I was hoping that the 601 Power Macintosh card in my Quadra 800 would go from 66 to 80MHz when I clockchipped the Q800 from 33 to 40 (since it will run at 80 in the 840AV). Alas, after a startup sound and a pause, it then goes into the chimes of death.

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Well, if it makes you feel any better, I found that the Turbo601 would not run any faster than about 34 - 35 MHz bus speed in the IIci. To overclock it I had to change the clock multiplier to the PPC601 chip.

Your case is more unexpected, given that the same card will operate faster in a different machine, but perhaps the logic is detecting its host and adjusting a PLL accordingly.