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New way to ask about Torque Vs HP

 
Old 10-20-2014, 12:04 PM
  #1  
Loa
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Default 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
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Old 10-23-2014, 01:52 PM
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Apocolipse
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Wait...what?

Torque is measured, HP is calculated.

Torque X RPM / 5252 is HP...
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Old 10-23-2014, 05:19 PM
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Originally Posted by Apocolipse View Post
Torque is measured, HP is calculated.
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
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Old 10-23-2014, 07:06 PM
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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.

Last edited by Apocolipse; 10-23-2014 at 07:14 PM.
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Old 10-23-2014, 07:41 PM
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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
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Old 10-24-2014, 03:49 PM
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Chevy Guy
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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.

Last edited by Chevy Guy; 10-24-2014 at 03:59 PM.
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Old 11-05-2014, 12:06 PM
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Bill Dearborn
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Originally Posted by Chevy Guy View Post
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.

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Old 11-12-2014, 02:47 PM
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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.
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Old 04-09-2019, 09:12 PM
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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.
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Old 04-11-2019, 09:49 AM
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Originally Posted by Chevy Guy View Post
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.
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
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Old 04-14-2019, 06:24 PM
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Originally Posted by Apocolipse View Post
Wait...what?

Torque is measured, HP is calculated.

Torque X RPM / 5252 is HP...
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.)

Last edited by NSFW; 04-14-2019 at 09:13 PM.
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