As you can see from the pictures in the gallery, BMW has color-coded a cutaway of the exhaust manifold. The following describes where the runners are connected:
|Green:||Cylinders 1 and 7||to||turbo L scroll 1|
|Red:||Cylinders 4 and 6||to||turbo L scroll 2|
|Yellow:||Cylinders 5 and 2||to||turbo R scroll 1|
|Blue:||Cylinders 3 and 8||to||turbo R scroll 2|
So if we plug in the firing order, we can map out which turbo experiences what, when:
As you can see, just like a regular twin-turbo V-8, there is an exhaust pulse going to one of the turbos every 90 degrees of crankshaft rotation. However with this manifold, each turbo gets a pulse every 180 degrees rather than at the uneven intervals.
If you look closely at which scroll receives the exhaust pulse when, you'll see that each turbo is fed by the same scroll twice in a row. That design likely maximizes any positive interference effect between the pulses.
And there you have it; a stroke of genius that helps the first turbo M engine retain M's trademark responsiveness. Not compared to its normally aspirated engines, of course, but compared to all other turbo engines. For those who love the power of turbos but loathe the lag (and that would be me) this small change makes the future seem a whole lot brighter. Because like it or not, we're going to see a lot more turbos in the future.