my thought is look at the crank on a 454 vs a 305. A 454 has more rotating mass and more torque. how high the motor revs rreally has nothing to do with it

 

i used this analagy, a 396 big block vs a 400 smallblock, basically the same displacement and stroke. But the 396 makes more torque, even though its a margianal amount because of the heavier crank.

 

the way i understand torque is "how hard is it to stop the motor". translated litterly from another language it would be "rotational momentum", the higher the mass, the higher the momentum at the same rpm. diesels make a crap load of torque, but they will never rev as high as a gas motor, so you would be correct on all points. but also weight is not the only way to get touque.

 

it all depends on how you're looking at it and what the system constraints are. basically torque is a "moment of inertia" from a static standpoint it is defined as " a force applied at a distance" like whn you're trying to loosen a stuck bolt. would you use the 10" rachet, or add a 36" cheater bar to it? add the extra distance and the torque is multiplied. so basically it works the same for rotation masses. given the same sized wheels. it takes more torque to spin a solid rubber wheel versus a pneumatic wheel. since it has less mass. this isa good example becasue the bulk of the mass is near the outer portion.
torque in an engine depends on a lot of things...compression stroke, bore...etc etc. but if two engines were identical and the rotating mass (crank and flywheel) were oif different masses. it would be easier to stall out the engine with the lesser rotating mass.
I"m not sure what it would be like in a real application thouhg. becasue it takes more energy to get the mass moving faster in the high mass engine. i gues it depends on how you look at it.
hmmm, ididnt answer the question really, but perhaps shed some light on the issue. this same issue ois why a brake upgrade is important when upgrading to larger tires. ti takes more power to stop the rotational mass of the bigger tires along with the increase in regular old mass.

 

Torque is the amount of "twist" applied to the tranny, bolt,
or whatever.
It has nothing to do with mass.

Take for example a torque wrench.
Pull 50 lbs on it at 1 foot = 50 ft/lbs.
Pull 25 lbs on it at 2 feet, - same = 50 ft lbs.
Pull 100 lbs on it at 6", same = 50 ft/lbs.

The bolt ended up with the same amount of tightness.
The differences was only leverage.

The weight of the wrench (flywheel) - inertial mass - never
came into play, whether the wrench weighed 1 lb or 100 lbs.
And the speed at which you pulled it never came into play
either, whether you jerked it or did it slowly, 50 ft/lbs
is 50 ft/lbs.

The heavier flywheel inhibits rate of change of engine RPM,
whether speeding up or slowing down, but does not change
torque.
Think about it - does removing a flywheel and installing a
flex plate affect the ability of the engine to generate
torque?

Horsepower is a theoretical measurment, calculation, using
torque and speed.

 

There are several things that come into play with a bigblock vs. smallblock debate. There's a reason you'll rarely ever find a smallblock in a farm truck.

Low RPM volumetric efficiency is important. You also need to look at bore vs. stroke. A very important thing is deck height.

 

I'm an engine expert by no means, but i believe that it is the throw of the crank that creates most of your torque. The larger throw increases mechanical advantage of the pistion on the crank centerline. Isn't this why people make 383's, to produce more torque using a crank with a longer throw?

Anyway, i think that it is engine geometery, not mass, that produces torque.

 

for more torque DEISEL is what i have heard. but that is expensive and i am in college, and have no money... so plan B.

i think that the more mass on the crank itself means that there will be slower throttle response time. Big blocks have tons o power. What i am thinkin of eventually doin on the K5 is putting in a 383 stroker. I hear those bad boys have a lot of torque. right now, with a 350 v8 and a sm465, my rear driveshaft yoke snapped from the torque that the tranny has on it. So, i think that to get more torque get better gears. Samuri guys with 44's and the I4???? yeah they have like 6.56 gears or so.

 

Well from what I have been told by every engine rebuilder I have been talking with lately, more rotating mass is bad for horsepower, and street. I have a 427 tall deck engine that I had hoped on putting into my jimmy. With the tall deck the rotating assembly is much heavier than a standard 427 light truck and car engine (its out of a 5 ton and has 4 ring pistons). The block it self is 'taller' if I remember what the guy told me. Torque wise they were monsters but they lack the ability to rev high, no horsepower, 320 - 340 horses. Every custom shop I called told me no don't do it. Find a 454 instead, or stroke the old 350. So heavier is not necessarily better.

