Race cars - Torque vs Hp - The Undiscovered Country (for many)

[russ_watters]

Long thread for a relatively simple concept, but Chris is right here: Power is typically calculated from torque and rpm in real life, so it generally makes more sense to discuss the concept from that angle than to say [paraphrase] 'if you know the power and rpm, you can calculate the torque and acceleration'. In other words - if you know the power and rpm, you probably already measured the torque directly. Sure, you can read the power off a performance curve, but where did that curve come from? It came from measuring the rpm and torque!

The wiki for a dyno says it pretty clearly:

A dynamometer or "dyno" for short, is a machine used to measure torque and rotational speed (rpm) from which power produced by an engine, motor or other rotating prime mover can be calculated.

http://en.wikipedia.org/wiki/Dynamometer

Yes, you can make life more complicated if you want and it'll usually work to approach the problem from the opposite direction, but it isn't really all that useful to do it that way. However, the way the issue was stated in the OP, though a little unclear, is clumsily worded and implies something that is not correct. And the conclusion statement in the last paragraph, with the graph that goes with it, is most certainly not correct: Even if, you gear the cars differently to account for the slop difference in the engine performance curve, the car that runs at a higher torque and lower rpm will accelerate faster because the drive losses are less in such a car.

The really therefore is that by overcomplicating the issue and looking at it backwards, you've confused yourself enough about the particulars to get the conclusions wrong.

 

[zanick]

It might be simple but it is one of the most confusing for most in the racing world. (except the very top engineers). You have to understand, that sometimes we only have dyno graphs to go off of, so that addresses you last point, because the resultant torque and HP measured, is at the rear wheels, incorporating the effeciency losses. So, if two engines with their drivetrains have the same rear wheel HP at any vehicle speed, they will have the same rear wheel forces. the advantage of one vs the other will follow the HP curve. (providing the same spacing of the gears) Now, of course, as I have said many times, using the gear output torque curves will tell you what you want to know, but you need to know the engine torque curves, the gear rations, tire diameters and vehicle speeds. With knowing HP curves and vehicle speeds ranges, all you need to know is gear spacing in %.

The conculsion I have come with , is that if two cars have equal HP curves, (as measured at the rear wheels) they will both have the same rear wheel torque at any vehicle speed. This is still true, even if i am coming at it from the wrong direction. The interesting thing about using HP is that it already incorporates engine speed, so in comparisons, it is easy to use to determine which engine is better, where the shift points should be and where its advantages and disavantages would be on a race track.

The entire point of the topic was to get the terminology correct to bring it back to the racing world. a you know, we want to optimize our time spent at the max hp range of the engine. (not max engine torque range of the engine). There is also a factor of the time spent at the higher rpm ranges. Just getting average power or even average torque found at the rear wheels, doesn't show the entire picture. I can even more accurately integrate the HP curve useable range and get a value that might not indicate which engine is going to create more HP-seconds, or Ft-Lb-seconds at the rear wheels. I think the only way to know this is to get a Ft-lb-seconds value based on known time over an operational speed range.

I only got on the chicken and egg discussion because the criticism was in the areas of actually using Hp curves to determine optimal rear wheel forces. Again, I understand that the force is what does the work, but was taking it a bit futher associating power with energy, as a potential that creates the force. Unit measures of work are also in HP-seconds, KW-hours, Watt-seconds and thought it could address the basic question brought up in the beginning. Yes, there was some terminology confusion in the beginning, but that was in areas of engine torque, rear wheel torque as measured at the drive wheels but calculated back to the engine, rear wheel HP and rear wheel torque and forces at generated at the drive wheels.

by the way, there are dynos that are called chassis dynos that do nothing but measure the rate of acceleration of the drums that the car drives. since it needs to know speed, rate of change of speed, and the mass, you might say it is measuring hp as well, right? It can't tell you engine torque, unless it has a engine speed signal. all it can tell you in that case is the tangental force at the driven tires, it can't even tell you final drive torque of the vehicle! However it can accurately provide a MPH vs HP curve.

