MotoringFile provides this quote from an interview with Dr. Friedrich Nitschke of the BMW M division with
Car and Driver:
The three-cylinder is an attractive engine. It is possible to reach around 185 to 200 horsepower per liter in a forced-induction three-cylinder and we have 1.5 liters of displacement. Such an engine, which, by the way, sounds very similar to a six-cylinder engine, would have over 310 horsepower. And we are not even at the limit there. Generally speaking, I could imagine such an engine.
