Quote:
Originally Posted by swamp2
Interesting.
Let's look at piston speeds. US units, sorry Uli...
N55: 68.6 ft/s
S65: 69.1 ft/s
S63Tu: 70.1 ft/s
Thus non-stroked S55(?) at 7500 rpm: 73.5 ft/s
Stroked S55(?) to 3.2l at 7500 rpm: 78.4 ft/s
Ferrari 458: 79.7 ft/s
I know, I know don't compare BMW and Ferrari. That complaint falls flat though when looking at the S65, it is the closest to a Ferrari engine in any other production car.
From this I think it is fair to say that even a stroked 3.2l S55(?) could support a redline of 7500 rpm.
Of course there is the question of Valvetronic and its rpm limit. I've only heard rumors about that.
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Of course it's possible to have a redline that high, but the 458's engine (whatever it's called) and the BMW S65 are naturally aspirated engines. With a turbo motor, increasing the redline means using a larger turbo which would mean increased lag. The principle reason (well, one of them main reasons) the N54 and N55 lose a lot of power above 6,000RPM is the small size of their turbos which was done on purpose to give them very quick spool times. An M motor needs to have very good throttle response so they are not going to want to strap large turbos on it, 7,500RPM is doable but don't be surprised if they keep the redline lower to really focus on responsiveness.
It's not just turbo size that has to increase, either, the intake manifold would also need to grow larger to support the increased airflow which would also negatively impact spool time.