I remember overclocking the 486DX2-66 in the early 90's. I got the idea after reading my brother's Intel data book and noticed that while the max clock speed was speced at 66 MHz, all of the timing diagrams implied it could run to 80. I borrowed a variable speed clock generator and sure enough it was stable at 80, and started to crash at around 82MHz.
When I started to help friends overclock theirs, I quickly realized the "silicon lottery" variance. Some would only run reliably at 78 or 76 MHz. I bought a bunch of fixed frequency clock generators (that were drop-in replacements for the original on the motherboard) in 2MHz increments due to the variance.
This was back before CPU's had heat sinks or fans, so we quickly figured out that adding those gave better margins. We even made some 10-LED bar temperature display that had a thermocouple glued to the CPU case and indicated 10 degree C increments (green=0-60c, yellow=70-80c, red=90-100c).
I remember overclocking my calculator (TI-85, 1992, Z80 CPU) ... its LO was a 2.7K/22pF RC oscillator which gave it an approximately 2.5MHz clock. To get this type of oscillator to speed up you'd normally lower the capacitance a bit.
The reason that this story is interesting is that in most cases you could just yank C9 entirely and with nothing more than a resistor between the clock pins, you'd get a roughly 300% performance increase. I guess the parasitic capacitance was enough to still oscillate a bit although mostly it would have been random. Looking back, this was basically a CPU being clocked with 50mhz noise and still running happily! Amazing!
Not quite the same but I once was bored enough to keep trying to see how low power a solar powered calculator could work with.
I held my hand over the solar panel at various lengths until the screen cut out and while doing this I just kept hitting random keys and while lifting my hand up.
One day when I did this I must have hit the one in a million chance. It started rapidly counting up by itself!
I think I only got it to happen once more. I suspect the fluctuating voltage and it trying to do calculations while I was pressing keys was just enough to get some gates latched into the wrong state, somehow.
Good memories... I ended up adding a switch under the cells cover because the mod was just draining these way faster. But curve plotting was finally snappy.
That was before binning really got to be a business model. Of course once a production line was stable, they could generate more high end chips than they actually needed, and so the chip you bought from bin 3 might actually be a bin 2 chip. It always seemed like AMD was really conservative that way, which is why hobbyists loved them.
I have a recollection of a guy who got a 486 DX-33 up to 133 MHz by putting the entire computer in mineral oil and floating chunks of dry ice in it. Watch out for asphyxiation.
When I started to help friends overclock theirs, I quickly realized the "silicon lottery" variance. Some would only run reliably at 78 or 76 MHz. I bought a bunch of fixed frequency clock generators (that were drop-in replacements for the original on the motherboard) in 2MHz increments due to the variance.
This was back before CPU's had heat sinks or fans, so we quickly figured out that adding those gave better margins. We even made some 10-LED bar temperature display that had a thermocouple glued to the CPU case and indicated 10 degree C increments (green=0-60c, yellow=70-80c, red=90-100c).