Motherboards from that era were hardwired for a single clockspeed. You can clearly see a 66 MHz crystal installed there.
This is divided by 2 to generate the 33 MHz clock.
This crystal has to be replaced by an 80 MHz one to make the CPU run at 40 MHz (but I can't guarantee that it will work properly, the motherboard and chipset have to be designed for it).

It wasn't until the 486 era that chipsets would have integrated clock generators, and you could select which speed they generated with a few jumpers.

But if it was the 486 era that allowed it and the socket on my mobo clearly allows chip replacement, what would be the point? To swap an SX with a DX? So if I get the 80 MHz, should it work in theory?

Also I have a 40MHz 387 floating point unit on the way in the mail. Does this mean it won't work now because it's not synchronous with the CPU speed? Oh despair...

Boards at that time were usually sold with the CPU already installed by the vendor (and thus the correct oscillator) or in complete systems. while there was no easy upgrade path at that time, from a manufacturing standpoint it was easier to just produce a board that could be used for as many different chips and just let the CPU and oscillator be the variable parts, either populated by request or by the vendor themselves. Also, later there were "Overdrive" CPUs released that were specifically designed as upgrades for existing 386 CPUs.

BTW, you can't switch between 386 SX and DX, the difference is the DX has a full 32bit Bus, while the SX only has a 16bit one. Advantage of the SX was, that manufacturers could use 286 chipsets or complete boards with minor to no redesign.

The 387 will likely run at 33Mhz as well, I believe they derive the clock from the same oscillator.

Thank you.

Hmm.. perhaps I should stick with the 33 mhz combo for a while and maybe 1 day I can mess with the crystal.

BTW, were AMD and Intel chips clocked at the same speed identical in performance or was 1 superior to the other?

Changing the frequency of a 386

I had just been discussing this topic in another thread.

Oscillators are very inexpensive on eBay or elsewhere. You would want a DIP-14 one of twice the frequency at which the CPU operates. In your case, you would need to replace the 66.6666MHz oscillator with an 80.0000MHz oscillator.

If you are lucky, the oscillator will be socketed and it will be easy to swap.

Either that or I'll have to dig it out of the PCB 😉 Oh well, if it was easy there'd be no satisfaction in it, would there?