Update: False. S55 engine has oversquare bore/stroke and shares "nothing" with N55!

[mkoesel]

Quickie search so far yields:

http://www.clublexus.com/forums/1731672-post60.html

Anyone have access to electronic back issues of that publication?

And that's N52, not N55, yes. But surely if the N55 had increased the bore spacing it would have made big news long ago. I'll keep searching. Feel free to join me or at least stop me when you are convinced.

[mkoesel]

Here's a VAC crank with a ridiculously long stroke (a length which incidentally could get us into 4L plus territory for this supposed new block). Not proof of anything, but does show the measures people have gone to in order to get more displacement from the S54.

http://www.vacmotorsports.com/catalo...troker-kit.htm

Granted they also sleeve, probably to keep the thing from grenading. Don't get me wrong - these guys are competent.

[mkoesel]

http://www.m3forum.net/m3forum/archi.../t-427444.html

Good enough for me. I read it on the internet so it must be true. Please feel free to research liberally if you are looking for more answers and need affirmation.

But yeah, I really just want to know the story of the S55 beyond all uncertainty. Amazing how much mystery this car has provided us, eh? Every time we learn more, we realize still how little we know.

[Boss330]

Originally Posted by mkoesel

http://www.m3forum.net/m3forum/archi.../t-427444.html

Good enough for me. I read it on the internet so it must be true. Please feel free to research liberally if you are looking for more answers and need affirmation.

But yeah, I really just want to know the story of the S55 beyond all uncertainty. Amazing how much mystery this car has provided us, eh? Every time we learn more, we realize still how little we know.

91mm seems correct for the S54 and N52, so should probably be the same for the N54 and N55 as well. And doing new measurements at a larger scale on the N55, I come up with 91,12mm bore spacing...

If so, BMW really has gone to town with retooling for the S55. They MUST have seen major benefits and possibly a long lifespan for the S55 engine

[Boss330]

CanAutM3 has pointed out that there seems to be a slight "problem" with the quoted HP and Torque figures:

As an observation, those numbers cannot be correct.

-Maximum HP between 5500-7200rpm (should be 5700-7200rpm)
-Maximum torque between 1800-5700rpm

The torque and power plateau cannot overlap (5500-5700RPM). Using the assumption of 430hp and 395lb-ft, the transition from the torque plateau to the power plateau should be around 5700RPM.

BMW's dyno chart actually shows that there isn't a overlap of HP and Torque... So, spot on observation



First post edited to contain this info

[mkoesel]

Originally Posted by Boss330

If so, BMW really has gone to town with retooling for the S55. They MUST have seen major benefits and possibly a long lifespan for the S55 engine

Clearly, but the fact that the S55 is only 3L still leaves me with doubts about the entire proposition. The time was ripe in about 1998 for giving themselves room to grow the cylinders. Now? We have turbocharging to increase effective displacement.

Maybe the S55 is larger than 3L. Maybe. But now we are throwing good logic after (possibly) bad.

[mkoesel]

And speaking of people commenting through other channels, a knowledgeable forum member sent me this regarding the earlier discussion about the relationship between stroke length and torque:

Anyhow, regarding torque and stroke, it is a common misconception that for a given displacement, a longer stroke will yield more torque. With a bigger bore, there is greater surface area for the cylinder pressure to work on and provide a greater pushing force. Torque produced by a piston is defined by the following formula:

T = P x (B/2)^2 x pi x S/2 x sin(Teta)
Where:
T=Torque
P=Cylinder pressure
B=Bore
S=Stroke
Tetal=Crank angle with Teta=0 at TDC

[Yellow Snow]

Originally Posted by Boss330

The great thing with a large bore/short stroke engine is that it's possible for BMW M and the tuners to develop "stroker kits".

With a 85mm bore this engine gets 3219cc
With a 88mm bore this engine gets 3329cc
With a 89,6mm bore this engine gets 3390cc

Don't you mean stroke?

Originally Posted by Boss330

See my previous post re bore spacing. http://f80.bimmerpost.com/forums/sho...3&postcount=52

Where do we have the 91mm bore spacing info from?
Is it a verified measurement or just what everyone believes?

It's definitely 91mm spacing. I've measured it myself.

[Boss330]

Originally Posted by Yellow Snow

Don't you mean stroke?



It's definitely 91mm spacing. I've measured it myself.

I meant stroke yes...

And if you read my later posts I also measured a 91mm bore spacing today

[Boss330]

Originally Posted by mkoesel

And speaking of people commenting through other channels, a knowledgeable forum member sent me this regarding the earlier discussion about the relationship between stroke length and torque:

Agree that it's not as simple as oversquare means low torque and undersquare means loads of torque.

However undersquare (with a large bore and short stroke) means the possibility to fit larger valves increasing flow.

