mark kibort

I think the problem here is that most of us are talking about the power being used, as measured at the wheels to the ground. we are talking about "Net" forces, not total power. if you talk about total power, then take into account the heat, noise and everything else if you are going to have the car on jack stands. when you are WOT, the engine is at max capability to produce power. its power curve is plottable. You can do a dyno run of an engine in neutral, by how fast it can get its mass up to and through any rpm. of course, now there are other factors from allowing you to get an accurate number, just as if you dump the clutch, you get an inflated power number, due to the stored energy in the engine and flywheel .

The only way to discuss this topic is to use agreed conditions of evaluation. we are talking, power to the driven wheels, engine torque, as measured from the driven wheels and rear wheel torque as multiplied through the gear boxe.

mark kibort

You shift it at near redline, most likely, which might not ever allow you to reach peak torque, nor would you need to for maximizing acceleration. think maximiing, HP-seconds or average HP.

mark kibort

Here is the problem with this topic as most understand it.

I think we all agree that power incorporates torque. that torque, produced by the engine, when combusting fuel, x a rpm will yield a power value. Power is the rate of doing work. torque, is just a rotational force. If someone says I have 500ftlbs of engine torque, that means absolutely nothing in racing, unles you know engine rpm and the shape of the torque curve. Also, if someone says I have 4000ft-lbs of rear wheel torque, it means little, unless I know tire diameter. If you have power as measured at the rear wheels, you know everything you need to. thats why I always fall back to :
acceleration= power/(Mass x velocity). power can be rear wheel forces too, but there are more variables to narrow it down. This is why if you dont have a speed signal on a dyno (spark for RPM) , what does a dyno tell you? it only tells you MPH and Horsepower. engine toruqe is calculated and it could give you rear wheel torque, but it would have to be divided out to find engine torque. the dyno in this case, is only measuring a rate of change of kinetic energy, which is power!

The other way I look at it, is that power is the rate of doing work. Torque is just a rotational force. you need more information to understand the "torque's" part in the acceleration equation.
Energy IS work. work is force x displacement. And the rate of doing work is power. you do more work faster, you accelerate faster. HP-seconds is a unit measure of work.

RKD in OKC

So, where do we get these seemingly elusive HP-seconds?

Roy928tt

Mark, you are so confusing to try and understand, in some statements you appear to understand the concepts then a couple of sentences you appear to contradict yourself.

I think I will stick to what I understand and you can stick to your understanding.

The original topic was Horsepower and Torque- nothing about acceleration, gearing, hp seconds etc etc blah blah. It wasn't about racing or vehicle performance, simply the difference or relationship between Torque and Horsepower.

Torque is what an engine produces, turning force, how you choose to use that is up to you. Different engines produce different amounts of Torque in different manners. Horsepower is only a derived figure.

Cheers Roy

EspritS4s

+1, Exactly my thoughts.

mark kibort

you get them by spending more time in the peak hp range, AND/or getting a stroker, tune job, or supercharger.

mark kibort

Roy (and Gary) Please help me understand what part of the subject matter is being presented in a confusing fashion.
the original topi is about HP and torque, but i think everyone seems to be talking about it for the understanding how they interelate, of how they both apply to acceleration . afterall, we all want to accelerarate as qickly as possible.

What concepts are you talking about that agree with you and which do not.

in your last statement, you say HP is a derived figure. How is it derived any differently on a dyno than HP? what "torque" are you talking about.
The relationship is very simple. power = torque x rpm/5250.
What is power though? the ability of torque to do work? Sure, we all know torque is the force in a radial plane, just as we all know force is a "push" on something. Just to be clear, we are talking about net force. (e.g. not a force like sitting in a chair is your weight, and the chair is pushing back with the same force. there is an example of no net force). Some talk about the force of acceleraton, in terms of engine torque, and that is just not correct in comparisons, and the rear wheel torque after gearing, needs to be looked at ONLY. (and of course the shape of the HP or torque curves)

Is the discussion, which is more important? more useful, more applicable to figuring out acceleration? I guess that is my question to you .

