The Ultimate Driving Machine
BMW has long since abandoned this tagline in favour of efficient dynamics, a worthy, but somehow distant cause.
Looking through the catalogue of normally aspirated machines built by the M-Division, there is a clear winner from a dynamic standpoint, the Z4M Coupe. Consider the following:
Torsional Rigidity
E30 M3 23,500 Nm/deg (w/ Roll Cage)
E46 M3 18,750 Nm/deg
Z4M Coupe 32,000 Nm/deg
Z4M Roadster 14,500 Nm/deg
The Z4 is also interesting from a weight standpoint, as it is one of the lighter modern BMW platforms. However, weight is not the end all and be all - the packaging and distribution of that weight is key to how the car behaves, particularly on the limits of grip.
This is where the choice of engine becomes a delimma, as the car came with the iron-block, high revving S54 (217kg), and the magnesium block N52 (148kg). The weight difference between these engines is too large to ignore.
Experimenting with the M52TU which was an aluminium version of the S50/S52 engines (20kg saved), deleting vanos (7kg saved), shaving the crank (2kg saved), the engine still is around 20-30kg heavier than the N52 from the factory, with a much narrower and compromised powerband.
While I have a lot of love for the S54, and it's raw, fire-breathing spirit, the N52 would make a better overall car. The focus on developing the N52 would have to be to move it's torque curve to the right of the chart, through careful study of new camshafts, matching headflow from the intake and the exhaust, and building the valvetrain to spin higher.
It will not match the S54 for rawness nor top end power, but the area under the curve has a fighting chance, and it would bring a more neutral balance to a chassis that has a reputation to bite. Using a conservative 5% weight reduction (-68.5kg), gets you to 1300kg which is 80kg less than a M3 CSL with significantly more stiffness. Here is a study of keeping the car street-worthy while reducing the weight further:
So after these weight savings, let's compare the Z4 Lightweight to the M3 CSL. Although the Z4 is outgunned, it puts up a good fight.
E46 M3 CSL (S54 Engine) : 1380kg / 330 hp (0.24 hp/kg)
E85 Z4 3.0 SI Lightweight (N52 Engine) : 1237 kg / 268 hp (0.22 hp/kg)
To match the lightweight Z4 for the CSL power wise, it would need to have around 296 hp, and again the peak numbers do not tell the story. The lighter weight should improve mid-corner and exit speeds, while the superior weight distribution and torsional rigidity would come into their own.