I have mixed feelings about writing this paragraph. One one hand, I promised you information on this subject ages ago, but on the other hand, much of the information regarding the 6x86 M1 and the upcoming M2 is about to change. Nevertheless, I will refer to the good old, well known 6x86 and it's later stepping versions.
Due to the massive heat production of the older versions (steppings of less than 2.7) and the overly high heat production of even the latest versions, this CPU is not as flexible as the Pentium for overclocking. The first important thing to remember is that you are indeed able to kill your 6x86 with overclocking. I've never heard of an Intel CPU with this problem, not even a 486, but I have heard several stories of fried 6x86 CPUs. Hence I DO NOT recommend you overclock this CPU at all. The only reason I don't refrain from writing about the 6x86 is that I've been promising this information for a long time.
Overclocking the 6x86 is quite a bit more restrictive than overclocking a Pentium. This is mainly due to its heat production but also can be attributed to it's limited multiplier settings of x2 and x3. You can more or less forget about the x3 multiplier because the only scenario where it makes sense to use it is at 3 x 50 MHz. Due to the pathetically low bus speed, this is completely uninteresting in the way of performance. Hence this only leaves the x2 multiplier.
If you really want to overclock your 6x86, think small! Think in small steps!! It is worth considering just one step up. This means P120+ (100 MHz) to P133+ (110 MHz), P133+ (110 MHz) to P150+ (120 MHz) and P150+ (120 MHz) to P166+ (133 MHz). The step from the P166+ (133 MHz) to P200+ (150 MHz) seems to be too big and has a fairly low success rate with quite a high risk of losing the CPU.
You'll achieve the highest success rate with 2.7 or 3.7 stepping 6x86 CPUs because they run more stable and produce less heat. Cooling is paramount for the overclocking of a 6x86, so don't even think about overclocking this CPU without a HUGE heat sink or a power peltier.
I hope all this will all change with the release of the split voltage 6x86. This chip will be run at 2.8 V and should result in much less heat production. Maybe the 6x86 will suddenly turn into a really great overclocking CPU.
Overclocking the AMD K5
The K5 has been late in arriving, but is proving to be very powerful indeed. The latest performance figures show fairly nice results compared to the Pentium and the pricing of the K5 is just wonderful! 'How about the overclocking then?' I ask, 'after all AMD is the manufacturer of the most overclocked 486 CPUs in history'. My first overclocked CPU was an AMD 486/40 (to 50 MHz) then an AMD 486/100 (to 120 MHz) and everybody knows about the beautiful and cheap AMD 5x86-133, which just runs great at 160 MHz and still is faster than many P100 systems.
In my initial experience, the older PR75, PR90 and PR100 K5 CPUs are not great fellows for overclocking. Most of the time, the system would remain dead after moving up only one step and these versions share the same serious heat problem with the 6x86.
The PR120 and PR133, however, seem to be much better candidates. These two guys run at the same external and internal speed as the PR90 and PR100!! This proves that the chip inside has been improved and altered. Obviously these CPUs don't produce as much heat as the first incarnations, which is one of the first requirements for overclocking. The new PR150 and PR166 (at 120 and 133 MHz) seem to be of the same design.
Unfortunately I haven't got enough information to tell you anything decent about successful overclocking of the K5.
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