Guys, Guys, Guys....

What really matters when deciding which vehicle will be faster is torque delivered to the wheels . This value can be increased in only one of two manners: 1.) produce more torque at the engine flywheel or 2.) multiply the supplied flywheel torque with gearing . The value that simplifies this idea of a rate of applied force is horsepower. Knowing the HP supplied to a given mass you can very closely calculate how fast the oject can accelerate and how fast it will take to move the object over a fixed distance... you simply can't do that knowing the peak torque value only.

Much like cumminsdriver365 stated, if HP #s are equal, and vehicle weights are equal, then the accelerations will be equal even if one vehicle has 10x the torque of the other.

Also, torque is a force , not work. It's the coupled moment delivered by the engine and through the drivetrain. Forces can be multiplied by levers (gearing) in exchange for distances.

 

It's a fun discussion, so I thought I'd throw in my two cents.

Torque is only a measure of force. Not work done. It's how much effort is being applied to a load, not how fast the load is moving. Imagine pushing a car by hand. You lean into it and it hasn't started to roll yet. That's effort, or torque, not work. Work is done when the car is actually moving. How fast it's moving relates to RPM and torque. The combination of effort and speed is horsepower. Horsepower accomplishes work.

A better way to see it is with a lever.
Imagine a long lever. You have a heavy load sitting on one end, a fulcrum somewhere, and you are pulling down on the other end to lift the load. Now look at how far and how hard you have to pull to lift the load. If the fulcrum is near the load you have to pull your end a long way to lift the load a small distance. That is a high speed low torque arrangement. The handle moves a long distance, with a small effort, to accomplish the movement of a heavy load. Now move the fulcrum closer to the handle end and the handle moves less distance to move the load but it requires more effort, more torque. In both cases the load moves the same distance so the work done is the same. The lever adapted the person, or the engine, to the load.
Torque is measured in foot pounds. In other words, how much effort is being applied to the lever (pounds) at one foot out from the fulcrum. It's not about speed at all, only force.
The transmission in a vehicle is simply a series of levers to adapt the available torque and speed of the engine to the load being moved. Gears are levers. Horsepower, or speed and effort combined, is what does the work. It is adapted to the load by the tranny that multiplies the torque and matches the speed by leverage.
I enjoy a high torque low speed engine but a small block Chevy, for instance, with the same horsepower, will do the same amount of work. You just have to run it at 5000 RPM or so to do it.
The important thing is that the term "work" describes how much has been acconplished. A combination of speed and effort. Torque is how much effort.
An engine that runs at 5000 RPM, with 100 ft lbs of torque, and a two to one reduction in the trans will produce 200 ft lbs at 2,500 RPM (minus gear losses). It will do the same amount of work as an engine that runs at 2,500 RPM and produces 200 ft lbs of torque.
I think we all agree that pulling a load up a grade with a Cummins is far better than with a small block Chevy, but if they have the same horsepower the speed will be the same because the work being accomplished is the same. A certain load being lifted at a certain rate.

Wetspirit

 

Actually Newton's second law Force = Mass x Acceleration is an accurate and repeatable way to measure HP. Then you can calculate TQ with the same equation quoted earlier only in changed order. TQ= (HP X 5252) / RPM will give you the calculated TQ number from measured HP. It's just a small medial function of algebra. Inertia chassis dynos work on this principal. Load dynos work by measuring TQ and calculating HP. Either way is acceptable.

Great discussion by the way.

 

Thank you....I stand corrected.

 

300 hp could describe a 1 liter drag bike, a hot 3L V6 gasser, a production 5.9L Cummins in a Dodge truck, or even a larger diesel like maybe an 11L Cummins. Since the HP is the same it doesn't describe much in the way of differences, but the the torque would, where it could range from maybe 100 to 1500 lbs/ft. The same work could be done by all if geared properly, with dramatic differences in fuel economy, driveability, and engine life. A friend has an older GM 20k lb flatbed that has a 350 Chevy, a splitter on the rear axle, won't do much over 55 mph, and needs to pretty much be floored all the time with a load. He's on the 2nd engine in less 80k miles.

 

Some of the fun parts of Physics 101. At least I think it was 101 and maybe fun, way too long ago to be sure. Not exactly a chicken and egg comparison, but you can't have one without the other, can ya?

