From: faruque@sun.soe.clarkson.edu (Abdullah Faruque,334CH,3752,2654247)
Subject: Re: Run a 486/33MHz CPU at 40 or 50MHz?
Organization: Clarkson University
Date: Fri, 8 Jan 1993 17:29:42 GMT

From article <1993Jan8.100550.9995@ac.dal.ca>, by francis@ac.dal.ca:
> I have a 486DX/33MHz system, and I have been told by the manufacturer that
> the motherboard is upgradeable to 50MHz by plugging in a new crystal and CPU.
> 
> Question:  	Is it possible to plug in (say) a 40MHz or even a 50MHz crystal
> 		into the board *without* replacing the old (33MHz) CPU? 
> If so, is there a performance improvement or will the 486DX 33MHz chip just
> burn itself out, ie, has it been internally hardwired for the specified clock
> frequency of 33MHz?  I would like to hear from anyone who has attempted this
> (with/without success).
> 
> Francis
> 

I am running a 486DX-33Mhz at 40Mhz without any problem. I put an
extra fan and heatsink to cool down the CPU and now the CPU is running
at a much lower temp that it used to run at 33Mhz (without any extra
cooling). So, probably the long term effect due to the extra heat
generated by over running it is near zero. Actually it may even be
better, because it is running much cooler (with a fan and heatsink)
than the max operating temp specified by the manufacturer.  Normally,
the manufacturer specifies the max operating temp of a CPU (or in
general any semiconductor device) under certain operating conditions.
If you increase the operating frequency then you must also provide
necessary cooling to keep it under the max rated temp.

Now the question is that how far you can increase the speed. There is
no rule here, it may run at only 1% or as high as 30% above the rated
speed. There are design limitations beyond which it will not work at
all, even if extra cooling is provided.  You have to do the experiment
and find out the max possible speed that can be obtained without any
soft error for a particular CPU. I found that a 486DX-25 can run at
33Mhz and a 486DX-33 can run at 40Mhz without any problem. But again
that may not be true for all 486DX25/33 chips available. So, far I did
not encounter any problem running DOS 5.0, Win 3.1, OS/2 2.0 and many
other diagnostic and application softwares.  I even pushed a 486DX-33
to 50Mhz just to see what happen. I was surprised that the system
booted up and most DOS based programs run (even Checkit passed all CPU
tests), but Win 3.1 and OS/2 2.0 did not run at all when pushed to
50Mhz. You have to consider the following things when pushing a CPU.

1. You must provide extra cooling, to make sure that it is running 
at a temp below max operating temp.

2. The motherboard should be rated at or above the intended speed.
Otherwise you are looking for troubles. If you push the whole
motherboard (which is not rated at the intended speed), then it may or
may not work. That depends on who good the motherboard components are
and if they can run at the increased speed.

3. Check extensively with different OS, diagnostic and application
softwares before you settle down at the intended speed.

4. The main memory and external cache (if any) must meet the speed
requirement necessary to run without putting any additional wait
state. If you have to put additional additional wait state, then you
will loose some performance and pushing the CPU may not even justify
in such case.

5. If you do not feel confident enough to do the experiment, then it
is not for you. Not recommend, if you do not even know what a crystal
OSC is and how to locate/replace it.

Please do not flame! I saw this topic several times, so I just
describe my experience here, which may or may not work in your
case.

-Faruque
Email: faruque@sun.soe.clarkson.edu