Nintendo NES Overclocking Guide
I've perfected a process by which to overclock the Nintendo Entertainment System (NES) to run games smoother without slowdown. The NES CPU normally runs at 1.79 MHz, I've reached a stable maximum of 4.2 MHz, about a 230% overclock. The games do not run faster than they should, the CPU never overheats, and most games are perfect up to 3.3 MHz!

1.79 to 4.2MHz on air cooling
That's pretty impressive - more than double clock speed increase.

I wonder how far it could be pushed with heatsinks & active cooling. Time to being those finnish guys and their liquid nitrogen in, see if we can push it past 6MHz

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