A question about long-stroke or undersquare engines

karabiner98k

As i have read in several articles, it is obvious that short-stroke or oversquare engines can rev higher and produce more power and less torque and usually used in sports cars but long-stroke engines are good for high low end torque and can't rev as high as short-stroke engines.

But i have seen some cars that have quite rev-happy engines (8000RPM) with so much power despite being undersquare.

Two examples are BMW M3 (E46) and Lamborghini Murcielago LP640

BMW M3 (E46) I-6 3.2L

Bore x Stroke = 87 x 91 (mm)

Lamborghini Murcielago LP640 V12 6.5L

Bore x Stroke = 88 x 89 (mm)

So, the above cars have long-stroke engines but with so much revs and power.

How can these cars manage to have the characteristics of an oversquare engine while being undersquare?

brewnog

They're only a bit undersquare, particularly the Lamborghini!

Aside from simple engineering factors (like reducing reciprocating mass and balancing components), the main factor here is that both I6 and V12 engines have perfect primary and secondary balance, unlike a straight four or V8.

karabiner98k

Thanks for your answer.
Could you explain this fact a little?
Why I6 and V12 engines have perfect primary and secondary balance?

S_Happens

Typically textbooks start with the generalization that rpm is primarily limited by mean piston speed. While this is not exactly true, (mean piston speed by itself can barely hint at accelerations seen by the rotating equipment nor does it give any idication of the geometry) it's a good place to start.

brewnog

I can, but the Wikipedia article is a good place to start. Let me know if you get stuck.

http://en.wikipedia.org/wiki/Engine_balance

Ranger Mike

good one brewnog..also con rod length ratio!

mender

When discussing oversquare vs undersquare, you have to talk about the same cylinder displacement, otherwise there is no basis for comparison. The discussion then becomes one of stroke length and other things.

jack action

Size wise, if you want to compare engines, only the bore area is important.
Speed wise, if you want to compare engines, only the mean piston speed is important.

Check this post I've wrote on another thread.

The advantage of an undersquare engine is the possibility or creating higher compression ratio (which is why practically all diesel engines are undersquare).

The advantage of an oversquare engine is a smaller engine (volume wise, hence weight wise as well).

Lsos

I'm surprised you didn't mention most extreme example that I know of: Honda/ Acura Integra type R (B18C5)

4 cylinder
Bore: 81mm
Stroke: 87.2mm
Redline: 8,500rpm

The simple answer to your question is engineering. If you use better materials and better engineering practices, you can end up with some extreme characteristics.

mender

Just think, if they'd only made the engine oversquare and bumped the piston size up to 95 mm or so the engine would have made much more power.

So why aren't newer engines oversquare?

xxChrisxx

No low down torque (which is what most people use for every day driving), as mentioned above.

If you meant keeping the stroke the same but increasing bore - then you are incraesing displacement...

mender

If the engine specs are the same other than the bore to stroke relationship, an oversquare engine will have the same torque as an undersquare engine.

So why aren't newer engines oversquare?

Lsos

I'm not enough of an expert to comment too much on your last post, but I believe undersquare do generally favor/ allow for more torque.

Whatever the case may be, sometimes there are other driving factors behind an engine design. The Acura/ Honda Type R that I mentioned, for example, is derived from the Integra GSR engine, which while still having the same dimensions, redlined at a lower 8000rpm and made about 25hp less. The engineers wanted a factory tuned "Type R" version of this car, and so they set themselves at squeezing more out of this engine. Despite its extreme characteristics, the engine performs extrmely well and reliably, and it was simply easier (and less expensive) to make more power this way than to completely redesign the entire engine/ car from scratch.

mender

That is a myth that just won't die. Oversquare engines have the advantage in both torque and power production.

So why are newer engines designed to be undersquare? The original question that started this thread hasn't been answered yet.

xxChrisxx

I think you'll find you are a bit off there. There are a coupe of reasons why stroked engines produce more torque, they are in Heywoods book on engine fundamentals.

One reason is that force decays away far quicker in an oversquare engine. So although you get a higher peak force value acting down the cylinder axis, most of the force is going into bending the crank rather than turning it. You get a pressure 'spike' close to TDC that decays away quickly, in a long stroke engine you get a force that decays away slower as the valume change per degree of crank angle is less.

There are a few other reasons but i'd have to read up on it in Heywood.

Think about it, if what you are saying WERE the case, all engines would have F1 style bore to stroke ratios. ie huge bores and tiny strokes. There would simply be no downside to doing it. It's for the very reason that they don't produce the same torque that engines are not oversquare.

You also can't really nail this down to one variable either, there are millions of interconnected reasons, as in an engine every variable affects almost every other variable.

mender

If the rod/stroke ratio is the same for both engines, the volume change/degree will be the same and pressure decay will also be the same.

Any other explanations as to why you think that? I see that statement a lot; I also build a lot of engines and dyno them. Did a new engine combo for a racing team just a few weeks ago that proves this wrong. Reduced the engine size by 5% to get a weight break yet lost only 2% torque and 1% power in the same rpm range despite increasing the bore to stroke ratio by 10%.

By the usual oversquare reasoning it should have lost at the minimum 5% torque output because of the displacement change and more because of the bore/stroke change.

If power and torque production were the only considerations, every engine would be designed that way. There is another consideration that is more important; any guesses as to what that is?

jack action

I suspect that undersquare engine is limited by weakened parts. If the piston bore is increased, its height must be too to prevent rocking. But because the stroke is not increased, the piston pin has to move closer (in proportion) to the bottom edge of the piston to respect crankshaft clearance. The higher forces due to the larger piston are not supported by the (still the same) amount of material between the pin and the bottom edge.

Just an idea, nothing to support it.

Mender, can you tell me more about this engine. I'm in a debate on another forum about the fact that power is only dependent on bore and that stroke don't change anything (like I say in this post). It looks like you have just prove that with your engine (it seems you only shorten the stroke without changing the bore, and by getting the same HP you are proving my point). Can you confirm that and give some numbers?

On the other forum, math is not seen as valuable tool to prove something, they want actual measurements.