Go over to the e23 page and ask the Turbo Boys. Lots of 745i talk over there.... much turbo knowledge. From what I gather the turbo uses a 3.2liter engine with something like 8:1 compression. Turbos usually do use lower compression engines, or blow the head off the block. I'm clearly not a turbo guy, but don't want to see you FU your engine. Research it well.