 

Rrich explained it well as did the alky engineer. More rotating weight is harder to stall or get it to rev but it wont make any different amount of power. Rotating weight is all about inertia.

Heavier things are harder to get to live at higher rpms though because of the added inertia(to a point anyway once you start talking about ultra lite exotic crap)

With a TD block you can run 3 ring pistons and longer rods and a big crank for a mongo stroker. TD blocks is how they get 632" outa a BBC. Only problem with TD stuff is the expense. So you can generaly get more power per dollar outa a standard deck block.

 

Adding rotational mass to the engine will no increase the torque output of the engine. If anything, the overall engine performance will decrease. The only thing that the added weight will help is that once the engine gets the weight spinning the inertia will help keep it spinning, basically meaning it will help keep it from stalling.....but only for a very short period of time like a couple of seconds.

 

Horsepower is only a function of Torque times RPM, and weight is not a factor, other than for the inertial aspect (More weight requires more work to be set into motion,and therefore more work to be slowed). Also, the main reason the 383 and deisels produce more torque is the compression ratio, which generally is related to a longer stroke and head chamber size. Basically, the more compression you can make, the more power you can get, but, as can be seen with deisels, RPM will be limited by strength of the internal components. The other side of this coin is the all out race engines where high octane fuel is used to actually slow the explosion through a longer portion of the power stroke to produce more power and prevent detonation.

 

He He - maybe a good analogy.

Push a Honda car as hard as you can.
Now push a 1 ton truck as hard as you can.
Are you weaker (less torque) when it's heavier?
(More mass to move.)

 

In reply to:

Also, the main reason the 383 and deisels produce more torque is the compression ratio, which generally is related to a longer stroke and head chamber size

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you're neglecting an important factor. The increase in stroke gives the piston a greater lever arm on the crank. Torque is force*distance, so if the big end of the connecting rod is at a greater distance from the centerline of the crank (ie larger stroke) you get more torque, with the equivalent MEP in the cylinder.

And rotating mass shouldn't affect torque output at all. There are two ways of looking at it, statically and dynamically. By statically, i don't really mean "static", i mean constant rotational velocity. From this standpoint, mass would have absolutely no effect on torque. However, when you are changing RPM, the engine has to use more torque to accelerate a greater rotating mass, which means you get less torque to the wheels. Extra rotating mass might help if you're loading the engine enough that it is causing a decrease in RPM, in this case the the extra rotating mass would be decelerated by the load, providing a little extra oomph until the rotating mass stopped decelerating.

I would personally try to reduce rotating mass, if anything. Although reducing reciprocating mass would be more beneficial.

 

I like you explaination.
You can have torque without any thing moving. Stall an electric motor, or pull on your wrench without the bolt moving and you still have torque. But you can't get any horse power this way.
Anyhow, take two cars exactly the same, except for flywheels. One car with a steel flywheel, the other with an aluminum light weight wheel. Line them up side by side. Rev them up and dump the clutch. The heavier flywheel should leave stronger and harder with less rpm drop. But after things level off the lighter flywheeled car should accelerate quicker. Until it is time to shift again, then it starts all over.

 

In reply to:

Also, the main reason the 383 and deisels produce more torque is the compression ratio, which generally is related to a longer stroke and head chamber size

There are two reasons a 383 (stroker) makes more torque than a 350:
1, more leverage on crank and 2, more displacement (not sure but I think if all else is the same a sb400 will make more torque than a 383 but not much)
if piston dome or dish, combustion chamber and compression height were unchanged then by the math you would gain in compression ratio by increasing stroke (or bore) but just being a 383 does not mean that the engine will have more compression

diesels make more torque because diesel burns more slowly and and so lower engine speeds are used which allows for longer strokes and thus more leverage on the crank shaft
More rotating mass will not increase torque but will allow the engine to run under load at a lower rpm without bucking and a lighter rotating assembly will make for a "snappier" engine this is a common trade off in high performance 2 stroke engines and is mostly determined by the power to weight ratio and operating conditions for knarly trails and tight woods a heavier flywheel helps prevent stalling when working up a hill at lower speed it can also make it easier to controll a big motor and keep the front wheels on the ground and thus the rider on TOP of the bike a lighter flywheel will let the engine rev quicker and spin the tires faster to clear mud or get the guy behind you dirty

ok so I went a bit off topic but i thought it was a good example