7.3L, I think someone on this site has a Brabus SL73 but I could be mistaken.

As a side note, not to be confused with the engine used in the Zonda, while that seems to be an M120 as well its an AMG effort.

__________________
2006 CL500
2009 C300 4matic
1969 280SE
2023 Ram 1500
2007 Tiara 3200