Also note that long stroke engines are typically tuned for good low RPM power, reason being that they don’t survive well at high RPM due to high piston speed, so that might provide the illusion that stroke has an impact on peak torque RPM.
And indeed larger bore does allow more valve area and more power, an effect that isn’t currently modeled but soon will be (next update). One reason to run longer stroke is that it doesn’t make the engine much physically larger, and also in the UK and some other places, cars are taxed on bore, not capacity, hence why long stroke British engines are so common.
I’m aligning with the longer stroke helps make more torque side. A longer stroke is of course not the only, and not the largest influencing factor; but I believe it is one. Without even going into theories; if you look at most engines produced; it holds true. It’s not a massive difference, but there is one. Generally undersquare motors produce about 10 ft-lbs of torque more than their horsepower rating, and oversquare motors produce about 10 horsepower more than their torque rating. 10 is just a rough number, but it’s about the average for what I’m talking about. Of course there are exceptions; as with anything in the world.
You are making it sound like there is no point in making undersquare motors at all; and if that were the case manufacturers wouldn’t produce them.
I think it would make for a good game mechanic as well; if you want a motor with more torque and at a lower speed, then you build an undersquare motor. If you are going to impliment the larger bore = larger valves mechanic, then there would be no point whatsoever to build undersquare motors.
The only thing is that none of the engineering textbooks that we reference play out that theory, so we’d just be making it up unless we can find something that quantifies the effect.
because gasoline burns very fast and power is apllied for a short duration of the stroke, making the longer lever somewhat useless. it could be used to produce more torque by retarding ignition and intake.
Well from what I have learned in physics, the stroke does not really act like a lever. The piston is only going up and down in the cylinder and is applying no horizontal force. That is if we think of vertical being up and down in the cylinder and horizontal meaning left and right. The length of the stroke would only increase the torque if it were to be moving left and right. Lets think of it like this, if you have a pole the is 1 meter long that is attached to something that you are pulling, lets say 20 pounds. If you increase the length of the pole to 2 meters, nothing happens, you still need to use 20 pounds of force to pull it. That is because the force you are using to pull it is not torque since torque is rotational force. Now if you use the same 1 meter pole to pry up something that is 200 pounds it will be difficult, but if you use the 2 meter pole it will be much easier since this time you are using torque or rotational force to pry this 200 pound object up. So stroke really just increases the height of the engine and the volume, since the volume of a cylinder is pi * radius squared * height. stroke being the height, bore being the diameter. I do not see how stroke could increase the torque. This is just how I think of it, I do not know if I am right as I am no expert with this stuff by any means.
The never-ending discussion of torque… sigh Water77 gets most things right, but in general I think this thread has so many misconceptions that it’s painful to read.
Let me start off by showing how stupid it is to even talk about torque in the first place. Weee!
On my bike (me being the motor) I get 130 Nm of torque, ain’t it amazing! Ain’t I accelerating fast!? Well… no because I only have a power output of ~300W while doing so. The only thing this torque accomplishes is bending the frame.
Talking about torque in regards to performance is like talking about speed in kilometers (or miles). Hey, he did 300km! Ehhh, per what? 5h? Not that impressive.
How does stroke affect torque? Simple answer: linearly. If you increase stroke by x%, at the same time you increase capacity by x%, which in turn is directly proportional to torque in first approximation.
If you want to read some more about it, I did a little write-up about this stuff a while back: viewtopic.php?f=13&t=198
killrob, you are saying that if you increase stroke on an engine and bore stays the saame, it produces more torque , explained by you to be a result of larger capacity.
what if you inrease stroke and decrease bore to mantain the same cap.? In the article posted by bishop it tells the exact thing i said; peek burn and peek pressure happen long before 70-80 degrees ATDC, therefore the longer stroke has very little influence on torque
[quote=“darkjedi”]killrob, you are saying that if you increase stroke on an engine and bore stays the saame, it produces more torque , explained by you to be a result of larger capacity.
what if you inrease stroke and decrease bore to mantain the same cap.? In the article posted by bishop it tells the exact thing i said; peek burn and peek pressure happen long before 70-80 degrees ATDC, therefore the longer stroke has very little influence on torque[/quote]
Yep, You, Bishop, Killrob and I are all correct that it stroke has little influence on torque. The only real torque increase is from any capacity you gain. But in terms of just the bore to stroke ratio, you’re more likely to make a little more torque with a larger bore if anything, as you’ve got room for larger valves that flow a bit nicer (or better placement of small valves)
Yes, true true. For very undersquare engines with little valve area you would run into breathing problems, thus losing torque while increasing stroke (keeping capacity constant). Assuming that there are no flow problems at all, more stroke does (to first order) exactly nothing while keeping capacity constant.
So (if the engine size stays the same), bigger bore = more hp and Nm. Bigger stroke = basically just better smoothness.
Am I right?
I suspect diesel engines gain more from enlarged stroke than petrol engines do, simply because diesel burns much slower than petrol, and therefore the peak in maximum burn takes place much later on than in petrol engines.
Honda proved what otherwise? Did Honda found a way to ignore, or just simply bend the rules of physics?
Im not too sure what you are trying to say there darkjedi, but I can assure you that those Honda engines you brought up would rev alot more if they would have made them with shorter stroke and bigger bore. Simply because in that case the distance the piston has to travel is shorter, there’s some physics for you.
So care to elaborate, what did Honda prove otherwise?
For constant size (breathing limited at valves)
bigger bore = significantly more power, more smooth, heavier, larger, more expensive, higher avg. octane requirement, higher max rev, but earlier valve float.
bigger stroke = even less power, less smooth, lighter, smaller, less expensive, lower avg. octane requirement, lower max rev, but later valve float.
At constant size, while not breathing limited at the valves, you take away the effect on power, otherwise things stay the same.
Daffy may correct me when I got something wrong, but that should be most dependencies.
[quote=“FordManFromHell”]Im not too sure what you are trying to say there darkjedi, but I can assure you that those Honda engines you brought up would rev alot more if they would have made them with shorter stroke and bigger bore. Simply because in that case the distance the piston has to travel is shorter, there’s some physics for you.
So care to elaborate, what did Honda prove otherwise?[/quote]
they made some engines, not all of them with very long strokes and 7500-8000 rpm limit. i’ve seen some modded versions hit 9500. i meant they were able to make long stroke engines that rev high and yet still be reliable. jd power said 3 engines in 1000 have a problem. i don’t know their exact reasons for making them like that , but this is what they did.
