Macbook 8.1 overheating
Solution 1:
To make this work, I have to boot my mac with efi .
My solution was based on EFI-Booting Ubuntu on a Mac.
The writer there says that you have 2 grub files on your efi partition but there should only be one (grubx64.efi) .
The mac is now 5-10°C cooler and the startup is now very short .