So from what I understand, your system is basically a PS/2 model 70 for the Japanese market. It sounds like 25MHz is going to be the best case scenario for your planar, but even that might be pushing too hard as it was designed for operation at 20MHz. Overclocking is worth a shot, but at the very least you'd probably have to upgrade the RAM.
Assuming you are stuck at 20MHz, then the answer is no, a DRx2-33/66 will not run at full speed in your board. It will run at 20/40.

The cheapest upgrade you can get with clock doubling is probably the 132-pin version of the 486SXL from TI. All of them support clock doubling even if they don't have the (2) in the marking.

There are also upgrades that use the 486DLC blue lightning chip from IBM, which are clock tripled, but those are normally kind of expensive.

They made 486 planars for the 5551, right? Have you checked how much those cost? It might be cheaper than a fancy CPU upgrade that doesn't deliver true 486 class performance.

Anonymous Coward wrote on 2020-12-05, 16:48:

So from what I understand, your system is basically a PS/2 model 70 for the Japanese market. It sounds like 25MHz is going to be […]
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So from what I understand, your system is basically a PS/2 model 70 for the Japanese market. It sounds like 25MHz is going to be the best case scenario for your planar, but even that might be pushing too hard as it was designed for operation at 20MHz. Overclocking is worth a shot, but at the very least you'd probably have to upgrade the RAM.
Assuming you are stuck at 20MHz, then the answer is no, a DRx2-33/66 will not run at full speed in your board. It will run at 20/40.

The cheapest upgrade you can get with clock doubling is probably the 132-pin version of the 486SXL from TI. All of them support clock doubling even if they don't have the (2) in the marking.

There are also upgrades that use the 486DLC blue lightning chip from IBM, which are clock tripled, but those are normally kind of expensive.

They made 486 planars for the 5551, right? Have you checked how much those cost? I might be cheaper than a fancy CPU upgrade that doesn't deliver true 486 class performance.

Thanks for the suggestions! Considering the fact that this is 1 of roughly 5 Japanese IBM MCA machines I've seen for sale over the past 6 years across every major online Japanese marketplace (and eBay), that last part isn't really an option unfortunately. 😜 And besides, that's not really what I'm going for, I own other 486 and Pentium machines - this is more just to see how far I can push it. Not many people own PS/55s and even fewer have seen them run mid 90's DOS games, I'm not that games from the 386 era but this thing is in such good condition that I'm trying to make up excuses to hold onto it. If it can run Doom at a playable framerate in low res with the view window shrunk 2 sizes, that's enough for me to keep it, especially now that I've got an Adlib and will likely be building a Sound Blaster next year.

So a DRx2-66 will work at 40MHz? That actually sounds okay, do you know what its performance would be like in comparison to an Intel 486DX-33? I've also been looking at the Cyrix 486DLC-40GP and TI486SXL-40 which are more or less affordable on eBay from Chinese sellers.

Ti486SXL has usually compatibility problems with some motherboards. DLC and DRx2 have good compatibility.

Also Ti486SXL is need to be PGA132 (386DX pinout) and not PGA168 (486 pinout).

kixs wrote on 2020-12-07, 05:36:

Ti486SXL has usually compatibility problems with some motherboards. DLC and DRx2 have good compatibility.

Also Ti486SXL is need to be PGA132 (386DX pinout) and not PGA168 (486 pinout).

The compatibility of the dlc and sxl should be practically identical, unless you have evidence to prove otherwise. The drx2 chips should have improved compatibility, since the integrate the flush circuit for boards that lack hidden refresh. The sxl/dlc can be made functionally identical if you build your own flush circuit. Otherwise I recommend the drx2 25/50. The 33/66 costs quite a bit more, and this system can’t take full advantage of it anyway.

Evidence... I have 🤣

I didn't do much testing with different motherboards... but the 5V Ti486SXL doesn't like this board for sure. While all the rest of PGA132 work fine.
3/486 OPTI495SLC VL-BUS benchmarks

Anonymous Coward wrote on 2020-12-07, 05:58:

kixs wrote on 2020-12-07, 05:36:

Ti486SXL has usually compatibility problems with some motherboards. DLC and DRx2 have good compatibility.

Also Ti486SXL is need to be PGA132 (386DX pinout) and not PGA168 (486 pinout).

The compatibility of the dlc and sxl should be practically identical, unless you have evidence to prove otherwise. The drx2 chips should have improved compatibility, since the integrate the flush circuit for boards that lack hidden refresh. The sxl/dlc can be made functionally identical if you build your own flush circuit. Otherwise I recommend the drx2 25/50. The 33/66 costs quite a bit more, and this system can’t take full advantage of it anyway.

Good to know, thanks guys. I'm not seeing any DRx2 at all right now on eBay, it's likely due to me being outside of America (I'm in Australia). Guess I'll post a WTB thread.

EDIT: looks like there's no WTB forum, unless I'm blind...

Use cpu-world. Lots of sellers have drx2s.

Anonymous Coward wrote on 2020-12-07, 08:22:

Use cpu-world. Lots of sellers have drx2s.

I just took a look and couldn't find any, unless the search function isn't great. I might post a WTB thread there and on Amibay.

Are you sure a DRx2 25/50 or 33/66 will work? The board's clockspeed is 20MHz and can't be changed with jumpers, it's fixed. I've never tried a CPU in a board set at the wrong clockspeed before so I just want to confirm that it works before buying one of these. If not, I just realised there's a DRx2 20/40 which should work fine.

Cyrix DRx2 family goes from 16 to 33MHz base clock. If you use higher rated CPU it will run X2 what your base clock is. But you waste the potential. In your case any X2 40/50/66 CPU will run and perform the same as X2 40.

There is no special circuitry on the CPU or in the motherboard to check the rated CPU speed, so the CPU will try to run at whatever frequency the motherboards feeds it.
Now, the bigger question is whether or not the upgrade CPU is compatible with your motherboard BIOS. With CPU upgrades it's kind of a crap shoot.

You need to post a WTB ad on CPU-World. I'm pretty sure you will get a reply quickly. The 20/40 and 25/50 models were made in far greater quantities than the 16/32 and 33/66 models.

BTW, don't expect too much from the DRx2. I believe the DRx2-33/66 is slightly faster than a 486SX-33, so maybe the DRx2-20/40 is similar to a 486SX-20.

I still think the SXL-40 is the way to go if you're willing to solder a flush circuit into your motherboard. These chips have 8kb cache vs 1kb on the DLC/DRx2, which will give maybe a 10% boost. I don't know what they go for now, but recently I saw some in the $20-$40 range.

There are also other CPU upgrades I forgot about. They have something called an MCMaster which is basically a system on a card:
http://ps-2.kev009.com/ohlandl/CPU/MCMaster_486.htm
And there are also upgrades that let you plug true 168-pin 486 CPUs into the 386 socket, like the Kingston 486/NOW. These probably aren't cost effective, but it doesn't hurt to keep an eye out because sometimes you get lucky.