The dynamically balanced nature of I-6 engines in general is very desirable, and the turbine-smooth rotation and pleasant exhaust note of the BMW engine in particular is commendable. One drawback of the I-6 design is, however, its long crankshaft, making the engine’s north-south length long and requiring strengthening of the crankshaft. Today, there are good V-6, VR-6 and H-6 engines. Since it seems the BMW engine’s bore and stroke are maxed-out, maybe BMW should try an H-6 or V-8 (both of which are balanced), if their engine designers abhor the V-6 design.
As for the soot in exhaust please note that soot (C-C-C-) and CO/CO2 are two different things. While CO, which turns into CO2 quickly, and CO2 emissions will be rather strictly controlled in the near future, they are not considered as health hazard like NOX and HC. Soot on the other hand is considered as carcinogen and should be controlled. That is the reason why the new diesel engines are required to have DPF. Soot is the product of fine droplets of gas or diesel exposed to high temperature in oxygen-starved combustion chamber. Under the high-temperature with no oxygen environment gas or diesel turns into C-C-C-, H and HC (H and HC are taken care of by the 3-way cat). I do not think BMW engineers want to breath lung-full of the 335i exhaust.
