Overclocking Guide

by Anand Lal Shimpi on April 30, 1997 7:24 AM EST
Overclocking - Now for the Kewl Stuff

With the release of the Cyrix 6x86 PR-200+ the 75MHz bus speed began appearing in motherboards first that officially supported it, and then in motherboards equipped with Intel chipsets which officially DON'T support bus speeds greater than 66MHz. As you may have figured out, this broadens the horizon for overclockers. Lets take that same Pentium Classic running at 150MHz (60 x 2.5) and put it on a board that supports the 75MHz bus speed. We now have the option of keeping the CPU speed at 150MHz but taking the bus speed up to 75MHz and the clock multiplier down to 2.0x. A 150MHz Pentium Classic running at (75 x 2.0) should yield greater overall system performance than a Pentium Classic running at 166MHz. And when doing this, you have less of a risk of damaging your CPU. With the advent of the 75MHz bus speed, the 83.3MHz bus speed was close behind. Take that same 150MHz Pentium Classic CPU and overclock it to 166MHz. This time instead of using a 66MHz bus speed and a 2.5x clock multiplier use a 83.3MHz bus speed and a 2.0x clock multiplier. While both settings would give you a Pentium 150 overclocked to 166MHz the one using the 83.3MHz bus speed would give you overall system performance equal to if not greater than a Pentium Classic 200!!!

Advanced Overclocking Table

Below is a list of the first and second options when performing an advanced overclocking of your CPU (NOTE: the AMD K5 PR/166+ runs using a 1.75x internal clock multiplier and the AMD K6 PR2/233 uses an internal 3.5x clock multiplier):

CPU Bus Speed Clock Multiplier Option 1 Option 2
AMD K5 PR/75+ @ 75MHz 50MHz 1.5x 75 x 1.5 = 113MHz 66 x 1.5 = 100MHz
AMD K5 PR/90+ @ 90MHz 60MHz 1.5x 75 x 1.5 = 113MHz 66 x 1.5 = 100MHz
AMD K5 PR/100+ @ 100MHz 66MHz 1.5x 75 x 1.5 = 113MHz N/A
AMD K5 PR/133+ @ 100MHz 66MHz 1.5x 75 x 1.5 = 113MHz N/A
AMD K5 PR/166+ @ ~116MHz 66MHz (Internal) 2.5x N/A 75 x 1.5 = 112.5MHz
AMD K6 PR2/166MHz 66MHz 2.5x 83 x 2.5 = 210MHz 83 x 2.0 = 166MHz
AMD K6 PR2/200MHz 66MHz 3.0x 83 x 2.5 = 210MHz 75 x 3.0 = 225MHz
AMD K6 PR2/233MHz 66MHz (Internal) 1.5x 83 x 3.0 = 250MHz 75 x 3.5x = 262.5MHz
Cyrix 6x86 PR90+ @ 80MHz 40MHz 2.0x 50 x 2.0 = 100MHz N/A
Cyrix 6x86 PR120+ @ 100MHz 50MHz 2.0x 55 x 2.0 = 110MHz N/A
Cyrix 6x86 PR133+ @ 110MHz 55MHz 2.0x 60 x 2.0 = 120MHz N/A
Cyrix 6x86 PR150+ @ 120MHz 60MHz 2.0x 66 x 2.0 = 133MHz N/A
Cyrix 6x86 PR166+ @ 133MHz 66MHz 2.0x 75 x 2.0 = 150MHz N/A
Cyrix 6x86 PR200+ @ 150MHz 75MHz 2.0x 83 x 2.0 = 166MHz N/A
Intel Pentium 100MHz 66MHz 1.5x 83 x 1.5 = 125MHz 75 x 1.5 = 112.5MHz
Intel Pentium 120MHz 60MHz 2.0x 83 x 1.5 = 125MHz 66 x 2.0 = 133MHz
Intel Pentium 133MHz 66MHz 2.0x 83 x 2.0 = 166MHz 75 x 2.0 = 150MHz
Intel Pentium 150MHz 60MHz 2.5x 83 x 2.0 = 166MHz 75 x 2.0 = 150MHz
Intel Pentium 166MHz 66MHz 2.5x 83 x 2.5 = 208.3MHz 83 x 2.0 = 166MHz
Intel Pentium 180MHz 60MHz 3.0x 83 x 2.5 = 208.3MHz 83 x 2.0 = 166MHz
Intel Pentium 200MHz 66MHz 3.0x 83 x 3.0 = 250MHz 83 x 2.5 = 210MHz
Intel Pentium 233MHz 66MHz 3.5x 83 x 3.0 = 250MHz N/A
Intel Pentium II - 233MHz 66MHz 3.5x 66 x 4.0 = 266MHz N/A

Overclocking - If You can't Stand the Heat, Get Out of the Kitchen...

As you might be able to tell, overclocking a CPU can yield a significant increase in heat production in addition to a speed increase. Therefore you need to make sure that you have adequate cooling for your case and overclocked CPU. If you are doing some severe overclocking or if you are overclocking a Cyrix/IBM 6x86 or AMD K5 (both of which run EXTREMELY hot in their unoverclocked state) a peltier cooler is almost a must. For those of you who don't know, a peltier cooler is a sort of mini refrigeration unit for your CPU, they have been known to decrease the heat levels of your processor tremendously. Their success rate truly justifies their cost which is normally in the range of $40 - $50. A tower case with a 2nd fan also is a great environment to be running an overclocked CPU in. Bottom line, make sure both your case and CPU have adequate cooling before attempting to overclock anything.


If you want a cheap and easy way to increase performance and aren't too concerned with the risks involved, overclocking is the right choice for you. Have fun with your newly overclocked CPU...

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