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

[Boss330]

Originally Posted by solstice

Weight. The E90 335i is 220 lbs heavier than the 328i sporting the NA version of the engine. With a similar saving from the F80 we are at 3300 lbs with no other lightening of the car.

The E92 M3 GTS is 300 lbs lighter than a E92 M3. Give this hypothetical NA F80 limited edition the same savings and we are at 3000 lbs ( with driver which close to an E30... ). Then increase bore as far as cylinder walls safely allows it and you will have quite a machine that will have little problems smoking a 335i and the sound of an NA I6 at 9000 rpm will be worth it alone.

E46 M3 with straight pipes

[mkoesel]

Originally Posted by Boss330

Wasn't the "original hypothesis" about two comparable engines?

No. Because that immediately makes the whole thing subjective. How do you mathematically model "comparable"? You can't. You need to define what that means first. And that definition will ultimately boil down to the need for a more specific hypothesis.

We can compare any number of interesting cases including motorcycles, turbocharging, and so forth all day long. Great. But all we need is one counter example to disprove the original statement.

The genesis of the discussion was regarding BMW's inline architecture, what might have kept it the way it was for so long and what might be prompting a new design now. The suggestion was that stroke needed to be a certain length to get a certain result. If we are now saying that this limitation applies only within the confines of some other parameters, it obviously changes the entire discussion. And clearly we then need to define what those parameters are and how they are being restricted to draw any meaningful conclusion.

[mkoesel]

Originally Posted by NISFAN

No, the GT3 last time I checked is 3.8 or 4.0 litres depending on year model

Ah, but like I said earlier, 3.8L is very feasible with a 98mm bore center. You can go over 4L if you push the stroke further, but it will, as we know, start to effect max RPM.

If this really is a new block I am 100% sure we'll see naturally aspirated versions built by tuners and enthusiasts within a few years. Furthermore, I'd put money on 500hp+ as I'd said. Still no match for an N54/N55 with a turbo upgrade, but nothing to sneeze at either.

[mkoesel]

Originally Posted by NISFAN

The internal combustion engine is a complex beast. Peak torque value and where it occurs in the rev range is controlled by so many different parameters.

Absolutely.

You know what, though? I have been impressed by how accurately people have been able to simulate complex systems with what seem to be, by comparison, relatively simple mathematical models. We've seen great examples of that just in this discussion forum alone.

I think we can all agree that the greater displacement (including effective displacement as with forced induction) an engine has, the greater its maximum torque potential will be. So an engine with infinite displacement, regardless of its maximum (non-zero) operating RPM, will make infinite torque. We can also probably all agree that the higher RPM an engine can spin, the more power it can make. So an engine spinning at infinite RPM will make infinite power irrespective of its (non-zero) displacement. Those, to me, are some pretty simple properties of very complex systems.

But unfortunately I've yet to see anyone fit bore dimensions at a fixed displacement into the picture. Maybe it's simply because they aren't ultimately what determines the engines output? That's a genuinely open question on my part, not a thinly veiled assertion.

[dhoggm3]

Length/strength of the crank is a limiting factor for rpm for an I6 configuration. A flat 6 has only half the crank length.

[Boss330]

Originally Posted by mkoesel

No. Because that immediately makes the whole thing subjective. How do you mathematically model "comparable"? You can't. You need to define what that means first. And that definition will ultimately boil down to the need for a more specific hypothesis.

We can compare any number of interesting cases including motorcycles, turbocharging, and so forth all day long. Great. But all we need is one counter example to disprove the original statement.

The genesis of the discussion was regarding BMW's inline architecture, what might have kept it the way it was for so long and what might be prompting a new design now. The suggestion was that stroke needed to be a certain length to get a certain result. If we are now saying that this limitation applies only within the confines of some other parameters, it obviously changes the entire discussion. And clearly we then need to define what those parameters are and how they are being restricted to draw any meaningful conclusion.

IMHO stroke isn't the ONE factor that can be isolated and all other parametres be different.

Other parametres means a identical engine, apart from bore/stroke ratio. Same cr, same volume etc. How can that be subjective?

Are you still seriously saying that comparing two very different engines Will tell us anything about different bore/stroke ratios?

[mkoesel]

Originally Posted by Boss330

Other parametres means a identical engine, apart from bore/stroke ratio. Same cr, same volume etc. How can that be subjective?

I'm pretty surprised by this response. It was you who posted the information earlier about the bore size dictating cylinder area which impacts valve diameter which impacts air volume. You cannot increase stroke without decreasing bore.

The discussion was about feasibility, not artificially restricting arbitrary parameters. If you don't like the limitations you've imposed on your hypothesis, feel free to revise it.

