New way to ask about Torque Vs HP
 

Hello,

Like most of you I read all sorts of ideas, theories, interpretations for a clear explanation of those 2 important terms. And, probably like most of you, I haven't found unanimously agreed upon answers.

Let's try it this way, with a question. If you already have the torque curve of a car on a dyno (not only the engine on a stand), from 1000 until its max rpm, what kind of information would getting the hp curve give you?

In other words, what info is missing when we don't have the hp curve?

Thanks

Wait...what?

Torque is measured, HP is calculated.

Torque X RPM / 5252 is HP...

That's what I thought as well. But just about everywhere you look/read/watch, they say that torque is something and that HP is something else.

If it were as simple as Torque X RPM / 5252 = HP, then it would be exactly like kg = 2.2 X lbs. We'd be treating them as perfectly equivalent.

I still haven't figured out why they're not "the same", like kg and lbs are "the same".

So instead of asking for definitions, I thought I'd ask it this way: what does the HP curve add that wasn't already known given the torque curve.

Loa

Nothing really gets added it's just a different unit of measurement. It gives you a power variable instead of a force variable by using the formula.

Here is a mind bender for ya.. 1hp = 746 watts since hp is a unit of work.

And yes, it is as simple as tq x rpm /5252, for the most part anyways.

That is why every dyno you will see will have the lines intersect at 5252rpm.

Hello,

You're right: at every point on the dyno curve we can calculate HP using torque. Sheesh... Why all the articles and youtube videos trying to tell us that torque is "what gets you going" and HP means "what keeps you going"? It's the same damn thing!

New question then: why the hell do we even bother talking about HP? Is it because cars usually list their peak HP and peak torque? (instead of the full curve)

Thanks for the info.

Loa

Lets not even get into the fact that wheel dynos are completely useless for anything except comparing before and after mods on a given day.

If you drove around to 5 different dynos on the same day you would have 5 wildly different readings, and could differ by over 100hp.

It isn't that they are not good for getting an accurate assessment of HP it is how they are used that limits the accuracy. Basically, they work the same way an engine dyno works. If operators of engine dynos used the same way shops use wheel dynos those results would be about equal in accuracy. DynoJet dynos have a +/-1% accuracy. Sounds great right. However, at the 300 HP level that means the measured HP can vary from 297 to 303 on any individual run. To average that out you need to make several runs in a row for each measurement. If the operator compensates for all of the environmental variables and uses a standard SAE method to calculate power, you make 10 runs in a row and average the readings you will be close to the actual power being measured. The first 10 dyno runs set the baseline and after making the mods the average of the next 10 runs will give you the results of the mods. Usually they just make a couple of runs and as you say all it is good for is making comparisons that day as long as you are looking for a power difference much larger than the 1% accuracy impact. If you just changed the mufflers hoping to get a 6 HP increase you would never be able to accurately tell whether or not they helped even though a dyno reading might indicate they did.

Bill

hp can keep you going but also the rate and which you make torque
If its bringing it on quicker and carrying it farther youre making more hp thats my take on it.

People use HP because thats whats been selling cars parts motors forever. It can be misleading actually either can. Dont get too wrapped up about it. :cheers:

I think on one hand, the less RPM in general the longer some stuff will live. Low rpm can save parts. Low can be 1500 in one context, and 6200 in another.

On the other hand, you have such disparity as 2.0L engines which cruise comfortably at 3400rpm and that number doesn't bother them. Whereas 7L engine wants nothing to do with that, it costs a fortune in rotating costs to maintain such a high RPM.

Sixth in a T-56 is 0.5:1 and it is the reason you can see 33mpg from a 7L engine. It is quite a difference over .70 of the 4l80e in economy for large engines with higher parasitic rotational losses. The 2.0L doesn't really care if final gear is .5 or .7 or .6 in fact it may prefer closer to .7.

Those kinetic rotating costs and RPM dependent cruise-ability (you can't cruise at 7k with most engines) are the real answers/reason to original question of why we use a large diesel engine at low RPM, or a smaller gasoline engine at a higher rpm.

This!! I cringe when I see guys choosing a build or selling their car because their dyno sheet didnt match someones on the web .WHAT?! lol

That sounds like it makes a difference, but it really doesn't. It's like saying that length is measured, but area is calculated. That doesn't make area any less legitimate than length, and in some cases it is much more useful.

Fundamentally, if you measure two things, and you can use arithmetic to calculate a third thing, then you have measured all three of those things. Legitimately. Usefully. When you measure length and width, and do math... you have also measured the area. When you measure torque and RPM, and do math, you have also, legitimately, measured the horsepower.

Inertial dynos don't measure torque, by the way. They measure time required to accelerate the rollers, and from that time measurement, they calculate power. Then from that power measurement, and RPM, they calculate torque. That's why when a DynoJet can't get a good RPM signal, it can still produce a horsepower curve. It's measuring horsepower. And it's doing it by measuring time and roller RPM, and doing arithmetic, because that results in a measurement of power.

Getting back to the original question...

