The validity that stroke is the main producer of more torque came under fire in the Windsor forum, so I defended it. Well, my engine terminology and descriptive abilities aren't getting me very far. I'll admit I am not so sure I understand all I know and have read on torque. Maybe someone can explain it better than me or set me straight.

The point made is that cylinder filling efficiency is where this torque comes from in longer stroke engines and not the leverage advantage. Well, I just don't by those apples entirely. I see more going on than this and it is all based on the stoke leverage and duration advantage.

So, where's the beef on stroke and torque?

 

Basic physics defines torque as force X lever arm. US terms would be lbs and feet. The longer the feet, (stroke) the more the torque. Done.
Old straight six engines would pull stumps. Newer V8 designs don't produce torque until they are at higher revs, just like the DOHC/Multi-timed ricer engines. You have to rev 'em up to get anything out of them.
Used to was you could start cars in high gear, and they would just walk away and gather speed with no protestation. The mechanical advantage of a long stroke will give them torque from the get go. Try that with a late model V8 or 4 banger. Won't happen. They need the rpms to produce torque.
Cylinder filling will only come into play at higher rpms. That is why they have the funny plumbing on the 4's, with variable length intakes, etc.
tom

 

That's pretty simplistic. However, the force part of the equation is made of many componants, not the least of which is the volume of air/fuel being compressed, and how much of the resulting force from the combustion of said air/fuel volume can actually be applied to the crank lever. When everything else is equal of course, the longer the crank lever; the greater the tourqe, but all other things are rarely equal. We can easily show that increasing the stroke of an engine will result in increased output, but the increase in stroke has also increased the displacement, so we can't use this as a proof for stroke alone being the reason.

It could be said that low RPM tourqe output is mostly a function of displacement, and not stroke in practice. Let's look at three different Ford V8 engines of roughly the same displacement, but with three different strokes. The 289 had only a 2.87-inch stroke. The 292 had roughly the same tourqe output, at the same engine speeds, with a 3.3-inch stroke. The 281 (4.6 modular) actually has slightly more stroke than a 351, and significantly more than a 289, but it's low RPM tourqe output is roughly the same as the other two above mentioned. The 4.6's tourqe curves are much like other modern V8's with overhead cams, and it really takes off with greater engine speeds were it's overhead cam head design gets great volumetric effiency.

The actual leverage distance to the crank centerline is 1/2 the stroke. A 302 has a crank leverage distance only 0.42 less than a 460. The difference in throw between most V8 engines ranging from 5.0 liter displacement to 6.0 liters displacement can be measured in fractions of an inch, and sometimes even that can be thrown away with poor rod angle. Imo, way too much empahisis is placed on stroke as the function for tourqe. It's an important related factor, but just one of many factors.

 

torque is a byproduct of chemical energy being turned to mechanical energy. Chemical energy (combustion) in turn creates cylinder pressure driving the piston downward. The piston is connected to the crank via a rod. The way youre rotating assy reproduces that cylinder pressure into mechanical energy is the single most important part of making good torque. Your bottom end has to be effecient at what it does, and loose the least amount of chemical energy possible while turning it into mechanical energy. Once you have done that, it comes down to how efficient your top end is at getting the a/f mix to the cylinder. Youre only able to make as much hp/torque as your induction system will allow. If your intake velocity is all out of whack you wont get proper cylinder filling. The same can be said for the bottom end, you can have the best top end, and an inneficient bottom end, and be lacking the in power department.