[Boss330]

Originally Posted by mkoesel

I'm pretty surprised by this response. It was you who posted the information earlier about the bore size dictating cylinder area which impacts valve diameter which impacts air volume. You cannot increase stroke without decreasing bore.

The discussion was about feasibility, not artificially restricting arbitrary parameters. If you don't like the limitations you've imposed on your hypothesis, feel free to revise it.

The discussion was, from my view, whether a larger stroke creates a different power characteristics as regards hp and torque.

I have then elaborated that larger bore also has other benefits, whereas long stroke has it's benefits.

We have probably been talking past each other in these posts. To me the reasons why a 458/Ducati 848 engine is oversquare and a high torque low rev engine (such as the HD 883) is undersquare confirms my view on the difference and advantages of over- vs undersquare.

[mkoesel]

Originally Posted by Boss330

We have probably been talking past each other in these posts. To me the reasons why a 458/Ducati 848 engine is oversquare and a high torque low rev engine (such as the HD 883) is undersquare confirms my view on the difference and advantages of over- vs undersquare.

I genuinely appreciate your belief, but the fact is that we still have no mathematical model that relates stroke length and bore size to torque at a fixed displacement.

What we do have are the equations I was sent offline and posted earlier which I think make for a good start. NISFAN suggests that while the formulas cover the mechanics of the rotating assembly, they ignore the combustion process which he contends is effected by the stroke length. So there is more to the picture. It may be too complex to boil down to a simple set of equations - I don't know.

Either way, absent of proof, I have to remain where I was before the discussion which is that I am not convinced one way or the other about how the bore dimensions effect torque. What I am fairly convinced about, however, is that a naturally aspirated engine approaching 90 ft-lb per liter has got to be coming close to what is physically possible or, by now - in the past 100+ years of evolution - we'd have seen that number go up and up and continue to as time goes on and engines have become more advanced. So, then if there were anything that could be done with the stroke to make a big advancement it would have occured by now even if it meant something seemingly ridiculous like using a 200mm stroke with a tiny 30mm bore. In other words, I think that in the end you reach a point of diminishing returns at a given displacement so rather than spending money on working out tiny gains at that displacement, an engineer will simply increase the displacement which could be done either by increasing stroke or bore (or both) since either will accomplish that.

[Boss330]

Originally Posted by mkoesel

I genuinely appreciate your belief, but the fact is that we still have no mathematical model that relates stroke length and bore size to torque at a fixed displacement.

What we do have are the equations I was sent offline and posted earlier which I think make for a good start. NISFAN suggests that while the formulas cover the mechanics of the rotating assembly, they ignore the combustion process which he contends is effected by the stroke length. So there is more to the picture. It may be too complex to boil down to a simple set of equations - I don't know.

Either way, absent of proof, I have to remain where I was before the discussion which is that I am not convinced one way or the other about how the bore dimensions effect torque. What I am fairly convinced about, however, is that a naturally aspirated engine approaching 90 ft-lb per liter has got to be coming close to what is physically possible or, by now - in the past 100+ years of evolution - we'd have seen that number go up and up and continue to as time goes on and engines have become more advanced. So, then if there were anything that could be done with the stroke to make a big advancement it would have occured by now even if it meant something seemingly ridiculous like using a 200mm stroke with a tiny 30mm bore. In other words, I think that in the end you reach a point of diminishing returns at a given displacement so rather than spending money on working out tiny gains at that displacement, an engineer will simply increase the displacement which could be done either by increasing stroke or bore (or both) since either will accomplish that.

As I wrote in a previous post, yes it's more complex than just one factor. But generally speaking a high revving low torque engine will be a oversquare engine like the 458 or a F1 engine. A low revving high torque engine will usually be undersquare. There are exemptions like the S54, but piston speeds are at the high end which is not good for reliability/wear.

I guess we will have to wait on BMW to release details of why they went this route (if info is correct).

Maybe we all have the wrong take on why they did it

[swamp2]

Originally Posted by NISFAN

Hmmm, I think I worked out VE of an S65 at around 91% @ 8000rpm, which is very good in today's CAT filled society. Always open to see a more scientific calculation.

Did you take measurements? How did you calculate it?

Post #83 on this page here. Show a very lightly modified intake system on a Z4 M achieving a VE of over 100% at 60 mph and a peak VE of about 108%. This fellow did all of the mods and testing himself. I totally trust his insight when he states how REALLY well designed the E9X M3's intake system is and he has NO doubts it also will easily achieve over 100% VE.

The background of this somewhat infamous thread (and collection of threads actually) is on whether or not intake scoops near the kidney grills on an E9X M3 can and do add hp.

[car_fan]

Well it's no arguing the fact that Turbo Technology will be much more prevalent in the main stream than it was in the 80's. I got a chance to drive the 2014 Panamera S TTV6 at the Porsche World Road Show/Miami. Power delivery is decent, as well as the thrust. I'm down to one N/A car now, I hope to keep it around for a while.....