Nothing I have said has been counterdictory. If so, I certainly would like to understand what it is and fix it. So please point it out.

and gary, you do a "+1" but then in your response you refer to acceleration quite clearly.

tveltman

I think I'll have a pee in this pool too. No arguments from me about what conditions to use, but the way I see it, the easiest way to think about it is that the situation boils down to this:

horsepower = top speed (based on aerodynamics of the vehicle)
torque = how long it takes you to get there

horsepower of an engine is constant, you can't multiply the power output by a gear ratio like you can the torque. This is how cars are drag-limited, and it's why people keep pushing the performance of their engines upwards, so that they can overcome a greater degree of wind resistance resulting from the higher speed. You know intuitively that this must be true because otherwise the folks going for top speed would just put in a 26-speed transmission and not bother with aerodynamics or engine power.

Both are useful metrics for *something*, and they are inextricably related. Horsepower is a common metric precisely because you can't fiddle with the gearing to increase the number, so when comparing different cars, it works as a nice basis for comparison.

However, anyone who has driven two production cars with the same horsepower can tell you that they can certainly provide very different driving experiences, mostly dependent on gearing. So if you are comparing how powerful a given car is before and after some modification (unless of course you are shooting for land speed records), torque is just as valid of a performance metric, with the added bonus that you can use torque curves to simulate acceleration.

From my perspective, torque is the statistic I really care about. I can do some math after looking at a torque curve and knowing some other info (like kerb weight and gearing/final drive) estimate how fast it is off the line. At the end of the day, horsepower only tells me a drag-limited speed and can also tell me about fuel consumption, provided that I have a BSFC curve to look at.

My 0.02, flame away

EDIT: Note also that horsepower and torque are both somewhat retarded since engines are rated at peak power, which only tells you one point on the torque/hp curve. I'd still rather know the torque than the hp, but in terms of ratings, horsepower isn't terribly useful either.

mark kibort

First of all , are you talking engine torque or rear wheel torque as it is multipled through the gear box?

so say you are in a bmw screaming down the main straight looking at you tach and you remember from your dyno run that the torque just falls off from 7000rpm to 9000rpm. (9000rpm redline). where do you "think" you will have the best acceleration in that gear. shifting as the torque falls off the cliff or run to redline? without a good undersanding of the torque at the rear wheels in each gear, its a challenge to figure this out. with power, you just need gear spacing and you have your answer, almost visually.

max power is the engines greatest ability to accelerate the vehicle at ANY particular vehicle speed. In fact, there is a very simple equation to find the force at any speed based on the power you have at that speed.

anyway, as far as your thoughts on top speed. Huh?? gear boxes dont give speed, or power, they only optimize the power you do have available. infinitely variable gear boxes can provide the ability to give 100% of the power all the the time, but the power dicates the top speed, and the forces available through the air, not engine torque.

as far as fuel efficiency, that is a loaded one as well. Max torque dosnt give you that, or maybe it does for that power level. it is not even an indicator of the max fuel eff point for that rpm point. (I dont believe)

tveltman

fuel consumption is rated in volume per power-hour. therefore horsepower tells you something about fuel consumption. My point about top speed is that there is no way to make an engine put out more power than it's peak capacity. You can gear a car to produce thousands of lb-ft of torque trivially. Admittedly you won't be going very fast at redline, but if your goal is to get a ton of torque (say to tow some heavy ****), then you can take a 13 lb-ft single cylinder diesel engine and get it to crank out 260 lb-ft at the output shaft of the transmission with a 1:20 ratio (discounting drive loss). You can't take that same diesel engine, put it in your porsche, gear it the same, and expect to go very fast at all, because you will still only be able to overcome about 10 hp worth of air/rolling resistance.

