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^ Great points regarding turbo lag on a high output 4-cylinder engine.
Another issue with 4-cylinder engines:
4-cylinder engines have a pause or dead spot between each power stroke, and 3-cylinder engines have even larger empty strokes. Thus neither will ever have the smoothness in power delivery of a 5-cylinder or larger engine that has overlapping power strokes.
I don't think the weight benefits of a 3-cylinder or 4-cylinder engine outweigh the smoothness and non-pulsating delivery of power that is inherent to 5-cylinder and larger engines.
It is only marketing, economics and regulation that puts these engines in road going sports cars. I don't think many sports car consumers if given the choice would ever chose the lighter but inferior engine configurations unless cost or simplicity were their primary selection factors.
For racing, I would assume such configurations are only chosen when required by class/vehicle size requirements or when those extra 10's of pounds of weights are of absolute paramount importance.
I have owned a TT-RS (2.5L Inline-5) and various Inline-6 BMW's and Flat-6 Porsches. In comparison to the 4-cylinder car's I have owned varying from standard 4-cylinder fare to a Subaru WRX and Mazda Miata, the 4-cylinder engined cars had noticeably rougher and less smooth/consistent power delivery. I have never driven a 4-cylinder car where the power felt "enjoyable" or "exhilarating" like a 5-cylinder or larger engine does. This is also totally irrelevant of whether a car is naturally aspirated or has a turbo, an no amount of advanced engineering can do a thing to change these basic characteristics of engine cylinder count.
Can a 4-cylinder engine be great in a sports car? Sure it can, but it is never truly ideal even in great cars like the E30 M3 (done to meet racing class requirements). If a car is too small in physical size to contain an engine larger than a 4-cylinder then sure I can understand (original Lotus Elise for instance), but the 2-series is not a small car and is clearly designed as a platform to hold an Inline-6 or probably even larger engine. It is always a economic/hierarchical compromise IMHO for a car as large as the 2-series, and consequently there are people who are justified in not being thrilled by 4-cylinder engines being used in such "relatively large" sports cars.
I don't think "fun/pure sports car" and 4-cylinder engine belong in the same sentence unless we are talking about something truly small like an Elise, Miata or Alpha 4C... or a homologation car like the E30.
Like that pistonheads article suggests, a de-tuned S55 would truly satisfy everyone. The other brands do the same thing to great success.
Or maybe to preserve the hierarchy BMW should just take the S55 and lop off 1 cylinder to create a 2.5L Inline-5 turbo to stick in the M2... if they can stomach it.
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