2014 Porsche Panamera S specs:
Technology Standard Optional
3.0 Liter Twin-turbo V6 engine
420 hp @ 6000 rpm
Torque: 384 lb.-ft. @ 1750-5000 rpm
Aluminum engine block and cylinder heads
Integrated dry-sump lubrication
Direct Fuel Injection (DFI)
VarioCam Plus
Cylinder-specific knock control
Returnless fuel system
Hydraulic valve adjustment
Engine Management System (EMS SDI 10)
On-board diagnostics for monitoring emission control system
Stereo Lambda control circuits
Electronic throttle
Vehicle electrical system recuperation

[Boss330]

Originally Posted by mkoesel

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

(I apologize for the rather unconventional chain of communications)

As far as I can tell, this formula just confirms my view. This quote from your quote ( ) says that:

torque will increase for a given displacement if the stroke is increased

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

What we have with long and short stroke can be simplified with the following illustrartion:



However, as the bore is smaller on a oversquare engine the force (F) acting on he lever (crankpin) will also be different, in some way negating the longer arm of a longer stroke.

These would be very complex calculations.

However I'm eagerly awaiting BMW's official reasoning for going down the oversquare route. Is it because of this engine and advantages? Or is it "simply" because it allows for larger bore and even more stroke in future engines?

Like future M5 engines with a 3,8l straight six... The N55 would not be able to do that, the S55 will (again if info so far is correct).

[swamp2]

I think some key points about bore to stroke ratio are being missed in this discussion/debate. If the following SAE paper also has torque results it probably has the answer to the speciic debate happening.

Engine Combustion at Large Bore-to-Stroke Ratios

I did not want to shell out the $25 bucks or so for the paper. Even my SAE membership doesn't get them for me for free...

Anyway, from my perspective, the interesting things about bore to stroke ratio are specific output (power per liter) and (fuel) efficiency. Take a look at the following graph (source: Lecture notes Prof P.M.V Subbarao, ITT Delhi)



Here is the plenty obvious conclusion. For maximizing the specific power output at the expense of fuel efficiency a large bore/stroke ratio (small stroke to bore) is required. Note that F1 and normal passenger vehicle engines probably produce about the same amount of torque say 200 ft lb, with a bore/stroke ratio of 200-250% different. Sure torque probably is affected by altering stroke alone but the focus on torque is missing the point. Power and redline rise dramatically with a very short stroke and (as mentioned prior) valve area can go way up.

Marine engines are mentioned here specifically because the big long haul type are indeed built for maximum efficiency like heavy duty trucks.

The bore/stroke ratios of most of the vehicles you've been discussing are all hovering so close to the 1:1 mark undersquare, oversquare, whatever. However, to split hairs, the S55 engine will produce in an apples to apples way a better specific power output than the N54. But at the same time it moves down the curve quite a lot to focus on efficiency over specific output compared the the S65.

[Yellow Snow]

Originally Posted by Boss330

However I'm eagerly awaiting BMW's official reasoning for going down the oversquare route. Is it because of this engine and advantages? Or is it "simply" because it allows for larger bore and even more stroke in future engines?

I think that's hit the nail on the head. It's simply to easily allow a whole new series of future capacity increases. (IMO)

[Boss330]

Thanks swamp2

As usual, very insightful and great explanations. You Sir, are an asset to Bimmerpost

[Boss330]

Our resident offline expert has corrected my post with the following info:

In your last post with the wrench illustration, you mention that the calculation to establish by how much the force reduces based on the bore reduction to be very complex. Actually it isn't. It is very well reflected in the formulas I provided.

From your wrench example:

T=F x d

Where the force is defined by the cylinder pressure acting on the piston area (P x (B/2)^2 x pi) and the distance is the vectorial product of the half stroke relative to the crank position (S/2 x sin(Teta)). Hence:

T = P x (B/2)^2 x pi x S/2 x sin(Teta)

What is interesting is that due to the geometric characteristics, any increase in stroke for a given displacement will be negated by the same exact amount by the bore reduction. For a given cylinder pressure, torque is therefore displacement dependent only regardless of the bore and stroke relationship. To demonstrate this, take the definition of displacement and substitute it in the torque formula:

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

and Torque becomes:

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

Tada , torque depends on cylinder pressure, displacement and crank angle only. No bore or stroke.

Which makes sense as I had done that calculation myself and realized that I came out with the same torque numbers for both the N55 and S55 config... Something which certainly didn't side with my view on this matter.

However, I suspect "cylinder pressure" might be a keyword here... And that's where the complex calculations are coming into play.

