I would rather go with i386sx-33, if Am386-40 requires soldering. I mean, it would probably kill the board anyway, and if you want to go faster, get yourself a 486SX PC 😀

Am386sx is a reverse-engineered clone of i386sx, so the performance is the same. Are you aware that apart from soldering the new CPU, your mainboard most likely needs more modifications (swap oscillators, adjust bus timings) to run at 40/50 MHz?

The oscillator i will take care of later, i have one soldered in right now, and intend to swap it for a 8p osc socket i already ordered and have.

Soldering a new qfp on the mobo is no real concern, as long as i do not have to keep soldering many times.
So i have no time figuring out which is best performer.
Maybe i will swap out the i386sx33 over to this 386sx25 board and vice versa.

This board is included very tiny casing and power supply, dimensions are around 30cm x 30cm x 6cm and i will use it in my tv furniture to do some big screen dos gaming. street rod etc.

your mainboard most likely needs more modifications (swap oscillators, adjust bus timings) to run at 40/50 MHz

And that's assuming the chipset can handle the higher speeds. Often on SX boards, the support chips were only rated for the speed of the original CPU (i.e., a board with a 25mhz CPU would get a 25mhz-rated chipset). There's usually not much headroom in 'em... 30mhz is probably doable, and possibly even 33 if you're lucky, but 40-50mhz is unlikely to be reliable if it even works at all.

If you really want to boost the speed of that system, the better route would probably be to skip the faster SX chips and track down a 486SLC2/50 instead. That'd give you faster clocks plus an internal cache, while still maintaining the 25mhz bus speed.

Does the board of the 386SX has any cache?
The faster 386 boards do have cache and adding L2 cache later is typically a too complex task compared to just buying a different board.

The board has no cache,did any sx board had such?
But it is neat and small.
it is at my work ATM, will make a picture later.

Old Thrashbarg wrote:

your mainboard most likely needs more modifications (swap oscillators, adjust bus timings) to run at 40/50 MHz

And that's assuming the chipset can handle the higher speeds. Often on SX boards, the support chips were only rated for the speed of the original CPU (i.e., a board with a 25mhz CPU would get a 25mhz-rated chipset). There's usually not much headroom in 'em... 30mhz is probably doable, and possibly even 33 if you're lucky, but 40-50mhz is unlikely to be reliable if it even works at all.

If you really want to boost the speed of that system, the better route would probably be to skip the faster SX chips and track down a 486SLC2/50 instead. That'd give you faster clocks plus an internal cache, while still maintaining the 25mhz bus speed.

If you're referring to those upgrades that clipped on over the top of the 386SX chip, those are incredibly hard to find now and expensive and he'd still be hampered by the 16-bit memory bus. It would be cheaper just to buy a 386DX motherboard with a 386DX-40 chip installed or a 486 motherboard with any compatible CPU.

No, I was talking about just soldering a 486SLC/2 onto the board, like he was already planning to do with a faster 386SX. The SLC chips aren't super common, but they're at least quite a bit easier to find by themselves than in clip-on upgrades.

I did a similar SX->SLC2 swap not too long ago, using a Cyrix chip I salvaged from a dead laptop motherboard.

I'm with Slider in getting a 386DX40 motherboard+cpu combo, they can be bought for quite cheap and readily available these days and the performance gain would be massive compared to a SX25->33.