by Anthony Olszewski

This material is presented only as a description of how I go about working on a system. It is not meant to encourage unqualified personnel to attempt computer repair. In any event, you assume all responsibility and liability for injury or damage.

Last modified:
The modification date refers only to this menu.

What is overclocking?
Overclocking is, by various means, coercing the CPU into running at a speed faster than that specified by the manufacturer.

Since there's nothing to lose, I might as well give it a try, right?
WRONG! OVERCLOCKING is, by definition, intentionally thwarting proper specs, as listed by the manufacturer. In the worst case, the microprocessor may immediately, and permanently, CEASE TO FUNCTION! In plain language, YOU CAN FRY YOUR CHIP! Such dramatic negative results, though not often encountered, are possible. Killing a CPU is generally the result of flagrant abuse - attempting to double a processor's speed, for example.

Other problems are less dramatic, but just as real. The overclocked CPU is sure to generate more heat than its more conservatively paced brethren. A heat sink and fan combo are definitely required for any overclocking project. A heat sink is a small set of metal fins, like the surface of a motorcycle engine, that fits onto the component. The conductivity of the metal and the extra surface area transfers the heat from the CPU and allows it to dissipate. Heat sink compound must be applied directly to the chip so that no air space is between the processor and the heat sink. Air is a very good insulator. An air gap will hinder the transmission of heat from the processor to the heat sink. The heat sink fans do wear out. This is particularly true in tower case systems for here the fan has to work against gravity. At any rate, DO peek inside from time to time to make sure that all is well. To really insure against anxiety and sleepless nights, get one of the special thermostats with an alarm to watch the temperature. For a really industrial strength overclocking operation, consider the special heat sinks available in electronic shops. The ultimate is a Peltier cooler -- truly a miniature refrigeration unit!

Why does overclocking work at all? How can just a few jumpers actually change the processor?
Ah, this is the heart of the matter! The different speed categories of CPUs are often all manufactured out of the same die! Intel, and other chip companies, then test the individual CPUs. If a chip fails at the highest rated speed, it is then tested at the next lower rated speed. If it works OK, it will be sold as that speed processor. If it again fails, it will again be tested, but at the next lower notch. Overclockers make two (not mutually exclusive assumptions). The first is that even though a chip failed for Intel, with extra cooling it may function at a higher rated speed. The second assumption is that since the manufacturers get more orders for the lower rated chips, the companies might sell faster chips at the slower rating, just to keep supply and demand in balance.

Alright, I'm crazy! I want to try it.
With new PENTIUM motherboards it's often very easy - just change the speed jumper. For example (with POWER OFF), if your processor is rated for 75 MHz., change the speed jumper from P75 to P90. See if it boots. Run some benchmarks and programs that you are familiar with.

to be continued!

CPU Clock Multipliers Table
BrandCPU name-
Rated Speed
Internal Speed
MultiplierClock (mem bus)
PCI bus
Intel/AMD/Cyrix 486 Class 486DX2-66662x3333



AMD Enhanced 486 Class 486DX4-1201203x4040??

486DX4-120100**2x50?2x mode (non-standard)
Intel Pentium Pentium-75751.5x5025







Pentium -180? 180 3x 60 30

Pentium 200? 200 3x 66 33
AMD 5x86 Amd5x86-1331334x3333P75+


Amd5x86-133150**3x5050*?n/a (non-standard)
Cyrix 5x86 Cx5x86-1001002x/3x



Cyrix 6x86 Cx6x86-1001002x5025*P120+



NexGen Nx586-80?information not available on this processor
** for overclocked modes - possible damage to the CPU!
Cyrix 486DX4, 5x86 and 6x86 have 2x/3x clock multiplier.
Amd 5x86 has 3x/4x clock multiplier.
PCI specs are for 33Mhz -- PCI overclocking to 40 or 50 Mhz. is not guaranteed
Three clocks (internal/memory/PCI bus) are only in Intel Pentiums and the Cyrix 6x86. In the rest the PCI clock rate, if not forced to 33 Mhz., is the same as the memory clock.

Possible 486 Overclocks
Processor Overclocked Setting Comment
i486dx33 40MHz
i486sx33 40MHz
486dx40 50MHz Possible VLB card problems
i486dx2/50 66MHz 33MHz external
i486dx2/66 80 MHz. 40 MHz. external
AMD486dx2/80 100MHz 50 MHz. external -- The 3.3v version is the best bet
i486dx4/75 100MHz 33MHz external
i486dx4/100 1 00MHz 50MHz external (using the 2x multipleir, instead of the 3x)
AMD486dx2/66 80MHZ 40MHz external
AMD486dx4/100 120MHz 40MHz external
AMD486dx4/120 150MHz 50MHz external (Many VLB cards and motherboards have problems with this speed)
AMD486dx4/133 160MHz 40MHz external (Cooling EXTREMELY important!)

(The 5.0v version is not a good candidate, but the 3.3v one manufactured after February is worth trying.)
*Early models of the AMD dx2/80 may work with a 3x multiplier. These chips would sometimes work at a 40 MHz. bus speed, producing 120 MHz. Apparently, later versions of the dx2/80 can't use the 3x multiplier. So, 120 MHz. operation is out of the question.

Overclock Speeds for Pentiums
Original Clock Speed Overclocked Speed
75 MHz 90 MHz, 100 MHz
90 MHz 100 MHz (120 MHz)
100 MHz 120MHz (133 MHz)
120 MHz 133 MHz
133 MHz (150 MHz or 166 MHz, only functions with CPU stepping 'C') 180 MHz, 200 MHz
150 MHz 166 MHz, 180 MHz, 200 MHz
166 MHz 180 MHz, 200 MHz
Stepping (along with voltage, timing and packaging) is one of the ways that Intel categorizes its products.

63MHz Intel Pentium Overdrive and 83MHz Intel Pentium Overdrive -- some people report success overclocking these chips. Others state that overclocking does not work and is likely to damage the processor.

The iCOMP (Intel COmparative Microprocessor Performance) Index

Intel's definition of their iCOMP rating system:

"The iCOMP provides a simple relative measure of Intel microprocessor performance. It is not a benchmark, but the results from industry-standard benchmarks measured on well-designed commercially-available systems rolled into a simple-to-use number. It is intended to help end users decide which Intel microprocessor best meets their desktop computing needs. The higher the iCOMP rating, the higher the relative performance of the microprocessor."

The Official Intel iCOMP Processor Scores
Processor iCOMP Score
486 SX 25 100
486 DX 25 122
486 SX 33 136
486 DX 33 166
486 SX2 50 180
486 DX2 50 231
486 DX2 66 297
486 DX4 75 319
486 DX4 100 435
Pentium 75 610
Pentium 90 735
Pentium 100 815
Pentium 120 1000
Pentium 133 1110
Pentium 150 1176
Pentium 166 1308

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