The issue with cylinder pressure, or "Power Density", is illustrated in swamp2's post above. This is where the dog is buried and the "secret" behind why one concept is preferable over the other in different areas (like a F1 engine vs a HD truck).
-

[RiverRunner]

Cylinder pressure can be built in different ways, including advancing timing (detonation before TDC), turbo charging, higher compression, etc., you're also not taking into account the torque loss differences from some of these. If you took the same engine, and only changed the bore or stroke, nothing else, you may be able to mathematically model the maximum potential change, but you still have to take into account changes intake and exhaust volumes (if cylinder volume or rpm) changes, etc.

I know none of us can wait to see, and put one of these on a Dyno, that will tell us exactly what we want to know.

[swamp2]

^ Sort of. Since dyno's vary by at least 10% due to a slew of both controllable and uncontrollable factors I would be much happier to see the results on the only really accurate, public chassis dyno I've seen. Unfortunately their database does not seem to be growing or adding many new vehicles...

http://rototest-research.eu/

[CanAutM3]

The discussion started when it was argued that an undersquare engine had a geometric/mechanical advantage to produce torque due to a “longer torque arm” (post #22). As was demonstrated mathematically, for a given displacement, the bore/stroke ratio has no geometrical impact on the absolute torque produced by an engine. Instantaneous torque is only dependent on cylinder pressure, displacement and crank angle.

Originally Posted by Boss330

However, I suspect "cylinder pressure" might be a keyword here... And that's where the complex calculations are coming into play.

You are correct, the key lies in the cylinder pressure. The discussion has now evolved a bit, so here are my thoughts. An oversquare engine will have better ability to produce cylinder pressure higher in the RPM range and an undersquare engine will in the lower RPM range. As anything in life, there is no free lunch, everything is a compromise. I will spare you the formulas, because as you stated, it becomes mathematically very complex.

A bigger bore allows for bigger valves which in turn improves flow and volumetric efficiency resulting in greater cylinder pressures, especially so in the higher RPM range. A shorter stroke also has benefits in reducing piston speed for higher RPM (not citing anything new here).

However, a large bore also come with disadvantages. Because the engine relies on a greater force produced by the cylinder pressure (due to the big bore) rather than a longer arm (larger stroke) to generate a given amount of torque, the forces that all the components and bearings need to carry are that much greater. So stronger components and bearings are needed everywhere (piston, rod, crank) adding more complexity and cost. Further, these greater forces also generate more friction losses at the piston/cylinder wall interface. Another disadvantage of the bigger bore is the shape of the combustion chamber. For a given displacement and compression ratio, the combustion chambers of an oversquare and an undersquare engine are the same volume, hence the chamber of the oversquare engine is comparatively very broad in diameter but very short in height. Because of this, oversquare engines cannot tolerate as much ignition advance at lower RPMs since the flame will collide the piston and cause detonation, hence lower cylinder pressure. Much less of a problem at higher RPM since the piston speed moving away from the flame is sufficient to avoid the collision. Another disadvantage is the greater blowby losses caused by the greater circumference of the piston (worse at lower RPMs), which lowers effective cylinder pressure. There is more, but I will stop here .

In short, an oversquare engine will have better ability to produce torque higher in the RPM range and an undersquare engine will in the lower RPM range. It is a question of where torque is produced in the RPM range, not how much of it (Note however that producing torque higher in the RPM range will yield more power).

As swamp2 mentions, the bore/stroke relationship is selected by trying to find the best compromise for a given application and/or engine characteristics. Low cost, durability, reliability, overall efficiency favor long stroke low RPM engines. High specific output, power to weight favor bigger bore high RPM engines. On an industrial or marine engine that runs non-stop for days, reliability, durability and overall efficiency are very important. On an F1 car, where engine displacement is regulation constrained, high specific output, maximum power are very important; fuel economy (efficiency), durability and cost, much less so. And then there is everything in between.

[Boss330]

Thanks and good to see you here

[mkoesel]

Originally Posted by Boss330

As I wrote in a previous post, yes it's more complex than just one factor. But generally speaking a high revving low torque engine will be a oversquare engine like the 458 or a F1 engine. A low revving high torque engine will usually be undersquare. There are exemptions like the S54, but piston speeds are at the high end which is not good for reliability/wear.

Note that the V8 in the 458 is also an exemption as it makes more torque per liter than any other engine currently on the market.

If we go the route of subjectivity and look just at the "normal" non-performance cars out there with typical engines, I think we may indeed see a tendency for the under square engines to show slightly higher peak torque and at a slightly lower RPM than those with over square geometry. But, we are talking about an M engine here. Plus as swamp notes, road car engines are all so close to square, relatively speaking, that there is little variation in geometry to begin with.

I guess we will have to wait on BMW to release details of why they went this route (if info is correct). Maybe we all have the wrong take on why they did it

For the record I still haven't been able come up with a truly compelling reason which is why I remain skeptical.