40MHz 387 chips are actually not that hard to come by. I see them all the time.

If you install a 33MHz FPU in a 40MHz system, it will run at 40MHz (not guaranteed to be stable or accurate), unless your board has an option for asynchronous operation. That requires setting a jumper and installing an extra oscillator.

Indeed, it is board dependent.

I suppose if you're really stuck for finding one but have the money to spare, you could search for the 4C87DLC (Such as the one IIT made) as 40MHz versions of those appear to be more common in that, they comprise a higher percentage of what comes up. The problem is, the 4C87 isn't really a common part and does cost more than it probably should.

The other advantage is that they work well with (and were designed for) the 486DLC upgrade chips, so it's future proof if you ever want to use one. Without one, I believe it falls back to being a 3C87 and I know it works fine in this configuration.

Edit: Huh, there are a few of them going fairly cheap on eBay right now actually.

I've tested several 33MHz FPUs (Cyrix and IIT) on 40MHz bus and they all work fine - tested with AutoCAD.

It depends on how old they are. I think generally the intel ones do okay, because intel shrunk their die process for the 33MHz model. Older models of Cyrix Fasmath, IIT or USLI might cough up wrong answers.

I also tested ceramic Intel 386DX-16MHz and it worked all the way to 40MHz 😁

Haven't yet found a processor/fpu that wouldn't work from 33 to 40MHz. I had some problems with 486slc-33 to run at 40MHz but only later found out it was actually a memory problem. When replaced 70ns with 60ns simms it worked fine.

As always you could get a sample that won't cut it. But that would seem pretty unlucky 😉

There were some later Intel 387DXs that were labelled "16-33", as in MHz. Those should have no problem getting to 40. I would not try it on an original 16MHz part based on the older core though.

I should probably also mention that if you have plans to use the asynchronous FPU configuration, it will only work on the Intel chips. It's not supported by Cyrix, ULSI, IIT etc.