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Old 12-12-2004, 06:59 PM
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Why do you need more low-rpm torque on heavy vehicles?

As i remember: Acceleration = Force divided with weight

Now, no matter how much the cars weight you would always want to strive for more Force, right?

And: Force = Torque x rear-end ratio (because what you loose in distance you win in force)

So a specific engine/trans/rearend combo would always provide the same Force through the powerband, which we want as high as possible, as often as possible?

So why would u want a different setup in a heavy car (often low-rpm power) than in a light car (often high-rev)?

I know i have missed something, since what i am saying appearantly doesn't apply, but what?

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Old 12-12-2004, 07:10 PM
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low end torque

Its a simple equation. The heavier the vehicle the more grunt you need to get it moving. There are two ways to do this. Increase the torque output of the engine or go to a deeper gear.

IE: an example: these numbers arent to scale but you will get the idea.
Take an engine with a maximum torque output of 350 Lb Ft at say 3000 rpm.

One car weighs 3000 lbs. the other weighs 2000 lbs.
Theoretically, with the same identical gearing, it will take the heavier car a 3rd again as long to reach a given speed than it will the 2000 lb car. To make the 3000 lb car accelerate as quickly as the 2000 lb car, you will need to amplify the torque by 1/3rd. or by dropping in a lower gear that is 1/3rd lower than the one in it. Moving a lighter load takes less power than a heavier load. That is why one of the crowning attempts in racing is to get rid of weight.
The alternative is to build up the engine to increase the torque output by 1/3rd at the same given 3000 rpm.
One reason that the lighter cars usually have a higher reving engine is due to the lighter car being able to react faster with a high reving engine that has less lower end power. There are many hi revving engines in heavy cars, as well, but you will notice that they will also have a lot deeper gears in the rear end than do the lighter cars. It just takes less force to start the light car moving.
I realize these figures arent real accurate but for example only. I hope you get the idea.
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Old 12-13-2004, 06:40 AM
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Thanks for the answer, but i still feel i havent got it.

Lets skip the whole deal about wanting two different cars to get the same acceleration.

Instead, lets say we just want two different cars to go as fast as possible, each and one of the by their own, with a given budget. Also include the fact that they both have the same rear gears and trans and we are not allowed to change them. Just build the motor.

Why would you build the motor of the heavier car different?

Why cant you, in both cases, just desire as high F(=Force) as possible, as often as possible?

I mean...all of my life i have "known" that you need more low end torque to get a heavy car off the line, but now that I am questioning my own knowledge, I'm getting really confused.

Am I making sence?
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Old 12-14-2004, 11:09 PM
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Torque is power.

Horsepower is work.


Torque is a brute measurement of twisting force.

Horsepower is the rate at which you do that work.

Your ability to move a 20 ton boxcar relies on whether or not you're making enough power or not. So you can put a moped with a strap hooked to that car all day long, there's just not enough torque there to move the load. Insufficient power. Cant' be done.

Put a diesel locomotive in front of that car, and you'll see it move, although not very fast (even at a couple thousand horse, 20 tons is a lot of weight, most trains pull 10x this amount).

The amount of speed you build by the end of a 1/4 mile, however, relies on a measurement of horsepower. SPeed is Distance/time. A rate. Horsepower is work/time. A rate. The faster the rate you can do the amount of work the faster you can get the job (getting to 1320 ft) done.

A motor's torque peak will be found when it finds its maximum volumetric efficiency. This is the point when the bores are most completely filled with mixture. If you can accomplish this at a low RPM (as most engines do) you can start applying your force sooner. You have more power, more twisting force on the axle.

Horsepower and torque can be found as mathematical derivatives of one another, they're two aspects of a similar task.

K
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Old 12-15-2004, 05:25 AM
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Yeah, i know...hp = rpm x torque...and so on.

I am able to give a similiar explanation myself, and have been able to do it for years. I also "know" that you need more low-end torque too move a heavy vehicle.

But none of you seem able to give me a correct explanation using the two formulas:

Acceleration = Force / weight

Why dont you ALWAYS strive for the most Force, which is:

Force = Torque (at the given moment) X total transmission/rear-end ratio.

Ofcourse we must presume there is enough torque to actualy move the vehicle, but all normal V8 engines have that....so why do you build different setups for 2000 pound and 4000 pound vehicles.

Again, use the formulas to prove your knowledge.

I already have the "knowledge" itself, i just cant prove it.
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Old 12-15-2004, 01:35 PM
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Quote:
Originally posted by Gilligan
Now, no matter how much the cars weight you would always want to strive for more Force, right? ...

