Why would you want to do that? If your goal is getting a "Super XT" when it might be easier to buy an XT clone motherboard with NEC V20 or high speed 8088 (I've seen up to 12 MHz). Or even consider getting a 286 or 386 motherboard instead
Just replacing 8088 with V20 won't give much speedup (anything from 0% to 15%, depending on application)... the more plausible cause for replacement is that V20 supports 80186+ instructions, so that real mode 286 applications might work on it.
Overclocking an XT is a tricky business. Many things are tied to 14.31818 MHz crystal frequency:
- 8253 timer gets 1.19318 MHz (OSC/12)... it used for timing memory refresh, system clock and sound.
- CGA uses system's 14.31818 MHz for timing (MDA does not, but I've seen VGA that have OSC connected).
- 8237 DMA works with < 5 MHz frequency... no more. So if you overclock you can forget about DMA.
So just replacing the crystal is not a good idea, and a better solution is something like this:
- Keep existing 14.31818 MHz crystal
- Use an additional IC to generate clock for 8253 (might be another 8284 clock generator or 7492 divide-by-12 IC fed with OSC signal)
- Connect an oscillator with the frequency of x3 more of the desired CPU turbo frequency (e.g. 24 MHz for 8 MHz CPU) to EFI input of 8284
- Use F/C input of 8284 to switch between normal and turbo frequencies
- Use more additional ICs to monitor DMA requests (e.g. when DMA's HRQ signal goes active) - switch system to normal frequency when it happens.
During 80's a few overclocking boards were available that would do all that is described above. They normally were connected in place of system's 8284 chip (that needs to be unsoldered, and replaced with a socket), and required some additional wires to be connected to DMA or the bus arbitration circuit to monitor DMA requests.