I've always thought (even heard) that the E46 M3 has the most powerful 6 cylinder, naturally-aspirated motor (without any forced induction, of course) until my brother-in-law called me to tell me that he just got into a race with a brand new Porsche 911 GT3. When I did the research, it looks like the M3's motor was dethroned by the more powerful 3.6L 380 HP (105.5 HP per liter). The M3 is 104 HP per liter, just 1.5 HP less per liter. Oh well, it was good to know that we were #1 for quite some time.