Re: 90 degree design stolen from Buick
Idea is to have the exhaust charge mass flow from the last cyl help accelerate the next cyl. You really do not want a vacuum because that could result in reversion.
One confusing issue is that US cars count cylinders on bent engines with odd cyl on one side and even on the other. MB seems to be different so for the crossfire it is 1-2-3 on the passenger side and 4-5-6 on the drivers so the odd-sounding firing order 1-4-3-6-2-5 is really alternating banks so each side has an exhaust event every 240 degrees (on an American engine it would be 1-2-5-6-3-4 still odd from a harmonic standpoint but more reasonable).
I agree, better than what went before but still last millennia.