Karbosguide.com - Module 3d.3

About Cooling and Over clocking (continued)

The contents:

Which CPUs can be over clocked?
Risks in over clocking?

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Which CPUs can be over clocked?

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The first CPUs which were dramatically over clocked were AMD's 5x86 series. That was a 486 CPU, which could be forced up to an excellent performance at 160 MHz.

Since then especially Intel's Pentium CPUs have been over clocked. Many of those seem to be sold with specs far from their optimum performance. Actually it was so easy that as a result many P133s were sold in 1996 as fake P166s. They worked fine, and the users did not know it. But Pentium MMX and Pentium II can also be re-clocked.

It appeared that Intel were aware of this activity, and they don't seem to care. Unfortunately their CPUs came in two groups:

Clock doubling works.

Clock doubling does not work - it is disabled by the manufacturer.

You cannot guarantee that it always will work. But let me show a couple of examples, which I have made work with good results:

CPU Manufacturers spec Tuning result

Intel Pentium 2½ X 60 MHz = 150 MHz 3 X 66 MHz = 200 MHz

Intel Pentium Pro 3 X 66 MHz = 200 MHz 3½ X 66 MHz = 233 MHz

Intel Pentium II 3½ X 66 MHz = 233 MHz 4 X 75 MHz = 300 MHz

Intel Pentium II 4½ X 100 MHz = 450 MHz 4½ X 117 MHz = 527 MHz

Looking at the three examples, number 1 and 3 show the best results, where both bus frequency and clock factors are increased. That simply moved the CPU up one class in performance.

Here is a table of the clock factors, which the CPU's theoretically can accept (according to my studies):

CPU Clock factor

Intel Pentium (P54C) 1½, 2, 2½, 3

Intel Pentium Pro 2½, 3, 3½, 4

Cyrix 6x86 2, 3

Cyrix 6x86MX (M2) 2, 2½, 3, 3½

Intel Pentium MMX (P55C) 2, 2½, 3, 3½

AMD K5 PR75 - PR133 1, 1½

AMD K5 PR150 and PR166 2

AMD K6-2 and K6-3 4, 4½, 5

Intel Pentium II, Celeron and Pentium III Up to 8 and 12 (latest models)

Some AMD and Cyrix chips were special, in that they did not always respond to motherboard settings. It is like they determined their own frequencies.

All modern Intel processors are locked at fixed clock factors (Multiplier Locking). They only operate with one specific multiply factor.

The Celeron

The original Celeron was a Pentium II without L2 cache. This CPU was very overclocking friendly. There are several reports about 300 MHz Celerons working at 504 MHz without any problems at all.

The Celeron A

The Intel Celeron line starting with models 300A and 333 (both with 128 KB L2 cache on-chip) are both protected against overclocking. They hold a "Multiplier Locking", which locks them to the clockfactors 4.5 and 5.0 respectively.

The Celeron 533 will only work with clockfactor 8, so if you want to overclock it, you have to go for a motherboard with adjustable system bus frequencies. This could be 8 X 100 MHz instead of 8 X 66 MHz increasing the CPU speed from 533 MHz to 800 MHz. Many users have found this in-expensive way to get a higher performance.

Disadvantages and risks in over-clocking?

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Many factors need to be considered, when you start tampering with these system settings. Watch out for:

Heat. Can the CPU dissipate the heat?

The L2 cache RAM of old Pentium II, III or Athlon cartridges - how fast can it work?

RAM speed. Can it keep up with the system bus?

The I/O bus. Can PCI and EIDE units keep up?

Will the software still work?

The last two problems are associated with increased system bus speed. This kind of over clocking gives the best results. However those also create the biggest problems, at least in my experience.

The CPU gets hot

First of all the higher CPU frequency causes more wear on the chip. It is said that a CPU can last 10 years. However do not count on that if you over clock it. Actually I am less concerned about the wear. Of course you should not allow the chip to over heat, but I have never heard about burnt out CPUs. In news groups you can read about various monster fans used for cooling of totally over clocked CPUs.

RAM speed

Another problem is in the relationship with the bus frequency. Here we are talking about the system bus, which connects RAM with the CPU. If you increase this speed, RAM must be able to keep up. Here is a guideline table for the maximum bus frequencies with different RAM types:

RAM type Speed Maximum bus frequency

FPM 60 ns 66 MHz

EDO 50 ns 75 MHz

SD 10 ns 100 MHz

SD 7 ns 133 MHz

Finally you could say that with cheap CPUs running at 900 MHz and above - you really do not need any overclocking. Most users will not experience any benefit from shifting from say 700 MHz to 1000 MHz.

Learn more
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See Module 3e - about the latest CPUs.

Read more about the boot process and system bus in Module 2b

CPU	Manufacturers spec	Tuning result
Intel Pentium	2½ X 60 MHz = 150 MHz	3 X 66 MHz = 200 MHz
Intel Pentium Pro	3 X 66 MHz = 200 MHz	3½ X 66 MHz = 233 MHz
Intel Pentium II	3½ X 66 MHz = 233 MHz	4 X 75 MHz = 300 MHz
Intel Pentium II	4½ X 100 MHz = 450 MHz	4½ X 117 MHz = 527 MHz

CPU	Clock factor
Intel Pentium (P54C)	1½, 2, 2½, 3
Intel Pentium Pro	2½, 3, 3½, 4
Cyrix 6x86	2, 3
Cyrix 6x86MX (M2)	2, 2½, 3, 3½
Intel Pentium MMX (P55C)	2, 2½, 3, 3½
AMD K5 PR75 - PR133	1, 1½
AMD K5 PR150 and PR166	2
AMD K6-2 and K6-3	4, 4½, 5
Intel Pentium II, Celeron and Pentium III	Up to 8 and 12 (latest models)

RAM type	Speed	Maximum bus frequency
FPM	60 ns	66 MHz
EDO	50 ns	75 MHz
SD	10 ns	100 MHz
SD	7 ns	133 MHz