The 486 DX2 66Mhz clock doubled cpu has a bus speed of 33Mhz and the
internal cpu speed is doubled and is running at 66Mhz.

So a 33Mhz clock doubled 68040 should have an internal speed of 66Mhz.

But if you compare the speed of both cpu's the
486 66Mhz is a lot faster and it can't just be the 486's L2 cache making the difference.

The reason seems to be only some sections of the 68040
are clock doubled.

Here is some info from various websites

On the 68040: there was an element of dishonesty because anyone
who had seen previous ads would think that the "doubled" clock speed
was a speed bump, which it wasn't. Also, it was never clear to me
that the 68040 actually clocked the pipeline at the "doubled" speed:
as I understand it, some internal components ran at
double speed which is not quite the same thing.
The convention in quoting speed of a CPU is to use the
speed at which the pipeline is clocked.

** (Electronic Buyer's News Aug 20, 1993; pointed out by Bradley Lamont and
Motorola 68040 data book in 92) Apple marketed 68040 Macs as 'clock
doubled' though speed is NOT in the 486dx2's class {see CPUs Comparison
List}.

So I would say the the 33Mhz 68040 is more like the
speed of a non clock doubled 486 40Mhz.

When I was doing the 68k version of mpg123 I was getting the equivalent speed of a non clock doubled 486 40Mhz on the 68040 33Mhz.

see

http://fabrice.bellard.free.fr/mpg123-486.html

But there is some misleading info about 68040 clock doubling

ie

http://www.lowendmac.com/archive/2k0605.html

And some accurate 68040 clock doubling info at

http://www.everymac.com/systems/apple/mac_quadra/stats/mac_quadra_840av.html

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