3-series is the BMW's most popular sport sedan and 6-series is a very famous BMW sport car in 80' so that their corresponding Ms are also more sporty. For 5-series, it used to be a mid-size executive sedan so that 5M is not as sporty as M3 and M6.因为量产的5系没有双门款的啊~~~3系和6系都有量产的Coupe(双门).....明白?