T or F: Crank weight affects torque?

jwhoss76

jwhoss76

Famous Member
VIP
I'm curious guys. I have been told that engines with heavier cranks can make more torque. Is this true? I know a lightened crank will spin faster making more horsepower, but does a heavier crank mean you can make more torque? I believe a cast crank weights more than a forged one, or am I wrong in my assumption? Thanks

-John Humphries
 
Well, sort of (the way I see it). A heavier crank would generally be in a longer stroked, slower turning engine. The longer stroke means the crank throw is pushed more, and thus can have more energy imparted to it per firing. Put a heavy crank in a high-revving engine and it better be well balanced! :shock:
 
Jeep owners are dropping 258 cranks in the newer 4 litre inlines 6s becasue they have a longer stroke and they have additional weights built into the crank. They have to be balanced before use and the snout has to be remachined to work with the late model balancer. :wink:

Can you tell I spend alot of time around Jeeps? :roll:
 
Hey,

Torque, and crank weight are indepentand of each other. Torque is noted as the turning force acting on the crank from the piston. The force the piston moves down with on the power stroke is due to the pressure generated from the combustion taking place.

When people say that lightening a crank reduces torque, it really does not. However, what it does do is decrease the inertia of the crank. Inertia is the opposing force to a change of motion, hence it opposes both acceleration and deceleration. So a lightened crankshaft is able to spin up faster under acceleration, but it also loses rpm quickly if the car begins to travel up a hill.

The main reason to lighten a crankshaft is to decrease centrifugal forces. Centrifugal force is related to inertia, and it also opposes the motion of an object. When you rotate your arm with a bucket of water in you hand you feel an outward force, and if you spin it faster enough you can spin the bucket over your head without the water coming out. This is the result of centrifugal force. While decreasing the weight of a crankshaft weakens it structurally, decreasing the centrifugal forces means that the crankshaft will undergo less stresses and strains. The objective is to find a balance between all these factors.

Hope that helps
 
Right on, Cam. And if you want an engine to pull strong at very low speeds it is helpful to increase rotating mass as it helps to smooth out the power pulses and keeps things moving. Remember the formula for kinectic energy is Mass x Velocity squared. So energy increases by the square of velocity. If we double the speed (RPM) then the energy is quadrupled. But if we double the weight at a given speed, then energy energy is only doubled. Taking this to the extreme at low speeds using my John Deere Model A tractor as an example, it is rated at 975 RPM and only has two cylinders, yet it develops gobs of torque. This thing has, and needs, a very heavy flywheel as the speeds are so low and there is a relatively long time between power pulses. At such low speeds the only way to get enough inertia is to add mass.
At very high speeds things start to stretch out of shape from the centrifugal forces. With high speed circular saws of large diameter the rim stretches enough to get very wobbly and unstable so we have to pre-stretch the central portion of the saw to match the rim when it is at speed. We call this "tensioning" the saw plate. An 84" saw won't lay flat when at rest, the eye will pop back and forth like an oil can.
Joe
 
:oops: Don't you hate it when you know some things and forget to put 2+2 together? I forgot about the inertia a heavier cranck would have :oops:
So am I correct? Is a castr crank heavier than a forged one? Just checking myself here, been a long time since I played with a crank. :wink:
 
The forged crank should be a little heavier than the cast since the metal is compressed.

John
 
REGARDLESS, the weight of a crank,(or any other part of the rotating assembly) will NOT increase or decrease the torque output.
On a dyno, a lighter assembly will spin up faster and not lose as much doing it. What I mean is, a 460 testing at 300rpm per sec with a light flywheel for example, will require a certain amount of torque just to increase rpm at that rate. Trying to dyno it at 600rpm per sec, more torque is wasted spinning up the weight that quickly. Go to a heavy flywheel and the difference is even more dramatic. But the point is that at a steady rpm the engine puts out 500 lb's FT. at 300rpm per sec, the engine peaks at 485lb's FT, and at 600 or 900 rpm per sec the same combo peaks at 450lb's FT. This is an extreme example, but all it shows, is that more weight equals slower reving, all else being equal.
I suppose another way to state it would be; does a 300hp engine have more power in a 2800# car or a 4500# truck? :lol:
Cam said it quite well actually, I just wanted to reiterate the point that one does not effect the other.
 
What Cam said.

Two examples of the intertia situation. The big wheels and low profile tires seen on alot of imports actually weigh more than the stock setup and have more weight farther from the center of the wheel. On an intertia chassis dyno they'll show lower horsepower readings. Not because the engine generates less horsepower with different wheels. It just can't spin them up as fast. They do, however, translate into a performace loss.

Second, alot of years ago there was a guy, Koffel, who raced a small block Chevy powered Lincoln in one of the lower gas classes. He went to a 60, and I believe 75, pound flywheel. Once he staged and got the RPMs up (don't know how long that took), he'd sidestep the clutch and, BANG, he was gone. Hard on driveline stuff, but he did quite well.
 
Replace the flywheel on my John Deere Model A tractor with a garbage can lid and I doubt it would even run. Flywheels are a necessary evil, don't use one any heavier than you need. Some applications just need more than others.
Joe
 
Is it rampaging old age, or am I just panicking? These days, I give far more thought to "If it breaks, will I get hurt?"... Which has the potential for more damage to you - the light parts at higher RPM or the heavy parts turning slow?
 
addo":25f6klbn said:
Which has the potential for more damage to you - the light parts at higher RPM or the heavy parts turning slow?
Click to expand...
An engineer would answer, "Neither! The explosion was inadequately contained." :lol: :wink:
 
SuperMag":332czjpi said:
JW":332czjpi said:
Remember the formula for kinectic energy is Mass x Velocity squared...
Click to expand...
Almost. Ke=1/2 x mass x velocity^2
Click to expand...

Yes indeed, I stand humbly corrected :oops:
I was trying to simply explain that if you double the weight, the energy only doubles, but if you double the velocity the energy is quadrupled.
Thanks again,
Joe
 
You must log in or register to reply here.

Similar threads

M
Individual wheel drive (two rear motors)
Replies
8
Views
2K
benchracer
B
M
200ci I put a 200CID engine in my '73 Mercury Marquis
2 3
Replies
42
Views
7K
karkev
K
62Ranchero200
Torsional vibration, harmonic balancers, custom machining
Replies
5
Views
2K
drag-200stang
D
M
226-H (234 cu in) build, + T5 conversion semi-documentary
Replies
7
Views
12K
bubba22349
bubba22349
D
Differences btwn Ford and Fordsix.com Valve Adjustment Proc.
2
Replies
20
Views
8K
bubba22349
bubba22349
Back
Top