Since the M135I have 320hp and 450NM. The proper M car will most likely have more HP. Say 340+. And probably more NM as well.
Having more than 125HP /L in a modern NA engine is not very realistic.
Neither is a NA 2,8L++ 4 or 5 cylinder.
NA (3L straight six) is probably out of the question due to emissions. And it would not be that easy to beat the engine in the M135I due to its torque numbers.
So we are back at 4,5 or 6 cylinder turbo engines.
Upcoming M3 will probably be 3L I6 with about 450HP.
150HP pr L
0,5L cylinder volume
75HP per cylinder.
That is roughly where good modern turbocharging technology is in terms of high output.
We might see a 2L Subaru engine with 300HP next year..
But not likely BMW will bother with a I4 2.0L turbo at 340HP++. Would probably be very expensive to make compared to I6 which is BMWs thing. It would most likely sound worse compared to M135I. And it would probably need to have 350++ hp to "compensate" for the lack of torque from the lower displacement compared to M135I. I think that will be a bit too much HP with todays technology from a 2L.
If they made a I4 2,2L it would be easier to make about 350HP and some decent torque for a modern M car. But not very likely.
0,5L per cylinder seems to BMWs thing. So is straight six. But im hoping and thinking one or more of the 3-cylinder engines will come in the 2-series. Would be funny if BMW made a new 2,5L I5 turbo for the M2. Chopped of one sylinder from the new M3.
M135I have 1 normal twinscroll turbo.
Im putting my money on a 3L straight six twinturbo for M2 (One electric turbo or someting for minimal lag?)
Maybe tri-turbo for M3 (one electric turbo or something for minimal lag?)