Some info I found doing some searching:

-------------------

A longer stroke will have greater port velocity at a given RPM, more torque due to more leverage on the crank, will achieve it's greatest efficiency at a lower RPM, and have less peak potential than a shorter stroke motor. Smaller combustion chambers are also more efficient, with the flame front having a shorter distance to travel- this leads to being more detonation resistant, and having an advantage for emissions.

Bigger bores with shorter strokes have the potential to turn higher RPM's, and larger/more valves will fit into bigger combustion chambers. Since the HP race involves turning ever higher RPM's to make more power, the oversquare motors have increased in popularity particularly when it comes to motorcycles.

Due to the emissions requirements, the lack of widespread consumer interest in 10,000 RPM cars, and the better reliability from lower RPM's, most car/truck engines are nowhere near as oversquare as motorcycle engines.

While it would be interesting to check how one bore/stroke combo would compare to another, it's important to remember that any results you find would be different with other engine combo's. Port size/shape, camshaft profile, exhaust backpressure, intake shape/length, piston style, compression ratio, and a thousand other items would affect the results. Each change would favor one bore/stroke ratio over the other. It's the overall package that counts, not one specific item.

Huge ports, aggressive camshaft profiles, short intake (high RPM type motor) would likely show a preference for an oversquare motor.

-------------

Agree that on a turbo motor many of the points above aren't that critical and a undersquare engine would have sufficed.

But, if bore/stroke info is correct, i give credit to BMW M for choosing a engine concept that is generally better for a high performance, high rpm engine. Especially given the work involved in this...

[mkoesel]

Originally Posted by Boss330

Agree that it's not as simple as oversquare means low torque and undersquare means loads of torque.

However undersquare (with a large bore and short stroke) means the possibility to fit larger valves increasing flow.

(I think you meant over-square)

... which would have come in handy on all BMW I6 engines in the recent past as well, including the S54 and other M engines. So, I stick firmly by my point that there is no compelling technical argument for avoiding an over square bore (and bore center increase) for so long. There may have been other reasons at play (costs of tooling or unavailability of more flexible tooling), sure.

Agree that on a turbo motor many of the points above aren't that critical and a undersquare engine would have sufficed.

Plus, 3L displacement is plenty for 430hp. If this were a 500hp engine, I would have already been reasonably convinced we have a larger block with more displacement, even though they've hit 3.2L in the past (using iron only, however) and even though Mercedes is now hitting 175hp/L in a $50k car.

But, if bore/stroke info is correct, i give credit to BMW M for choosing a engine concept that is generally better for a high performance, high rpm engine. Especially given the work involved in this...

For sure. Especially when I've believed all along they would go with the current I6 architecture to keep cost down.

[Boss330]

Agree that a short stroke/large bore engine would have made even more sense back in the NA days.

Possibly a sign of larger budgets and/or larger production volumes that makes it economically viable to do this now.

I also see the advantage of being able to increase stroke and/or bore in future versions of this engine. Might be an indication of a long term engine platform.

[Jonjt]

Originally Posted by mkoesel

And speaking of people commenting through other channels, a knowledgeable forum member sent me this regarding the earlier discussion about the relationship between stroke length and torque:

Not claiming to know anything about generalities in engine performance but, by this forum members own formula, torque will increase for a given displacement if the stroke is increased. That formula is just T=F*r*sin(theta), with some extra terms to find force and the moment arm from the stroke and average cylinder pressures.


I see no reason why torque won't increase for a given displacement and more stroke. Just saying.

[solstice]

From experience with bikes where the difference between bore & stroke is a lot bigger than among cars a larger stroke/bore ratio is used for and results in torquier, low revving engines.

Here's a vivid example of my current V-twin bike vs. a similar sized V-twin of very different character.

Ducati 748 Bore x Stroke: 90x58.8 mm ( My bike )
Harley Davidson 883 Bore x Stroke: 3.0x3.8 inch

In my practical experience from bikes of all types the bore x stroke makes a huge difference in the engine character where long stroke yields an engine with low end grunt and lower peak rpms while a short stroke yields a much racier character with a weak low end but high peak rpms and hp.

The S55 vs N55 difference is however relative minor but still the lower piston speed at a given rpm and larger fuel intakes (valves ) should make room for higher rpms. It makes me wonder if there is a sigfiicant room left that will be utilized to push peak power and redline significantly higher up in the rpm range with a comp. pack. Are the small Mitsu blowers up to feeding the required pressure?

The reason BMW has not implementd a shorter stroke before is likely to find the best balance of both HP and TQ for a road car. They've done a good job at that. The shorter stroke is likely also part of why the M4's TQ is rated lower than that of an Alpina B3.

[aus]

I'm thinking this will be the replacement block for the N55. That block has been around for some time now.

.

[swamp2]

Originally Posted by Boss330

CanAutM3 has pointed out that there seems to be a slight "problem" with the quoted HP and Torque figures:



BMW's dyno chart actually shows that there isn't a overlap of HP and Torque... So, spot on observation



First post edited to contain this info

Nothing is inconsistent in this because power and torque have different units and therefore must have different scales. The curves will cross at different points depending on units or not at all. In US units if the dimensions of the units are removed, the curves cross at 5252 rpm.