So a specific engine/trans/rearend combo would always provide the same Force through the powerband, which we want as high as possible, as often as possible?

So why would u want a different setup in a heavy car (often low-rpm power) than in a light car (often high-rev)?
You've already answered your own question, kind of.

You probably DO want the most force as possible, as often as possible. But the heavier car will spend more time in the lower end trying to accelerate, so "as often as possible" happens at a lower rpm than on the lighter car. You haven't defined the requirements for the vehicle so it's hard to prove anything that can't be disproved by pointing out another circumstance.

If a car spends 75% of it's time below, say 3500 rpm because it's heavy and trying to accelerate, then it'd be best to address that rpm range. The lighter car probably won't be spending as much time in that range so there's not as much need to improve that range.

Quote:
Originally posted by Gilligan

But none of you seem able to give me a correct explanation using the two formulas:

Acceleration = Force / weight

Why dont you ALWAYS strive for the most Force, which is:

Force = Torque (at the given moment) X total transmission/rear-end ratio.

Again, use the formulas to prove your knowledge.
You haven't chosen the right formulas to prove anything. Example; I analyze where my car is losing the most time and decide it's because the top end is too low. What force do I need to maintain my desired top speed? Well, F=ma. At top speed, a=0 so the required force must be zero too? Hmmm... air resistance is a drag... (pun intended)

You've oversimplified things by A) not giving any criteria for what you want the car to do, and B) restricting the answer to two formulas to "prove" things, yet the two formulae don't fully describe the requirements the torque curve would be designed around.

There are different set ups because there are different ways to compromise the pros and cons of each option. A heavier car may bias the factors one way, a lighter car may bias them another.
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Old 12-15-2004, 09:40 PM
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Quote:
Originally posted by killerformula
Torque is power.

Horsepower is work.


Torque is a brute measurement of twisting force.

Horsepower is the rate at which you do that work.

K
Torque is work being done.

Horsepower is the rate at which work is being done.

Building for torque gets more work done easier.

It ain't rocket science.
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Old 12-16-2004, 08:20 AM
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Quote:
Originally posted by lluciano77
Torque is work being done.

Horsepower is the rate at which work is being done.

Building for torque gets more work done easier.

It ain't rocket science.
lol lluc,

For it not being rocket science, it sure tripped you up. Torque is most certainly NOT work. Torque is force, by definition. Work, on the other hand, is a rate. That's what horsepower is: a rate. Torque is not a rate, it is measured at an exact moment in time. Sounds like a bunch of nit-picking, but when you're trying to use your physics equations, it makes a difference.

K
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Old 12-16-2004, 07:56 PM
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Quote:
Originally posted by DesmoDog
You've already answered your own question, kind of.

You probably DO want the most force as possible, as often as possible. But the heavier car will spend more time in the lower end trying to accelerate, so "as often as possible" happens at a lower rpm than on the lighter car. You haven't defined the requirements for the vehicle so it's hard to prove anything that can't be disproved by pointing out another circumstance.

If a car spends 75% of it's time below, say 3500 rpm because it's heavy and trying to accelerate, then it'd be best to address that rpm range. The lighter car probably won't be spending as much time in that range so there's not as much need to improve that range.



You haven't chosen the right formulas to prove anything. Example; I analyze where my car is losing the most time and decide it's because the top end is too low. What force do I need to maintain my desired top speed? Well, F=ma. At top speed, a=0 so the required force must be zero too? Hmmm... air resistance is a drag... (pun intended)

You've oversimplified things by A) not giving any criteria for what you want the car to do, and B) restricting the answer to two formulas to "prove" things, yet the two formulae don't fully describe the requirements the torque curve would be designed around.

There are different set ups because there are different ways to compromise the pros and cons of each option. A heavier car may bias the factors one way, a lighter car may bias them another.
I must say i really liked your explanation. I never thought about the fact that "as often as possible" could vary when the cars have different weights.

And yes you are right. I have oversimplified things without explaining it in my first post.

Firs off, i havent taken any notice to air reisstance (simply because it has no affect on the subject i want to discuss).

Secondly, the criteria for the cars are to accelerate as fast as possible on the quarter (or lets say 1/8 mile to skip the hole air resistance thing as much as possible).

However, I hope you are right, because the answers sure satisfied me. =)

Thanks!
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Old 12-17-2004, 12:44 AM
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Quote:
Originally posted by killerformula
lol lluc,

For it not being rocket science, it sure tripped you up. Torque is most certainly NOT work. Torque is force, by definition. Work, on the other hand, is a rate. That's what horsepower is: a rate. Torque is not a rate, it is measured at an exact moment in time. Sounds like a bunch of nit-picking, but when you're trying to use your physics equations, it makes a difference.

