I have one that I tested for a day at 133mhz on a LS486E.
Voltage used before the long term testing was 5V, which was a bit heavy, so I "installed", a voltage regulator and fiddled around until I got it stable with "minimum" voltage.
The problem that I had was the ram and cache which would not play along at 66mhz fsb. So I had to disable the cache and turn down the ram settings one notch(fastest to faster).
The system, even without 2nd lvl cache and tamer ram timings, bites pretty hard - 64fps in Doom timedemo demo 3 and 97.8fps in 3DBench2.
Write speeds are between 120- and 140Mib/s.
It is also one of the 486 that manages to play demo4 of the Ultimate Doom above 35fps which normally requires a higher clockable AMD and cache at the fastest settings.
I'll put together a 120/40 system with cache and see how this one goes as I have found no scores for such a system in my notes.