 

If it helps, I did some testing in Desktop Dyno 2000. I set up a theoretical engine, no particular type:

~352 cubic inches, 8 cylinder
Identical head flow, 1.9" x 1.4" valves
9:1 compression
600cfm carburetor, dual plane intake
small tube headers, mufflers
cam with the following specs: 260/272 degree adv. dual pattern, .503"/.533" lift

Here's the numbers: (program only reads torque down to 2000rpm)
4" bore, 3.5" stroke, 351.9ci:
Peak HP: 322 @ 5000rpm
Peak torque: 384 @ 3500rpm
Torque at 2000rpm: 368 ft-lbs

3.5" bore, 4.573" stroke, 352.0ci:
Peak HP: 308 @ 5000rpm
Peak torque: 378 @ 3500rpm
Torque at 2000rpm: 369 ft-lbs

Now, lets look at the first one when the stroke is increased to 4".
4" bore, 4" stroke, 402.1ci:
Peak HP: 319 @ 4500rpm
Peak torque: 429 @ 2500rpm
Torque at 2000rpm: 428 ft-lbs

It would appear to me that torque is more a function of cubic inches, not stroke.

 

Yup, it's all in how the cylinders fill. Long stroke, small bore, short stroke, big bore. One has the lever, the other has the bigger piston pushing down on it.

Long stroke motors tend to have more flywheel effect due to the greater mass of the crank. This could create the impression of more torque, as they may launch with less throttle.

The closest apples to apples comparison is the Ford 300 six, at 4 x 3.98, vs. a GMC 305 V6, a 60 degree job with a 4.25 bore and 3.58 stroke. Virtually the same hp and torque of the Ford straight 6, in the same rpm range.

http://www.6066gmcguy.org/EngineData.htm

 

run something with a 2" stroke, and then a same sized engine with less bore and 4" stroke. youll get more low rpm torque with the longer stroke.

however, like I said. Stroke is not the only thing that gets you torque, theres stuff like rod:stroke ratio, intake manifold air velocity, type of fuel being burnt, exhuast.....

take diesel fuel, it doesnt have the initial combustion force of gasoline, and burns slower. The combustion takes place during more of the power stroke, allowing a longer duration of downforce on the piston. Where as gasoline is an initial boom and thats it. This is why diesels make way more torque than horsepower.

If your rod:stroke ratio is so crappy it takes all the initial combustion just to get the crank to turn, youre going to loose a bunch of torque because the rod can turn the combustion into winding effort easily enough.

Exhuast, if you have major back pressure its going to take more energy to push the gas out of the chamber than if you had a free flowing exhuast.

with your intake, if you can tune the runners to the right length and diameter they will actually help in filling the cylinder, as air and fuel are both masses, and when a mass gets moving it will continue to move once the driving force is removed (ie intake stroke) due to momentum. If your intake sucks ***** its going to take more energy from the rest of the engine to draw air into a cylinder, thus reducing torque/hp

it takes HP to make torque

 

ok lets look at this in another light shall we. some have stated it's about the stroke length others say it's about the additional volumetric efficency. Ok lets look at stroke, and even add in CID but the torque figures will not hold up. 3.85" stroke. torque output 390ft/lbs, other engine 3.74" stroke 620ft/lbs, first engine ford 460 second engine international vt365 (6.0L) set at 230hp. now yes I understand the comments will be apples to oranges but not really for this discussion. what it shows is that cylinder preasures have as much if not more to do with the torque than either the stroke or the CID.

 

Let me take a whack at it too. Don't laugh too hard now because I'm probably going to be very crude and probably simplistic as well.

I see that a longer stroke engine not only has the leverage advantage but also a longer duration of that leverage. Is or isn't the force of a combustion stroke applied to the piston longer in a longer stroke engine? If this is true then an engine with a longer stroke has (even if they are small), a total of eight added power producers which together equal a big one.

Also, I do see where a longer stroke engine allows for the intake valves to stay open just a bit longer which makes an engine's induction system more (now Im saying it too) efficient?

Am I getting it yet?

 

well think about it this way, gasoline combustion is a sudden boom and if you cant get the rotating assy transfering that boom to torque well enough youll be half way(hypothetical) down the stroke and all out of energy to spin the last half. All you have then is the power the other cylinders make to move the piston the last part down its stroke, taking more energy away from the power stroke of the next cylinder to fire, eating up potential torque output of the engine.

 

It's not perfect, but it's as close to apples to apples as we might get, and that's to compare the 427 FE to the 428 FE. Here we have two FE engines of similar designs, the same displacement, but with different strokes. The 427 has a 3.78-inch stroke and 4.23-inch bore. The 428 has a 3.98-inch stroke and a 4.13 -inch bore.

