It takes torque to get an object to move from inert to a certain speed, but horsepower to keep it at speed. The faster you want to go from 0-60 the more torque you need and the higher the MPH you want, the more HP you need. That is simplistically about it really.
The relationship between the two is what makes vehicles of all types move about...

 

Not accurate at all.
Driven properly, 0 to 60 time or acceleration, in general is a function of power as is top speed. Power is a function of torque and RPM.
Peak torque is actually a meaningless figure, however, the torque AND the RPM that this peak occurs is meaningful in the way the car drives.
When someone says they have a torquey motor or it "feels torquey", what that usually means is that the engine has a lot of low RPM torque which makes it feel fast without having to rev the crap out of it. However, the car with more peak horsepower is usually going to win the drag race and have a higher top speed.

 

Steve, the laws of physics behind what moves our cars is relatively simple, speaking strictly. Most engines out there have pistons which move in a reciprocal motion and turn a crank shaft, thus producing rotational movement. This rotational movement generates the torque and to varying degrees, horsepower. The torque is the "twist" of the crank shaft which really provides the initial motive forces to move a vehicle through the rest of the power train (transmission, drive shaft, differential, etc). Horsepower is not meaningless in that it is a unit of measure which allows us to quantify how much power is being generated at a certain engine RPM. Peak torque is usually generated at lower than maximum RPMs, where the explosive forces pushing down on the pistons and generating the rotational energy in the crank, and the inertia generated by the moving engine parts work together to overcome most efficiently the losses due to friction and other parasitic factors. As RPM rises above this point, torque drops. The most efficient power production of any standard piston driven engine is the point at which the HP and torque curves cross.

And you are correct about electric motors - - we'd probably all be driving cars with them if power and power storage options and methods were better, but right now, the best we can hope for are lithium-ion (or similar) battery packs and back-up gasoline or diesel engines to recharge them for long-range operation.

 

I've never really thought about it for cars I've driven, but I know it's true for motorcycles. Low end usable torque is important as RobMunach said because you have acceleration without high RPMs. For example, race bikes (the new Kawasaki zx6r) don't even really start until 8k RPM; the tach barely counts 0-8k as well. My bike, a Suzuki Bandit 1250, doesn't have a lot of high end HP as my redline is at 9k but I have 80% of my peak torque at 3k. That translates into very easy (but fast) riding around town.

HP is an accurate gauge of work potential. Once you have that number, you can always gear up or down to satisfy your force (torque) or rotational speed (RPM) requirements.

 

Horsepower is simply a unit of work, which takes in account force (torque) over a given time (rpm). HP=Torque*RPM/5250. So cars with low end torque may feel fast, but are usually gutless in the in top end. However, top end power leads to higher engine stresses and more rapid wear due to increased friction. With naturally aspirated cars max torque output per liter is pretty much even across the board. For example a F1 engine is limited to 2.4 liters but makes almost 900hp, they do this by having a max rpm of 19,000, however their peak torque is only about 200lbs, which isn't that different than the 170lb from the 2.5 Outback. So really low end torque only makes a car feel fast, which isnt a bad thing, but doesn't mean it really is fast.

 

acurasquirrel great analogy can I add that the torque is what gets it rolling and HP keeps it going - would that be fair to say?

 

Yes, I think most of us recognize horsepower is a function of torque and rpm. That is the crux of my argument. That being said, torque alone doesn't move anything. I can apply 100 ft- lb of torque to a bolt tightened to 150 ft-lb and nothing moves. It isn't until rotation happens that we have movement (of course I know that's redundant). But, once rotation occurs, we are talking about power now, not torque, right? At least to keep the bolt turning once inertia is overcome. Power is a function of torque and rotation speed (at least in turning a bolt or in an automotive application).

I agree with most that when people talk about torquey engines, what they are usually talking about is low-end torque. But, if torque isn't maintained as rpm increases, you will eventually lose power (depending, of course upon how much torque is lost over a range of RPMs). So, torque AND rpm combine to be what matters in driving, not torque alone, right? And, since it is torque and rpm that matter, it is POWER that is meaningful, right? When talking about automobiles, it seems meaningless to talk about torque without specifying rpm (torque curves) since you need both (power) to move your car.

 

Steve your beating a dead horse man. 99% of the auto drivers in the US couldn't tell you the difference between 150hp and 200hp in the same car but if you put them in a 150hp diesel didn't tell them anything about the car they would hands down tell you WOW this car has power and is fast. Hence the point HP is pointless when it comes to real world application. The HP battle was created by Automotive marketing to up sell product and it continues today. Except if you were to track how much HP people are using ie the % of power they actually need it comes no where close to what they think they need or whats they think is good per the marking BS on the TV.

A 250hp super bike which doesn't even hardly generate power till your hitting 13,000 RPM means nothing to the guy riding around town or even on the highway - my bike was 65bh at 6500 rpm and riding down the highway at 90mph at 5000rpm got tiring real fast. The BMW high torque low rpm bikes are some of the sweetest rides around when it comes to every day use.

HP in the terms of standard legal street use in the US with your normal every day driver really means nothing but a car that has little torque till you have hammered the throttle to the wall is also not viewed as a very good car by the every day JOE. So again the basic street use and reference to the term HP is completely garbage when pretty much everyone is referring to the torque and their cars ability to move and to move quick with little effort. Again the marketing spin of horses is FAR FAR easier to sex up and sell to people than showing them or trying to explain how much twist power their car has LOL.

 

99% of the auto drivers in the US couldn't tell you the difference between 150hp and 200hp in the same car
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I dare you to post this on NASIOC forum!!!!

 

Here is a better explanation..Horsepower and Torque: A Practical Explanation

 

The problem is that HP alone doesn't say how well the car preforms. You need to know how the engine preforms at different speeds and how the transmission plays into that.

