Revisiting after 2 years (excuse me for the necropost), I succeeded in pushing the 386 to 50mhz, with good timings no less. The speed difference is very noticeable in games and in Windows. Ultimate Doom became ALMOST playable without reducing the screen size. Benchmark results really improved as well; using a Diamond Speedstar 24 ET4000AX and the timings shown in below photos:
3DBENCH 1.0c = 20.9
PC Player = 4.7
Doom = 8179 realtics (9.13 fps)
Speedsys CPU= 11.59
Hardware-wise, no new magic. I'm using the same setup, albeit different serial #s: the FX-3000 motherboard and CPUs I originally tested are long gone (sold). But after coming across another FX-3000 and more DX-40 parts, I decided to give it another shot.
Since I'm not using the exact same motherboard, 15ns SRAMs, or SIMMs, they can't be ruled out. But I'm almost certain CPU's themselves were the limiting factor in previous attempts.
I tried 4 CPU's this time, 2 CPGA's and 2 PQFP's. Both the CPGA's pass at 50mhz, while both the PQFP's fail. Last time I had 2 CPGA's and 1 PQFP (all different), all of which failed.
So takeaways for anyone losing sleep over a 386DX-50 build:
-FX-3000 motherboards are your best bet, as they officially support 50mhz bus speed and not TOO rare
-When playing the CPU lottery, ceramic parts may have a higher success rate than PQFP parts (and are easier to cool)
-15ns cache and 60ns memory are hard requirements
Now for the fun part— photos!
The glorious 386DX-50 setup. Note that I used a 12ns tag SRAM to be safe, but I think 15ns will do. Heatsink + fan are necessary however; fortunately 486 coolers can be permanently mounted using a custom adapter bracket.
Tightest timings, almost: DRAM needs 1 WS. Maybe 50ns SIMMs can do 0 WS, if 30-pin versions even exist? ISA bus overclocks to CPUCLK/4 = 12.5mhz, really boosting video speed.
Windows 98 (yes, 98) runs surprisingly decent, though it does take some tricks to install on a 386. The hardware also passes a final stability test: installing NT 3.51.
Overclockers on the right, non-overclockers on the left
A very clean AT case to top it off