I don't know why there was such a controversy about the M2 engine being an open deck VS the M4 engine being a close deck as BMW's own technical documents show their different engine block designs (hence the engine denominations S versus N).
Ok I understand that the Press Releases were misleading saying that the M2 engine is of "close deck design". But that was easily debunked. Maybe M2 owners can use the below information to financially settle with BMW if they feel cheated. I believe if the "close deck" information is still officially available from BMW or was available for a long time, there is grounds to be upset because close deck and open deck designs are very different!
Let's have a closer view with pics and diagrams taken from the engine informations provided by BMW...
M2 N55 engine information here (click on the photo of the engine to download PDF file):
http://f30.bimmerpost.com/forums/sho...d.php?t=854477
M3-M4 S55 engine information here (click on the pdf file with the name "S55 Engine Technical Docs":
http://f80.bimmerpost.com/forums/sho....php?t=1015603
So let's go a little bit more into details.
The N55 engine family:
The M2 engine is part of the N55 inline-6 engine family, which is an evolution of the N54 engine family. Both N55 and N54 have very similar engine block designs and share the same bore and stroke: 84 mm (3.3 in) bore, 89.6 mm (3.5 in) stroke and 2,979 cc (181.8 cu in) capacity. Both engines are of
OPEN DECK design, meaning that there is a space between the engine block and cylinders sleeves to cool the engine. This engine design is the cheapest to manufacture and provides the best cooling efficiency. The difference between the N55 and N54 lies primarily in the fact that the N54 has twin turbochargers whereas the N55 has only one twin scroll turbocharger. Another difference is that the N54 engine did not have the variable valve lift (Valvetronic) whereas the N55 does. Also the N54 had piezo injectors and the N55 has solenoid injectors...
Below are pics of both engines and you can see that they are of very similar design.
Pic of N54 engine block
Pic of N55 engine block
Now, let's go back to the M2 engine. Its official denomination is N55B30T0 (272 kW). That means it uses the N55 open deck engine block. The difference between this N55B30T0 to the "regular" N55 engine (or N55B30O0 in its latest and most powerful version (240 kW)) are, according to BMW, "the M2 gets the pistons and crankshaft bearings from the BMW M3/M4 and also their valvetronic variable valve timing system. The BMW M2 also receives an upgraded cooling and oil sump system."
The M3/M4 engine is a different beast altogether and has a different denomination (S55B30T0 (317 kW)). This engine block is of
CLOSE DECK design which means that there is no space between the engine block and cylinder sleeves, which makes for a much stronger engine, but harder to cool. A close deck design engine is a design made for high performance engines and engines used in competition. since there is no coolant space between the engine block and cylinder sleeves, the engine is much stronger and can endure higher loads for a longer time. The drawback is that this kind of engine needs a much beefier cooling system.
Pic of S55 engine block showing close deck design, see how much thicker and sturdier the engine block is...
Now let's have a visual comparison of these engines with images taken from BMW's own technical information:
Visual of N55 engine block showing the open deck design:
Visual of S55 engine block showing the close deck design (and text from BMW confirming it)
S55 engine external view
N55 engine external view