Just saying that car A gets XXX better HP than car B doesn't say much. More telling is Car A gets say 200 HP at 3000 RPM while car B gets 175 HP at 2000 RPM.

Taking the listed equation and rearranging it you can look at tit this way:

HP*5275/RPM=Torque.

Car A would have 351 Ft Lb of torque while car B would have 461 Ft Lb. Given the same tranie, Car B with the lower HP, but at a lower RPM would give more acceleration power.

That is ignoring how the tranie plays into it. A powerful engine with a bad tranie will have bad performance, while a mid grade engine with a good tranie can preform well.

 

The problem is that HP alone doesn't say how well the car preforms. You need to know how the engine preforms at different speeds and how the transmission plays into that.

Just saying that car A gets XXX better HP than car B doesn't say much. More telling is Car A gets say 200 HP at 3000 RPM while car B gets 175 HP at 2000 RPM.

Taking the listed equation and rearranging it you can look at tit this way:

HP*5275/RPM=Torque.

Car A would have 351 Ft Lb of torque while car B would have 461 Ft Lb. Given the same tranie, Car B with the lower HP, but at a lower RPM would give more acceleration power.

That is ignoring how the tranie plays into it. A powerful engine with a bad tranie will have bad performance, while a mid grade engine with a good tranie can preform well.

Agreed. The power curve and the gearing matter a lot.

But, my original question was, "Is horsepower meaningless?". I think this thread shows that it isn't.

 

Neither are meaningless. That should be obvious since they are completely related to each other. HP is just a different way of measuring torque. Just think about how you want your car to drive. If you have all hp and very little torque, you will have no power at low rpm. If you have all torque and little hp, you have a lot of low rpm torque with a drop in power at high rpm (think turbo diesel). It's easier to visualize when you look at dyno charts of cars similar to what you have driven.

 

Those kind of arguments usually result from not understanding the inexorable link between horsepower and torque.

You will not have horsepower without both torque and rotational speed. When people say they think torque is better, usually what they mean is they like their horsepower output to be at low rpm, i.e. more like a diesel engine.

 

Those kind of arguments usually result from not understanding the inexorable link between horsepower and torque.

You will not have horsepower without both torque and rotational speed. When people say they think torque is better, usually what they mean is they like their horsepower output to be at low rpm, i.e. more like a diesel engine.

This sounds exactly like the argument I'm making.

You said it quite clearly so I hope the argument that "torque is all that matters" is now clearly nonsensical to all. Torque without rotational speed doesn't mean squat in an automotive application. Therefore, the real question is, "where do you prefer your horsepower in the engine's RPM band?".

 

I read where torque gets you up the hill. With my old legs (65yrs)I sure could of used a horse. lol

 

So, it sounds like you agree, horsepower isn't meaningless.

I would even argue that specifically, peak HP figures are not meaningless when the differences are large.

I don't need to see a torque or power curve to know a 100 peak HP car is going to drive a lot differently than a 400 peak HP car regardless of direct injection, gas/diesel/turbo, etc.

 

An important synthesis here based on previous posts is that most drivers rarely call upon the engine to deliver its peak HP because its generally so high up in the rev band (high RPMs). Thus, they operate closer to peak torque, which makes them think 'torque matters, HP doesn't'. Also why most drivers couldn't readily tell the difference between 150 and 200 HP in the same car - they never get where that difference is actually expressed.

Honestly, most drivers under-rev their engines. They overuse their brakes, and they underuse their suspension. The way most people drive, they ought to be in golf carts - if only they had the top end speed for highway use.

 

An important synthesis here based on previous posts is that most drivers rarely call upon the engine to deliver its peak HP because its generally so high up in the rev band (high RPMs). Thus, they operate closer to peak torque, which makes them think 'torque matters, HP doesn't'. Also why most drivers couldn't readily tell the difference between 150 and 200 HP in the same car - they never get where that difference is actually expressed.

Honestly, most drivers under-rev their engines. They overuse their brakes, and they underuse their suspension. The way most people drive, they ought to be in golf carts - if only they had the top end speed for highway use.

Another problem is that modern automatics are programmed to keep RPMs low for MPG. I've driven 6-speeds that are almost always two gears too high and really hesitate to shift down far enough to get close to peak torque or HP in quick order.

 

Electric motors are truly fantastic for moving vehicles they have torque curves that are flat as a sheet of glass from the moment they start to the max end of their RPM ability. Plus they can do absurd RPMS beyond anything combustion driven. Which eliminates the concept of a transmission and gears. Which is why HP numbers for Electric motors are nothing like Gas engines - and the same concept also applies to diesel engines compared to gas engines the HP needed from a Diesel engine is far less than whats needed from a Gas engine to get the needed power to move a vehicle.

 

You want zero flat spots in power with a gas engine or even a Diesel the only real solution available today is a CVT and even these have limits on both ends at the start and at the top end.

You want none of that and perfect power as close to 100% of the time electric motor is the closest thing we have to this on the planet.

 

Anyone know at what RPM the HP & Torque curves cross on the 2.5?

 

Same as any other engine - 5,252.

 

5252. It's how hp is calculated on all engines.

 

Torque and HP curves as a function of RPM for 2.5 and 3.6 engines are attached

 

OK Thanks

 

Looby, your all time favorite boxer list should read:

......
1942 - 1964 Cassius Marcellus Clay Jr.
1964 - ??? Muhammad Ali .....

 

Looby, your all time favorite boxer list should read:
1942 - 1964 Cassius Marcellus Clay Jr.

Thanks! I'll fix that as soon as I see a campaign
TV spot for Willard Mittamorphosis Romney.

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