Something I came across which made me cease interest in 266 MHz. http://www.cpu-world.com/forum/viewtopic.php?t=23838

On a side note, a long time ago i made the mistake of trying to overclock a 1M cache PPro on an Intel VS440FX motherboard.. It ran fine, until i ran Prime95 on it, then the VRM MOSFET exploded into shrapnel.. Hence why i'm using a 256K cache chip to do this little experiment. I might try a 512K L2 PPro, but a 1M cache chip is probably asking too much.

Might need to add a fan to the VRM heatsink with 1M at 233 MHz. Another comment that was interesting,

I would suspect that the 512k cache versions would be better since the sram was mfg at .35u rather than the .5u of the 256k cache.

Some statistics from another user,

Once upon a time, I gave all my 200MHz PPro's a run @ 266MHz. I had a dozen of 256K, 4 512K and 6 1M. I kept the voltage at the default 3.3V for a reason. One 256K could pass Memtest86+ and another 256K hanged up in the process. The other 10 were unable even to start it. Two 512K hanged up in the process, the other two didn't even make to it. None of the 1M could even POST.

And this poster had to add the missing capacitors to the top of the PPRO-256 to get stable operation at 266 MHz and 3.4 V. http://www.cpu-world.com/forum/viewtopic.php?p=263604#263604 He used 1uF caps from a dead s370 copermine.

My conclusion is that getting 266 MHz to run stable to the point of satisfaction (memtest, prime, etc) requires too much effort, especially for a VS440FX board (need to hack the multiplier jumpers; need to hack the VRM to get greater than 3.3 V; need to solder on 10 ceramic capacitors; need to get lucky).

Even with liquid nitrogen you can achieve only around 300mhz.

Well ,i run my ppros at 233 stable all day on my micronics w6-li mobo.I have 2 sa1 cpus and one sb1 .I managed to run these on my s1668 tyan mobo but i sold the board back in the days in favor for the micronics which has edo dimm ram sockets.

I could only run 233 MHz reliably on 512K or lower CPUs. Were you able to get 233 MHz out of a 1024K chip without increasing the voltage?

Without voltage, I never did.

The Serpent Rider wrote on 2019-10-21, 08:37:

Even with liquid nitrogen you can achieve only around 300mhz.

I vaguely recall a few PPRO boards with 70mhz, 75mhz,80 or even 83mhz FSBs (sometimes undocumented)

In theory you could then test some inbetween speeds like 240mhz , 245 or 250mhz assuming the FSB and cards are stable.