Now that being said there are inconsistencies with 395 ft lb and 430 hp at 5500 rpm, that point is simply impossible. I think its obvious that when BMW said more than 369 ft lb they were not being inconsistent. ~410 ft lb actual is a much better and self-consistent estimate if ~430 hp is the actual power (i.e. not underrated).

[Yellow Snow]

I've just had it from a very reliable source at BMW that the S55 cylinder spacing is 98mm.

[Boss330]

Originally Posted by swamp2

Nothing is inconsistent in this because power and torque have different units and therefore must have different scales. The curves will cross at different points depending on units or not at all. In US units if the dimensions of the units are removed, the curves cross at 5252 rpm.

Now that being said there are inconsistencies with 395 ft lb and 430 hp at 5500 rpm, that point is simply impossible. I think its obvious that when BMW said more than 369 ft lb they were not being inconsistent. ~410 ft lb actual is a much better and self-consistent estimate if ~430 hp is the actual power (i.e. not underrated).

In metric units the lines don't cross as they do at 5252rpm in the imperial system. What CanAutM3 pointed out was that you can't have overlapping max torque and max hp rpm.

In the first post it was claimed from one of the sources that max hp began at 5500rpm while you had max torque until 5700rpm. According to CanAutM3 that can't happen. And the dyno graph backs this up. Max hp starts at 5700rpm, or at the point where torque starts falling off.

Hp and torque also doesn't add up but that makes sense as BMW has said substantially more than 369ft lb, which makes sense.

[swamp2]

Originally Posted by Boss330

What CanAutM3 pointed out was that you can't have overlapping max torque and max hp rpm.

I must still be missing something. I can draw a torque and power curve where the peaks in each are single valued and at the same rpm. You must be referring to a special case where there are large plateaus in the curves.

Originally Posted by Boss330

Hp and torque also doesn't add up but that makes sense as BMW has said substantially more than 369ft lb, which makes sense.

Yes, still agree on this. Torque is probably going to be ~410 ft lb.

[mkoesel]

Originally Posted by Yellow Snow

I've just had it from a very reliable source at BMW that the S55 cylinder spacing is 98mm.

I appreciate the input, but we've heard from enough "reliable sources" over the past few years that we could have reliably been talking about the next M3 being a reliable diesel pickup truck. Or an intra-stellar skateboard with fairy wings.

Hopefully we get a full engine deep dive soon from the horses mouth.

[mkoesel]

A rebuttal from out mystery scientist (in his theatre?)!

"Not claiming to know anything about generalities in engine performance but, by this forum members own formula, torque will increase for a given displacement if the stroke is increased. That formula is just T=F*r*sin(theta), with some extra terms to find force and the moment arm from the stroke and average cylinder pressures.

I see no reason why torque won't increase for a given displacement and more stroke. Just saying."

The displacement of a cylinder is defined by:

D = (B/2)^2 x pi x S

Substituting this in the Torque formula I provided yields:

T=P x D/2 x sin(Teta)

This demonstrates that Torque produced is displacement dependant regardless of bore and stroke

This formula remains theoretical as there many other parameters that impact the torque prodcuced, but proves the concept nonethelss.

(I apologize for the rather unconventional chain of communications)

[NISFAN]

Originally Posted by mkoesel

A rebuttal from out mystery scientist (in his theatre?)!




(I apologize for the rather unconventional chain of communications)

The formula given is a mechanical one. What is not shown is the combustion process and engine characteristics for different stroke lengths. It is these that also influence torque.

A tidbit to ponder over, is that in a petrol engine, the combustion process is over by around 14-16 degrees crank shaft angle ATDC. It is this critical stage that help the extra leverage at small crankshaft degrees make a significant difference in torque. The view is helped by the thought that no matter what force you apply to the top of a piston at TDC, the torque read at the crankshaft will be zero.

Also.....

The main long stroke = higher torque basis is derived in part, in the commonly accepted view that long stroke engines produce peak torque at lower revs than a short stroke engine. They are simply more efficient in this range.

We also know that it is easier to produce higher torque figures at lower engine speeds.

For example, if we were to take a particular engine fitted with a medium cam profile and measure the torque, it would typically be higher than the same engine with a high duration camshaft. The high duration camshaft would develop the torque higher up in the rev scale, and the engine would physically produce more power, but peak torque would be lower (typically). (Of course a short stroke engine would favour building torque at higher revs, but the increase in torque would not be as significant)


Finally, on turbo charged engines, peak torque/power is limited to the onset of detonation. It is commonly understood that a wide flat disc (short stroke) is worse than a smaller higher disc (when talking about the shape of the squish area). This allows a long stroke engine to have more ignition advance, or more boost pressure for a given capacity. These two factors produce more torque in an engine.

The above is a general view, of course, there are exceptions.