K

Killer, I went to school for 4 years to become an electrician. We went pretty heavy into torque and horsepower. Torque is a measurement of how much work is being done. It is a force, but in this context it is work.

Read the 5th paragraph down:
http://www.westechperformance.com/pa...ng/hpvstq.html

You remind me of a salesman sometimes Killer.

They never truly understand what it is they are trying to sell. They only learn enough to sell it. And even worse, as soon as they begin to get an idea about what they are selling, they immediately try to sell you on the fact that they know what they're talking about.

Don't take offense. Salesmen make the world go 'round. They can be very rich or very poor.
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Old 12-17-2004, 08:01 AM
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Quote:
Originally posted by lluciano77
Killer, I went to school for 4 years to become an electrician. We went pretty heavy into torque and horsepower. Torque is a measurement of how much work is being done. It is a force, but in this context it is work.

Read the 5th paragraph down:
http://www.westechperformance.com/pa...ng/hpvstq.html

You remind me of a salesman sometimes Killer.

They never truly understand what it is they are trying to sell. They only learn enough to sell it. And even worse, as soon as they begin to get an idea about what they are selling, they immediately try to sell you on the fact that they know what they're talking about.

Don't take offense. Salesmen make the world go 'round. They can be very rich or very poor.
This is hilarious, you think you're educating me and telling me you know what you're talking about by referencing a hotrod article that's written for people who have never taken a physics class and is therefor dumbing down terms? Don't come at me because I corrected you lluc, my point is this guy obviously has his crap together about physics and he wanted equations from us to figure something out. Telling him that torque is work is flat out wrong. You used the term wrong and I called you on it, go crack open a physics book and read the first three chapters.

I don't know why you're so condescending and act like you're always right, lluc. You don't have any more credentials building these motors than any of the rest of us do. You have hardly any data save a few magazine articles to prove what you're talking about. We're all hobbyists here, and you're not the instructor. You have no idea what the rest of our experience is and you have the gull to tell me I don't know what I'm talking about?
definition of torque, the word "work" is nowhere on the webpage at all.

definition of horsepower

and here, a physics reference database
nice definition

These are simple ideas dude. Torque is a force, measured at a moment. Torque over a given amount of distance is work (remember, T*D....) and power, as I said before, is the rate at which work gets done, which is measured in this case, in horsepower or 33,000 lb ft per minute.

Gawd.

K
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Old 12-17-2004, 09:47 AM
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Back to the original question, This statement is quite true.

"we want as high as possible, as often as possible."

In the world of Hot Rodding, we are always looking for more power. Quite a few have more power than they can use.

Now the fact is, a heavy car can take more power than a light one. That is, provided we use a specific engine/trans/rear end combo that would provide the same Force throughout the power band. A stock Chevette with a LS6/TH400/4.11 would be a hand full at best to try and get down the track. A stock Caprice Classic Estate wagon would swallow the power with ease and make it a bracket hero. You can build the Chevette to handle the power, but if you do the same suspension and tire mods to the big wagon even more power could be added.

Another thing to consider is the time it takes to get to your power band and shift recovery times. A lighter car accelerates at a faster rate allowing the motor to reach it's sweet spot quicker. Shift recovery time is also reduced in a light car. In a heavy car, you add low speed torque to improve your acceleration rate. You would also use a close ratio trans to help shift recovery. Dan
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Old 12-17-2004, 10:28 AM
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Thanks for getting us back on topic, Dan. Guess we got a little carried away-

K
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Old 12-19-2004, 02:56 PM
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Quote:
Originally posted by killerformula
This is hilarious, you think you're educating me and telling me you know what you're talking about by referencing a hotrod article that's written for people who have never taken a physics class and is therefor dumbing down terms? Don't come at me because I corrected you lluc, my point is this guy obviously has his crap together about physics and he wanted equations from us to figure something out. Telling him that torque is work is flat out wrong. You used the term wrong and I called you on it, go crack open a physics book and read the first three chapters.


K
So, telling him torque is power and horsepower is work is right?

I am merely correctly your first statement.
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Old 12-19-2004, 03:16 PM
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No, that might not have been the best choice of words for me to use, you're right. But you also took what I said out of context, because I wrote four more paragraphs below those two lines to explain what I meant, and in what context I meant those words. SO with the supporting paragraphs, I think they're understandable terms.

K
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