Both engines made 460 ft-lb's of tourqe, so it seems that cid is once again the main determinant. However, the 428 makes it's peak tourqe at only 2800 rpm, and the 427 makes it's peak tourqe at 3200 rpm. Although I took the specs for the hydralic cam 427, the cam probably has something to do with this, but I think this is probably mainly the rod ratio.

The 427 has a rod ratio of 1.715. The 428 has longer stroke and therefore a different rod ratio of 1.63. This means that the 428 is going to pull the piston a little further down the bore by 90* crankshaft rotation. This should result in a little better cylinder filling at lower engine speeds. The 427 has the better rod angle at 90* rotation, so it may be able to apply more force to it's slighty shorter crank throw. Indeed the the general rules for rod ratios seem to be in effect here. Lower rod ratios result in peekier power curves lower down on the rpm scale, and longer rod ratios should give flatter tourqe curves, but with the peeks coming a little higher up rthe rpm scale.

Both engines seem to be optimized for different rpm ranges. The 427 has more valve area to match it's bottom end dynamics, and it's shorter stroke allows it to reach greater rpms before critical piston speeds are reached. The 427 makes about 10% more HP on the high end. The 428 should have slighty better volumetric efficiency at lower engine speeds, and it's piston speeds and rod angles should still be okay up to it's lower, but more streetable redline.

 

Ok, I've gotten stuck on this sudden boom of the air fuel mix in a gasoline engine and the power produced is over by half of the piston's combustion stroke. If this were true then I shouldn't see flames being shot out of an engine with no exhaust system except for headers? Where is the energy coming from to produce those flames if all of the energy has been exhausted before the exhaust stroke even starts? In my world you simply just can't have flames shooting out of something that doesn't have energy.

Here is something else to think about. I have been working on an engine that was having ignition and timing probems. I had finally fixed the problem and while rotating the distributor (with the key on) I caused the engine to fire once with only the #1 sparkplug wire attached. This single combustion stroke turned the crankshaft around almost 180 degrees. Ok, now the ability of this single combustion stroke I have witnessed is obviously very different than what has been described in this thread. Why such a huge difference?

 

That is a good comparison P51D!

Both of those engines have been optimized to enhance the characteristics also but the basic comparison is valid. The factory engineers did know what they were doing.

 

the power stroke takes up 180 degrees of crank travel. The crank rotates 720 degrees for a complete cylce on one cylinder. Now doesnt it make sense to you that if in that same situation with results of 270 degrees in crankshaft rotation, that your engine would be more efficient in making power? The further one combustion stroke can carry the rotation, the more power it will free up on the rest of the cylinders, and vice versa.
Now look at an explosion, dynamite for example....when the first boom happens there is an immense shockwave emmitted, blowing crap up all around it, and shaking things that are further away..now a split second after the initial boom/shockwave you see a billowing fireball/mushroom cloud emerging. Same goes in your chamber, you get the boom, then you get the majority of the fireball aftermath, which blows out your exhuast.

As for the combustion running out of energy halfway down the power stroke, I do believe I stated the situation was purely hypothetical. It was used for you to grasp the concept on how longer stroke can hurt overall performance, by screwing up your engines geometry. Look at all the stroker kits out there, theres only one I know of that uses stock length rods, all the rest use longer rods. Take a 302, stock rod length is 5.09", stroke is 3.00".... in a 347 stroker application rod lengths in kits are 5.4" with a 3.4" stroke. This is to keep the rotating assy as efficient as possible to allow full engine potential.

 

Pud, you really confused me with the hypothetical scenario because I am looking at this realistically (what we know happens).

So in reality, does the power being created get applied longer in engines with a longer stroke? With the added cubic inches along with the longer stroke it seems to me that the combustion of more air fuel mix applies power longer than a samilar cubic inch shorter stroke engine which will need to rev higher to create the same power because the power is being applied a shorter amount of time with it.