The engine is also 22 pounds lighter than the S65 V8 and this is with fluids. The total engine weight is 452 pounds. The engine also revs to 7600 rpm. The compression ratio is a fairly high 10.2:1 and the same as the N55 inline-6 that sees duty in the 335i.
It is interesting to note that the bore and stroke are 89.6mm and 84.0mm respectively. Compare this to the bore and stroke of the N55 inline-6 which is at 84.0mm and 89.6mm respectively. That is not a typo, the N55 and S55 share the exact same 2,979 cc displacement although their bore and stroke figures are reversed.
BMW designed the S55 oversquare giving a bore to stroke ratio that is greater than 1:1. This allows for lower piston speeds at higher revs. The S55 develops its peak torque as early as 1850 rpm and maintains it until 5500 rpm meaning the oversquare design has not hurt the low end much (although the N55 hits peak torque earlier) and BMW has done a good job balancing high RPM power and low end punch.
BMW's turbochargers are produced by Honeywell and generate 1.25 bar (18.12 psi) of boost. Will there be room to push the turbos past this level? Most certainly. The question will be by how much.
The downsides? The fuel tank is only 15.8 gallons. Why BMW continues to give their M3's such small fuel tanks is anyone's guess.
Enjoy the photos below.
DISPLACEMENT (CC): 2,979
BLOCK/HEAD MATERIAL: ALUMINUM/ALUMINUM
BORE × STROKE (MM): 89.6mm × 84.0mm
HORSEPOWER (SAE NET): 425 @ 7,300 RPM
TORQUE: 406 LB.-FT. @ 1,850 - 5,500 RPM
SPECIFIC OUTPUT: 141.6 HP/L
COMPRESSION RATIO: 10.2:1