Presumably the boards these things come with have
their own crystals. I would not expect a '486 upgrade
for a '286 to run at 8mhz or anything comparable (but
Tony could tell us).
The answer to that is the same as Rutherford's comment on the only
possile result of research in the social sciences, namely 'Some do and
some don't' :-)
I have some upgrade oards that turn an 8088 into an 8086 or 80286-based
machine. Those are ISA cards with a liead that plugs into the 8088
socket, and said boards certainly contain their own clock oscillators. On
the other hand, the oard in this machine -- the 486SLC oard that goes
in the 80286 socket, does not contain a clock crystal, it takes the clock
signal from the 80286 socket.
-tony