That is why I was saying that hp is useful for comparing apples and oranges, while torque is maybe better for comparing apples to apples. Of course in any real-world scenario you need to consider both, and each tells you something useful regardless, but for the average consumer, HP = top speed, torque = acceleration.

ptuomov

Here's how I am thinking about this. There's a traction limit which means that maximum acceleration is achieved at certain rear wheel torque. Set the boost profile for each gear in a way that at all points, the rear wheels receive the torque that gives you the highest acceleration rate. Maybe that's a flat 300 ft-lbs curve on the first gear, a flat 450 ft-lbs curve on the second gear, flat 675 ft-lbs curve on the third gear, etc. This I believe gets you from point a to point b in the minimum amount of time.

RKD in OKC

Oh, ju ju dust! :-)

EspritS4s

Mark,

I can follow most of your thoughts on this topic; however, it's confusing because you are, I think, sometimes loose with the terms that you use. You also introduce too many variables into the discussion, so that we end up arguing/discussing different points at the same time. In other words, your response to a point isn't always directly relevant but you present it as if it is.

The original topic was the relationship between torque and HP. That was why I posted the definition of HP because that gives all that you need to know about the basic concept. My post was tongue and cheek, but also true. The reality is that we're not discussing or debating that definition; however, to expand the discussion, you have to rephrase the question.

When you teach introductory physics, you usually start with an idealized relationship. F=MA or P=F*D/T. You then introduce variables, usually one at a time. It takes time to model a real world system, but you learn the building blocks so that you don't go around in circles debating the variables.

In my posts, when I mention torque, I've only been talking about engine torque because it eliminates complications such as gear ratios. In your posts, I think that you have applied that term to both engine torque as well as wheel torque. At least once you've replied to a point that I've made about engine torque with a comment about wheel torque without qualifying the distinction.

You've asked a couple of questions (which engine would you prefer-flat torque curve or flat HP curve for example), but you need more information to answer these hypothetical questions.

Maximum acceleration? Need to know the torque curve, rpm range, gear ratios, tire traction, automatic traction control, ...
Best torque/HP curves for track use? Again, gear ratios, number of gears, ...

The ideal car would have infinite traction, no rolling, aerodynamic, or drivetrain losses, and a continuously variable transmission where the torque peak occurs at the RPM peak (would allow maximum torque multiplication at the maximum torque point).

A discussion of maximum acceleration, best racing engine, or best towing engine isn't a discussion of HP vs. torque. You cannot boil these questions down to HP vs. torque without stating all the conditions. Once you do state all the variables, then HP vs. torque still turns out not to be the question.

Anyways, sorry for the rant, but I think the reason that people have trouble following you (or debating you) is that you don't always boil the discussion down to the relevant point. Maybe it's a reflection of your racing experience and focus on how each of these points work together to minimize your time around the track. You introduce tangents and variables based on your racing experience that don't always help to answer the original question.

mark kibort

actually, I see what you are saying. if we are talking average consumer, yes, the big torque (engine ) number, shows that you can chug a lug with a big diesel, to tow your big trailer up to the mountains. you dont want to put the same power to use with a gas engine, as you will be redlining it and burning more gas. there we got that out of the way. the discussion is in that there is nothing different between a 500ftlb 300hp engine o a 300hp 250hp engine as far as acceleration. (as in racing, yes) Sure the 300hp 250 torque engine could have a little more of a peaky hp curve, but with close ratio gears, the difference would be minimal. The point of all this discussion, is to differentiate the difference between HP and torque for the common consumer and avide racer. thing like " flat torque" curves, or torque monsters, really dont mean much if the hp is not there, expecially if there are close ratio gears avialable (or flat HP curves, were close ratio gears are not needed and Ive seen some of those on very low torque, high HP engines, like BMW e36 euro motor'ed racers)

Thats why I always just look at power, at slow speeds, high speeds , it doesnt matter, because at any speed, acceleration, is proportional to power.