Now, I've been doing this a long time and have a lot of experience optimizing race cars for the track to be more competitive.
What I do know is that what I am talking about works, even if I am coming at it from the wrong or "odd" direction. Thats why I came here, to get the terminology straightened out.

Mk

Long thread for a relatively simple concept, but Chris is right here: Power is typically calculated from torque and rpm in real life, so it generally makes more sense to discuss the concept from that angle than to say [paraphrase] 'if you know the power and rpm, you can calculate the torque and acceleration'. In other words - if you know the power and rpm, you probably already measured the torque directly. Sure, you can read the power off a performance curve, but where did that curve come from? It came from measuring the rpm and torque!

The wiki for a dyno says it pretty clearly: http://en.wikipedia.org/wiki/Dynamometer

Yes, you can make life more complicated if you want and it'll usually work to approach the problem from the opposite direction, but it isn't really all that useful to do it that way. However, the way the issue was stated in the OP, though a little unclear, is clumsily worded and implies something that is not correct. And the conclusion statement in the last paragraph, with the graph that goes with it, is most certainly not correct: Even if, you gear the cars differently to account for the slop difference in the engine performance curve, the car that runs at a higher torque and lower rpm will accelerate faster because the drive losses are less in such a car.

The really therefore is that by overcomplicating the issue and looking at it backwards, you've confused yourself enough about the particulars to get the conclusions wrong.

 

[zanick]

You said this:
This defines the amount of work a given engine can do. The POWER OUTPUT IS SET IT DOES NOT CHANGE DEPENDING ON THE GEARING. So if you geared it 1:1 you'd have the same power at the rear wheel as if you used a 100:1 gearning. It would be going this work more slowly, but it would be doing more work.

This is not correct, but you still stand by it?

The bottom line, is I might not be using the right analogies. I only used the gravity, meteor for an analogy that there could be something that is responsible for the force, in terms of energy or the change in energy.

In my mind power is just torque with a rpm attached to it. Just a short cut to find out what i need in comparative environments.

mk

Please buy some books and read them.

No you didnt create a force from the energy. It was the momentum stored that caued the force, due to conservation. Energy stores are not good examples of what you are trying to say as they require forces in.

You can express the energy that its storing, but that energy doesn't create the force.

Read any statics book. Or the flywheel section in Shigley - Mechanical engineering design.


YOU DONT USE KE OR RATE OF CHANGE OF IT TO CALCULATE FORCES! YOU USE MOMENTUMS! USING KE IS NOT REPRAT NOT WHAT HAPPENS IN REAL LIFE!



Tell me how do we measure power or energy directly. That means not using any more fundamental principle? I want to see proof of this.


I'll respond to your odd thinking about gravity at a later time.

 

[zanick]

Chris,
You did say this below, you know. I think we started out agreeing and then ended up in disagreement. thoughts?

mk

Yeah, i'll admit I don't know a super amount about racing boxes. Its been a while since I've done anything with transmissions.

Getting slightly back on topic. I'm glad this thread has swung around to what really matters for acceleration. The gear ratios! Yes peak torque and peak horsepower figures are great for willy waving in the paddock, but careful selection of gear ratios and usefulness of the powerband is king.

 

[xxChrisxx]

The thoughts are, you are wrong about the technical side. And I don't think you are going to be able to break that 'old skool dyno room habit' of using incorrect terminology. I'm now kind of resigned to that fact.(was a bit stresses out about other things yesterday) The subtle difference between things like kinetic energy and momentium is case in point. You talk in terms of KE when its really correct to say momentum. I've given you some books to read if you wwant to get the technical stuff correct.However, from a practical stand point, even though want you are saying is not what is acutally happening, mathematical equtions have been developed to allow you to calculate it. What you do says if something will work, but not WHY it works. And i suppose if all you are after is a 'will it work' then your calculations are fine. Just use HP and calcualte the area, talk in terms of kWh or Hpseconds or sometihng else. Thats perfectly valid for explaining what you want, but it does skew your thinking.

you are correct, I view HP and simply an rpm corrected torque curve. But I know that both variabels are important. For what you want just use HP as that's valid.Just for the hell of it, you keep saying you have more power in a lower gear. And that I am wrong in sayin that you have the sme power whether you have 1:1 or 100:1 ratio.

