I have actually seen a photo of an adapter that has an am386DX-40 that plugs into a PGA-168. I think it was posted on VOGONS in the last year. That adapter is made for a specific motherboard though, and I'm pretty sure you'd need to change a few jumpers to get it going. I believe the chipset was in the OPTi 495 family (a hybrid chipset).
Any kind of passive adapter isn't going to work on a regular 486 board.

I'm not so sure. I have a hybrid board (that doesn't work unfortunately) and it seems that all the pins that exist on the 386 side also exist and in fact are connected to the 486 side. I think the 386dx shares a lot in common with the 486. This needs more testing, but I'm really curious about this. Running a 386 on a late 486 board could bring some interesting results and perhaps let people do some crazy overclocking (66MHz 386DX on a PCI board with Edo ram ?)

TI486SXL (a 386 bus compatible CPU) comes in both 132 and 168pin packages. The two chips are functionally identical, with pretty much all of the extra 36 pins not connected to anything. The 168-pin version is not compatible with standard 486 sockets though, and requires a special board to get it running. I would guess if it could have been made a drop in part, TI definitely would have gone that route.

Anonymous Coward wrote on 2022-03-21, 14:43:

TI486SXL (a 386 bus compatible CPU) comes in both 132 and 168pin packages. The two chips are functionally identical, with pretty much all of the extra 36 pins not connected to anything. The 168-pin version is not compatible with standard 486 sockets though, and requires a special board to get it running. I would guess if it could have been made a drop in part, TI definitely would have gone that route.

Pipelining, burst, sequential read/write, cache, multiplier pin, bus cycle, timing
Are all different or not present on a 386, unless the chipset is designed for the slower 386 bus it’s unlikely you could get a 486 to communicate with a 386.

http://www.pchardwarelinks.com/486pin.htm

No that's the opposite we want to do. Run a 386DX in a 486DX motherboard, not run a 486 in a 386DX board.

Why not track down one of those hybrid 3/486 motherboards that supports both CPU's?
You can then swop out the CPU's as you desire.

I have two of these type of motherboards and most of these were based around the OPTI495 chipset.

I have this adapter, I can't tell if it will only work on specific boards/chipsets though as I have not yed had time to try it out.
the way it's constructed it doesn't fit on a lot of boards though.
but seeing that such a thing clearly exists, the chances of success are there ^^

I would say this is doable. You wouldn't *need* to add SRAM as cache. I remember reading about 286 to 386sx adapters being made with just a few PALs. The interfaces are different but not too dissimilar.

jesolo wrote on 2022-03-21, 19:33:

Why not track down one of those hybrid 3/486 motherboards that supports both CPU's?
You can then swop out the CPU's as you desire.

I have two of these type of motherboards and most of these were based around the OPTI495 chipset.

One that I have got is SiS and Contaq, center board in this pic download/file.php?id=115198&mode=view

Deksor wrote on 2022-03-21, 19:01:

No that's the opposite we want to do. Run a 386DX in a 486DX motherboard, not run a 486 in a 386DX board.

I know
but the issues for a 486 into a 386 are solved by an off the shelf transputer module

While the other way isn’t solved

The 386 into a 486 are the same problems as your chip is missing features and compatible timing a true 486 board will require but no “transputer “ module was made to put an older 386 into a 486 board