example, changing the running speed of a P54 from 166 @
2.5x66MHz to 180 @ 3x60MHz will actually degrade your overall performance. This can be bad
news for Cyrix/IBM 6x86 users, who should really only overclock their CPUs to a slightly
higher speed than the original. The 6x86 only has multiplier options for x2 and x3.
Believe me, I've tried every jumper setting on my FIC VA-503+ to no avail.
Changing the Bus Speed
To comprehend the methods by which you can overclock a Pentium, 6x86, or K5 processor, it helps to understand that their internal clocks run at a different speed than the external clock or bus speed. The external clock is the speed at which the cache and the main memory run and when divided by two (or three on a "Super 7 board) generates the speed of the PCI bus. There are only three different approved bus speeds used by the Pentium and the K5 processors - 50, 60 and 66Mhz. The 6x86 is sanctioned for five bus speeds: 50, 55, 60, 66, and 75MHz. Super 7 motherboards are capable of 60, 66, 75, 83, and 100MHz, with some capable of 90, 95, and 112MHz,
To change the bus speed, consult your motherboard manual for External (BUS) Frequency Selection. It will tell you which jumpers or dipswitches you will have to change to select the frequency you desire. If you are lucky and happen to have a motherboard with the new SoftMenu technology, you can change these settings in the BIOS setup. You don't even have to open the case.
It's probably best to increase the bus speed incrementally, one step at a time, until you can successfully boot the machine at the speed you desire.
Changing the Clock Multiplier
An internal clock multiplier in each CPU, which is programmed via CPU pins, controls the internal clock. Intel Pentium CPUs support the following multipliers: x1.5, x2, x2.5 and x3. Cyrix/IBM 6x86 CPUs so far only support x2 and x3, but the M2 will support x2, x2.5, x3, x3.5. The AMD K5 doesn't seem to be affected by the external settings of its multiplier. So far it only uses the x1.5 multiplier for each of the PR75, PR90, PR100, PR120, PR133 CPUs and the PR150 and PR166 K5 CPUs use the x2 multiplier. There are usually only two or three jumpers used to change these settings. Again, you can do this in the BIOS setup menu if you have a SoftMenu motherboard.
Changing the Processor Supply Voltage
Here's where I always get a little nervous, but it's something that is often required to make the processor run more reliably. Often it will be the only way to a successful boot of the machine.
If you aren't lucky enough to have obtained a SS7 motherboard and still want to attempt overclocking you old Pentium, 6x86, or K5 processor, often the change from STD to VRE voltage can lead to successful overclocking. This is because of a greater voltage difference between the digital HIGH and LOW conditions results in error free signals for the processor and chipset. If you can't run your CPU reliably at one particular clock speed, try changing to a higher supply voltage. The dielectric of STD and VRE processors is identical, so it damage your STD voltage CPU by running VRE voltage. The CPU will run a little hotter. So, be sure of your cooling unit.