Torque plotted over RPM is theoretically exactly as useful as power plotted over RPM, because it contains exactly the same information. But nobody likes to do arithmetic, so it's nice to have the power plot, because that gets you closer to knowing how much acceleration you can expect. To really know, you need to factor in vehicle weight. (...and with enough acceleration you have to start wondering about being able to maintain traction as well, but that's a whole other matter.)

I disagree. Acceleration is the result of a force applied to a mass. F=ma, or in terms of angular quantities T=I*a_{c}
A total change in kinetic energy over a time interval is the result of power applied to a mass over that time interval, which is perhaps what you were trying to articulate, but even that IMO is a sort of butchering of the physics because powers aren't really "applied", at least not in the physics sense as far as I see it. Forces and torques are applied.



The instantaneous value of angular acceleration of a body is most certainly the instantaneous value of of the of the torque curve. Similarly, the instantaneous value of the change in angular acceleration of a body is the instantaneous value of the slope of the torque curve. This is not high-level silliness, it is basic intro mechanics, which I've taught.

This statement I believe to be false. Instantaneous Acceleration can most certainly be considered and measured physically over an infinitesimal time interval, because it is simply proportional to applied force (or torque). Acceleration can occur without an interval or time. Power on the other hand is a calculated (as opposed to directly measured) quantity that requires a non zero time interval over which other physical quantities are measured, and then the values of which are used in a calculation of power.

This is false as stated. It should be restated:
"a 4-cylinder engine that puts out the same average power over an interval as a V8 will on average accelerate the car just as fast if all else is equal."
There is a very high chance that with the V8, there will be sub interval over which the V8 has a greater value of instantaneous acceleration, thanks to its larger values of torque produced.

I disagree with this, and it goes back to our earlier conversation about measured vs calculated quantities. As far as I'm concerned when it comes to causality, determinations of things, and unique solutions to differential equations that specify a particular velocity as a function of time v(t) curve as a solution, power is not what determines changes in acceleration, rather the instantaneous slope of a torque (or force) vs time curve is what determines instantaneous values of change in acceleration. This I believe was greg's "gobbledeegook" point about slopes of torque curves.
I would agree with your statement if you instead had said:
"Calculated average power can be used to calculate average acceleration"

This is not true. The true statement could have been "The car that puts down the most average power over a time interval will have the largest value of average acceleration as calculated over that interval"
The point that I would like to make to you now is that the car that puts down the most average power over time interval might not necessarily be the car that experiences the greatest value of instantaneous acceleration in that interval. That trophy goes to the car with the largest magnitude value in its torque curve, and that is why people like big torque numbers over short RPM ranges. This means a steep torque curve, and larger instantaneous values of acceleration.

I would like to point out here that your language here seems to imply a difference between power and work, and there is no physical difference between those two things.

First I would like to point out that language like "Acceleration is fastest" is really quite poor to use in this sort of technical discussion. Your statement might have been "Acceleration has the greatest instantaneous magnitude when instantaneous power has the greatest magnitude"
But even then, that statement is not necessarily be true. It can be true, but it is not necessarily true. What is necessarily true is "Acceleration has the greatest instantaneous magnitude when instantaneous force/torque is greatest"


First I would like to point out that language like "Acceleration is fastest" and "torque CLIMBS fastest" is really quite poor to use in this sort of technical discussion. Your statement might have been "Acceleration has the greatest instantaneous change to its magnitude when the instantaneous slope of the torque/force curve has its greatest magnitude" and that sort of statement would be entirely and always true, and I think really captures the essence of why people do, should, and ought to care about torque curves.



To sort of bring this all back to reality, I love my L98 car. It wears its "1989C4" vanity plate with pride! Sure I get my doors blasted off on the freeways and interstates by brand new challengers and mustangs, but stoplight to stoplight, onramps, or any other place where the total time interval being considered is rather small, the L98 is able to hang with newer, faster and higher horsepower producing cars thanks to its steep torque curve with large maximum values of torque, which allow the motor (for a given gearing) to propel the car with large instantaneous values in the change in acceleration, as well as large magnitude instantaneous values OF acceleration. Oh did I mention the donuts?

Please delete

Torque
 

Torque is how much pressure / force the crank will produce. Note that moving is not required normally, but it is with engines.

HP is how many times a second can you apply that torque. One time a second? Or 2 x a second? etc. Twice a second gets more work done. That's HP. HP does work.

In the context of a car's performance, torque and horsepower are both important factors to consider. The torque curve shows how much twisting force the engine is producing at different RPMs, while the horsepower curve shows how much work the engine is doing per unit of time. When you have the torque curve of a car on a dyno, you can see how much power the engine is producing at different RPMs. This can give you an idea of the car's low-end and mid-range acceleration capabilities, as torque is what helps a car accelerate from a stop or at a low-speed torque curve alone and does not tell you the car's overall power output. Horsepower is a measure of the engine's total power output, and it takes into account both the engine's torque and its RPMs. So, if you have the horsepower curve in addition to the torque curve, you can see how much power the engine is producing at all RPMs, which can give you a better overall picture of the car's performance capabilities.also, visit carwooow