>> ...or simply use MorphOS 3.10 or newer, which disables the unused CPU ;-)

> No, not really - idling CPU nevertheless consumes energy

Yes, an idling CPU consumes energy, of course, but as far as I understand, a disabled CPU is disabled, not idling. If you say "disabled" means idling, then what did the 2nd CPU do from MorphOS 3.2, which introduced G5 support, through MorphOS 3.9? What's the difference regarding the 2nd G5 CPU between MorphOS 3.2-3.9 and MorphOS 3.10-3.17? If it's simply idling like before, then what's the point in having it disabled since MorphOS 3.10?

> I tried to kill 1 CPU in OpenFirmware - but makes no difference.

Yes, this was discussed in the linked thread. Setting the "boot-args" variable in OpenFirmware to "cpus=1" is not affecting OpenFirmware itself but just Mac OS X.