I guess everyone here know shit about BMWs, because your original thread question is still unanswered. So, I will explain to you why there is a BMW 520i and a BMW 538i both using a 2 litre engine (1977 cc to be exact).
For decades now, going back to the E28, there have always been a 520i and a 528i. In the past the 528i was usually a 6 cylinder engine while the 520i was always a 4 cylinder engine, That was how buyers could differentiate the 2 models. What BMW did was to introduce the twin turbo into their standard 4 cylinder engine. So, the regular 4 cylinder engine produced 181 hp, while the turbo version of this engine produced 240hp. In the past, 240 hp could only be produced by BMW's 6 cylinder engines. In other words, for a normally aspirated BMW engine to produce the same 240hp, it would have had to be a small 6 cylinder engine like the 2.8 litre inline 6. Therefore, BMW kept the 528i designation to denote NOT ACTUAL ENGINE CAPACITY, but theoretical engine capacity required to produce the same horse power of 240hp. Don't worry, most BMW salesmen don't even know this.