A neat fact I recently learned: Bill Herd (designer of the C128) said in a presentation at VCF East earlier this year, that the Z80 in the C128 was done because

A neat fact I recently learned: Bill Herd (designer of the C128) said in a presentation at VCF East earlier this year, that the Z80 in the C128 was done because the C64 Z80 cartridge had power and heat problems. It was easier to just put it in the C128 versus trying to make the C128 compatible with the C64 Z80 cartridge.