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      07-04-2012, 04:27 AM   #107
Uli_HH's Avatar

Drives: AW M3 E90)/ 523i Touring
Join Date: Nov 2005
Location: Hamburg Germany

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Originally Posted by swamp2 View Post
I know the new M3 and M4 will perform. Nonetheless one can not deny there is just something wrong with a lightly massaged n54 with close to 500 hp in a 65-70k base M4 (guessing the M4 will have even more power than the M3 and will also exhibit engine "reuse" to maximize profit). I guess as long as the cars continue to dominate their classes I can't complain too much. Nonetheless small details like this make the cars undeniably less special and have me looking seriously outside the M brand for the first time in a long time.
100% agreed ... thatīs the point in the engine issue ... I donīt beleave that the new M3/M4 in any kind would be an bad performer - performance would be better than any precessor !

BUT for me an M3 (until today) was an very special car and that not only meens performance wise ... I am ready to pay an much increased price for an car like an M3 because I get something special an unique ... less than 20% common part are an credo for the M-GmbH (until now) ... an the unique, special and fascinating engine is an important part (if not the most important!) of the Myth M ... I am not really willing to accept an simple "pimped" AG-engine in an M3/M3, what an S55 based on the N55 in any kind would be ... there could the N54/335i(tuning)-Fanboys could say what they want - they just donīt understand the Myth M ... itīs much more than hp, performance and sound !!!

Originally Posted by swamp2 View Post
Let's look at n55 being the base engine for the new M3/M3, specifically the bored vs. stroked issue for a larger displacement. I thought I may be able to conclude that the engine simply can not be bored alone to 3.2 liters, but this quick calculation does not quite show that, neither from pure geometric argument nor from a more engineering based argument. It almost shows it but not quite...

Bore: 84 mm
Stroke: 89.6 mm
Bore spacing: 91 mm
Displacement 2.979 l

Cylinder wall thickness between cylinders = 91 - 84 = 7 mm = .28". Just over 1/4 inch.

To get to 3.2 l (a rumored displacement) with bore change alone requires a bore of:


Or 87.1 mm. Let's call it 87.

That would leave .15 inch of material between the cylinders. A shade more than 50% of what you can see in this picture (n54 block shown, these features are identical on the n55 block). Although this looks and feels too thin, engineering is not about look at feel. The stress in this portion of the engine will be roughly

σ = peak combustion pressure * (bore/2)/(thickness between bores)

(Note: This is the simplest thin walled cylinder approximation, which should be decent in this case.)

The peak combustion pressure (really rough estimate) is probably around 1000 psi (full load, high rpm). Anyone want to help me revise that estimate? This IS NOT BMEP, but peak dynamic combustion pressure.

Thus going from 3.0 to 3.2 liters by bore alone will increase the stress in this region by the same ratio as the wall thicknesses, nearly 200% in this case (x 1.86)!

The wall is certainly strong enough to handle a single combustion cycle. The limiting factor in design here is very likely "high cycle fatigue". This is just the repeated loading of the combustion cycle continuously weakening the material on the thin walled aluminum in this area. A practical limit for the allowable stress in typical aluminum alloys used for heads and engine blocks (say A356 alloy at an elevated temperature of 300 deg F) is in the 13000 psi range.

In the new 3.2 l hypothetical design the cylinder wall stress would be about 11000 psi. Very close the 13000 limit.

So in short, without sleeving, the n55 could likely enable a 3.2 l engine by bore increase alone. However, it appears to be producing a design very near fatigue limits. Thus, unlikely but not impossible.

My prediction for the engine is > 3.0 liter displacement and that will be gained by bore and stroke. I think that was also my prediction before this simple investigation...

As I've noted before even 400 hp from a 3 liter engine is already reaching the same hp/l as the Porsche Turbo. An additional reason I think the displacement must be going up.
First of all ... nice review!

But I simple donīt beleave that BMW/M-GmbH on the one hand would re-inforce the engine by puting reinforcement bars on the outside of the crank-shaft-housing and eventually on the engine block and on the other hand would reduce the thickness of the cylinder walls ... so increasing the bore of the engine for me is no suitable option to increase the displacement. Also if BMW is doing so, they need complety new pistons which would have nothing common with other engines ... no way much much too costly!

The only relativ cost effectiv way to increase the displacement of the N55 is to stroke it ... that this make less sence for an high-pressure and high-reving turboengine is clear.

Greets Uli_HH
P.S. ... but there are recent rumors, that the rumored 3.2 I6 could be the engine of choise for the coming M340i, which would have probably an much greater Hp-increase than current M...i models.

Last edited by Uli_HH; 07-04-2012 at 06:17 AM.