Didn't buy it just yet... but was wondering if with additional 5V-to-3V adapter you could run any 3V 486 on a 386 board or is the design just to simple?

I'm pretty sure the design is limited to running 486SXL CPUs. For the most part, its a straight one-to-one pin mapping. The limited circuitry on the adapter is only for hardware flushing of the SXL chips (FLUSH# pin only used).

Very interesting read. I just picked up a IBM PS/2 Model P70 that I was wanting to see how much extra i could squeeze out of it. It came with a 386DX-20 and a 33mhz rated FPU. I snagged a TI 486DLC-33 for $10 off EBAY and put it in and was able to enable the 1k cache via software and it does run a bit faster but the onboard Xtal is a 40Mhz one so the new CPU is really only still running at 20Mhz and not 33. I ordered a DIP 14 crystal socket as well as a 66mhz crystal (for the 33mhz since it runs at half that speed) and plan on swapping it out to see if it would run at the full 33Mhz. While waiting for it to come in i was wondering what other possibilities would work, and did not realize there were this many different upgrades available for it. I wonder if i could overclock it to 40Mhz from the 33 next if the 33 works AT 33mhz and the rest of the system is stable. Here is how it came.

I don't have experience with IBM PS/2's, but I'm sure someone here does. Can the motherboard's PCB handle 33 and 40 Mhz reliably? If wait states are needed, does the BIOS have these options?

There should be plenty of TI 486DLC-40 chips around.

There are no options to change anything since it is a Microchannel bus machine. It has to load it's "Bios" from a floppy disk. It is all "Plug and pray that you have the disk for the card you just inserted" 🤣 thats why im getting the DIP 14 socket and installing it so if it complains ill just put the 40Mhz crystal back in. Even enabling the L1 cache and it working was a crapshoot. MCA bus runs at 10Mhz but im not sure if it is divided off the CPU crystal speed or if it has its own to drive the bus speed.

MCA is really uncommon; should make for a nice collectors item. I guess you could test and see what happens. I suppose you could probe the MCA slot with a scope to see what freq. it runs at with 66 and 80 MHz crystals installed.

I'm surprised your L1 cache works. Have you tried it in Windows 3.11? Some advice on L1 cache config settings is here: Register settings for various CPUs

Yes, everything works find with it on.

Well i socketed the crystal and then tried different ones but they all gave a 216 error on boot =/ 216 seems to be a memory related error. Only the original 40mhz crystal would boot.

So I guess the PCB design or chipset cannot handle faster front-side buses?

Well whats interesting is that they made both a 16 mhz version and a 20 mhz version so it must be able to take some differences.

Most (all?) chipsets cannot be run faster than some threshold. PCB design also takes into consideration operating frequency.

Looks like only decent upgrade will be a clock doubling one but those will be like striking gold.

I'll let you in on a little secret, the TI 486SXL will run at 2x if you limit the FSB to not greater than 20 MHz. So you can use a 40 MHz crystal and set the SXL into clock double mode. This will give you a 40 MHz upgrade. You can also run a 32 MHz crystal and get a 32 MHz SXL. Not that if you use a 50 MHz crystal (25 MHz FSB), the SXL will not clock double. In that case, you'd need an SXL2-50.

The clock single PGA132 SXL is a lot easier to find than the SXL2. Be warned, though, that there are also PGA168 versions of the SXL and SXL2. There is one on eBay now which is the PGA168 version.

and next you should say you have one of those Ti 486SXL's that will work in clock double mode with that 40mhz crystal i have that you could sell me, right? 🤣
🤣

I, unfortunately, only own one TI 486SXL-40 PGA132. If you put a wanted request on CPU-World.com, you might be able to get one. Also, if you diligently watch the listings for sale over the next year, I'm sure one will show up. If the wanted request fails to yield anything, take note of the top 10 sellers of CPUs on CPU-World and message them. I suspect the cost of one of these from CPU-World will be around $25 USD.