Hobby Boss moulded 16 exhaust stacks but the BMW 801 is a 14 cylinder engine.
So, after quite a bit of research I came up with this:
I managed to identify 13 exhausts (numbered 1-6 and 8-14 on the above picture) but wasn't sure about the last one, so added the one numbered 7. I cut off most of the exhausts and arranged them as you see in the picture.
Now, I have found this picture:
Here is a close up of the engine:
It looks to me that the 7th exhaust is on it's own and a bit more central than I placed it, and is probably close to the one I cut off. So, I cut number 7 off again and repositioned it. Fortunately, I haven't painted the exhaust stains yet.