This is an S54 that is definitely pushing the naturally aspirated envelope. 3.5 liters of displacement, Active Autowerke tune, Alpha-N intake setup, AC belt removed, and the engine fan clutch removed. The S65 V8 in the E92 M3 is completely stock as delivered from the factory.
The S54 shows an incredible torque advantage especially in the mid-range. It hits a peak of 286 lb-ft at the wheels on the Active Autowerke Mustang Dyno compared to the S65 239 lb-ft at the wheels. More torque from less displacement is just incredible to see.
The peak horsepower figure difference between the two is also in the S54's favor with it putting down 362 horsepower to the wheels against the S65 336. An advantage of 26 horsepower at the wheels for the S54.
So which is ultimately more capable naturally aspirated? The S65 of course. This is a stock S65 as it comes from the factory with all accessories connected for a street car. This S54 verging out of streetable territory and is more of a race application. That said, the S54 can match and exceed the S65 V8's stock output. The advantage goes right back to the S65 V8 once it is tuned, modified, and especially if it is stroked out to its 4.6+ liter capability. A very interesting comparison of one of the best naturally aspirated inline-6's and V8's BMW M has ever created nonetheless.