 

Back in the 60's, a person could buy a chevelle with either a 327, 375 hp or with a 396, 375 hp. The 396 would always out run the smaller motor. and last longer too. By the way the 327 was, in my opinion a much better motor. The 396 was out of balance. And an oil burner.

Reason was , the bigger motor was in it torque range more, meaning it was a broader curve.
It is about getting the torque curve where it is usable, and gearing to use the torque.

 

Now who am I to disagree with Sir Issac Newton Great conversation guys Especially about the inertial vs load dynos. It struck me as odd on another thread when I guy told he calculated his torque- makes sense now.

Is there any other ways to quantify power besides the conventional hp/t? Maybe a percentage of torque increase over a second as opposed to the usual rpm measurement. Maybe I should refer back to my first statement

 

Please see my rebuttal to your premise under the OTHER posting of your lead post in the "How disappointing" thread.

 

This is a common myth. Simply increasing the load that the engine must operate against has NO EFFECT at all relative performance, at least in terms of physics.

If one car is quicker accelerating than another, adding 500lbs or 5000lbs to both cars will NOT change the outcome unless there's some other variable unaccounted for (like torque converter behavior at different loads).


The other myth is that torque is at low rpm and power is at high rpm. All you have is torque. Engines don't make power, they make torque. You have low rpm torque and high rpm tq. That's it.


Power is how we describe (and quantify) an engine's tq output relative to it's operating speed. Period.


Repeat:
"Engines don't make HP, they make TQ"

 

You're mostly right. But you're confusing the "physics" sense of "work" (force times distance) with the rotational equivalent.

The important difference is that in Physics, it's only work if 1) it moves, and 2) it moves in the direction of the force applied to it. For example, if you climb stair to a height of 100ft, you only did 100ft times your weight of work-- no matter if you walked 20yds or 20 miles in the process.

With rotation, you "get credit" even if the object doesn't move. It's just force X distance like your lever example. I like your phrase: "combination of effort and speed is horsepower".

Another way to phrase this whole mess is that if we know the workload, we can calculate results when we know effort. Likewise, we can also calculate effort if we know results for the same workload.


Good post, wetspirit.

jh

 

"The other myth is that torque is at low rpm and power is at high rpm. All you have is torque. Engines don't make power, they make torque. You have low rpm torque and high rpm tq. That's it."

If I had to rely upon one measure of performance in selecting an engine I'd pick peak torque, otherwise I'd like to see a torque curve vs rpm, as I can then calculate hp vs rpm if desired. Years back when Yamaha was running 350cc 2 stroke twins in road racing while others were running 750cc 4 strokes (Yamaha was typically winning), one cycle mag writer tried to run one on the track. He said that he couldn't do it, as the hp curve was so steep that he couldn't keep it in the narrow rpm band that produced manageable power. It was producing about 200 hp per liter, which was good for an unsupercharged/non-turbo engine back in the late 60s /early 70s. In 1975 I bought an RD400, and three others in my battalion did too. Within six months the three other guys had wrecked theirs, as even the street bikes were peaky and they got into trouble too quickly.

In terms of moving loads it helps to have lots of torque down low, just off of idle, instead of lots of peak hp at a high rpm.

 

You guys are genius, pure genius.

Great thread...

 

So, what Rusty is saying is "don't buy a half fast horse"

 

It's not so much the way we measure it. It's our *concept* of what HP is.

Rusty's explanation of HP using the example of "high torque" horse and "high rpm" horse is brilliant. It perfectly explains the concept of power vs tq.


The problem comes when we try to relate this to ACCELERATION, which is a typical measurement of "performance".

Unlike the horses at the mine, our engines have the effect of rotational intertia. Rotational inertia skews power output because it can serve as a reservoir of energy.

For example, you put your foot to the floor. Your engine is not putting all of its power to the wheels. Some of the waste power is converted to kinetic energy stored in the mass of the rotating assembly.

Now say you upshift without lifting from WOT. Some of this stored energy will be released to the driveline as the clutch engages. Some will be wasted as heat in the clutch as it engages.

Obviously, this can skew our measurements of HP in different frames of reference. An intertial dyno will read artificially low, and a load dyno would read artifically high in certain cases-- due to rotational inertia or flywheel effect.

The bottom line is this: if we are interested in performance as measured by acceleration, then all that matters is how quickly an engine can move the tach to the shift rpm shift after shift when under load.

jh