Prove me wrong, do some calculations and post them. Please TRY to show I am wrong. Dont just state it.

 

[zanick]

I never said that you would have more power in a lower gear, I said if you had the same power at 1:1 or 100:1, the speed of the vehicle would be totally different. but, if the speed of the vehicle was the same, Yes, you would have much less hp with 1:1 gearing. How can you compare the two? You also said that If you did gear it, one would be doing work more slowly but "doing more work". (see your quote below). I said, at the same HP, both would be doing the same amount of work over a given time. I think you might have just made a small unintentional error below. did you ? what I was saying is that for any same speed of the vehicle, the 100:1 gear ratio could allow for operaton at near max hp, while at that same speed, using 1:1, you would have much less hp at that same vehicle speed. See the difference? I am keeping my comparisons at the same vehicle speed, you are not. I don't think the discussion makes much sense unless we keep the speed and mass the same.


You said this:
"This defines the amount of work a given engine can do. The POWER OUTPUT IS SET IT DOES NOT CHANGE DEPENDING ON THE GEARING. So if you geared it 1:1 you'd have the same power at the rear wheel as if you used a 100:1 gearning. It would be going this work more slowly, but it would be doing more work."

So, what you are wrong about, is that if a car had 1:1 or 100:1 gearing and the same HP with each, the rate of doing work would be the same. one would be doing the work more slowly, but the work (Fs) would be the same over the same time period even though one would use a higher force less frequently vs a lower force more frequently

Just for the hell of it, you keep saying you have more power in a lower gear. And that I am wrong in sayin that you have the sme power whether you have 1:1 or 100:1 ratio.

Prove me wrong, do some calculations and post them. Please TRY to show I am wrong. Dont just state it.

 

[xxChrisxx]

you are correct i didnt mean more work, i meant more load. just going round this iritating debate is now staring to annoy me and I am beginning to make stupid errors in my haste to correct you.

so more load at lower speed,
less load at higher speed.
all the same work.
you don't have to keep the speeds the same to compare power.You are correct in saying that 100:1 would allow you to have more power at the same speed. THE ONLY REASION YOU HAVE MORE POWER IS MORE TORQUE.

I cannot state it any simpler than this. you can find this out from the HP curve, but once you are dividing by the speed you are really looking at torque at the reat wheel. just like if you were using torque nd then applied an angualr velocity you are taling about power.

call it what ever the hell you like. do it howeverthe hell you like. You wanted to be correct in the physics and several poeple have attempted to correct you. You clearly don't give a flying arse about being correct in the physics, you just wanted someone to affirm what you are saying.so purely to lay this thread to rest.

What you are doing with the horsepower curve will work, maximise Hpseconds for maximum go. Use that terminology.

good day

 

[russ_watters]

It might be simple but it is one of the most confusing for most in the racing world. (except the very top engineers). You have to understand, that sometimes we only have dyno graphs to go off of, so that addresses you last point, because the resultant torque and HP measured, is at the rear wheels, incorporating the effeciency losses. [emphasis added]

That's not the scenario you gave in the OP:

I posted a set of engine HP/torque curves... [emphasis added]

In the first quote, you're talking about performance measured at the wheels, and in the second you are talking about performance at the crankshaft. Now the wording in the paragraph from the OP (the rest of it) is clumsy enough that I can see you might have meant the measurements were taken at the wheels, but that isn't what you said. As chris has said, most of this thread has been an attempt to get you to correct your clumsy thought process and presentation of the issues. I think we've beaten this issue to death. Thread locked.

[edit] Actually, just to clarify that one last little confusion between you and chris - if the output of an engine in HP (and therefore rpm and torque) is fixed, the gearing (assuming equal drive losses) produces equal HP at the wheels. A 100:1 gear ratio produces 100x as much torque and 1/100th as much rpm at the wheel as a 1:1 ratio. But the speeds and accelerations are different, which is the point being made: You can't ever view HP as independent of other factors. You must combine it with rpm and when you do that, you're just rearranging that equation that relates the three.