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This may not be of much use to people who know what they are talking about, but I was a bit confused about the whole torque/horsepower thing so, I decided to do a search on the internet and get a good explanation in more simple terms. I knew that torque is what really makes a car move, but I didn't know why people would always quote horsepower.

Here is the article:
http://vettenet.org/torquehp.html

Correct me if I am wrong, but I believe that torque is the only thing that we ever feel when driving and horsepower does nothing for the feel. Now two engines with similar torque curves, but one with a higher hp will accelerate the same. The advantage of high horsepower (in this situation) is that the car can rev higher, so you do not have to shift and can continue to accelerate with the same torque at the wheels. Once you shift the torque at the wheels decreases and acceleration slows. Now, the thing that deviates from what I have said is that an engine with higher horsepower typically doesn't doesn't maintain the same torque curve. In reality, both the torque curve and the max rpms will typically be higher, so even at the lower rpms the higher hp engine will get going faster.

Although in real world street driving most drivers don't typically bring the rpms over about 5000 (now I know that some of us on here will go higher than that), so unless you are racing all that really matters is the torque curves, because you are not using the advantage of the higher rpms.

Does what I said seem to be correct?
 

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SolsticeDesire:

You don't feel the "torque" or the "power" in a car. What you feel is the "acceleration" or change in speed.

Acceleration is gotten by dividing the force (in this case the thrust at the tires) by the mass. (comes from Newton's 2nd Law of motion, F=m*a)

If you are travelling at a given speed, and wish to change the speed, you will require a certain amount of force to do so. This force is simply determined by the torque produced for the engine at the given speed and gear, worked through the gear ratio and tire moment arm to the force (or thrust) at the tire patch pushing you along.

The reason power is important is the fact that internal combustion engines have a very specific curve of torque generation throughout the range of RPM.

Not sure that helped :confused:
 

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Torque is important, don't get me wrong. What really counts, though, is the amount of thrust at the wheel.

If you have an engine that has no low end torque, then when operating at low (<2500 RPMs), the available thrust at the tire is not much. Therefore, less acceleration.

The other characteristic of an internal combustion engine is that the amount of torque produced is HIGHLY dependent on the RPMS of the engine, and therefore highly linked to the speed of your vehicle (unlike, say, a rocket engine, turbine jet engine, or electric motor). So, your acceleration is also highly related to your vehicle speed (since the speed is linked to the engine RPM by gear ratio).
 

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First of all, from a driver's perspective, torque, to use the vernacular, RULES :)
This sums it up (from the article link) ;)

Thanks for the link !
 

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Would it be fair to say that torque creates the acceleration, and horsepower maintains a given speed once attained?? Sort of a continual one-two punch effect.
 

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I'm no engineer but I think it is quite easy:

Power = Torque * RPMs [in some units]

Example:

Your engine has a Torque of 300 Nm at 3000 RPM. Convert into SI-units:

300 Nm -> 300 Nm
3000 RPM -> 3000 RPM : 60 sec/min = 50 RPS (rounds per second) -> 50 RPS * 6.28 -> 314 radian/second

=> P = 300 * 314 [Nm/second] = 94200 W = 94.2 kW

reconvert into hp:

P = 94.2 * 1.36 [hp] = 128.1


So you can see that torque and power are directly connected.

The acceleration you feel is caused by the engine power that is not consumed by the losses. At the top speed of your car, engine power and losses are equal and therefore you have no more power available to accelerate.

You want to have a high acceleration under all circumstances and therefore you need a high power at all RPMs. As you can see from the above formula you would achieve a constant power if you'd manage to have a linear FALLING (!) torque curve because then the product of torque and RPMs would be constant.

Of course this can't be achieved with a combustion engine. Therefore a good engine should have:

a) high peak torque
b) peak torque at lowest possible RPM
c) a torque plateau after the peak torque
d) a wide torque plataeu

In the area of the plateau the power will increase steadily. Once the torque drops after the plateau it usually does too fast for the still increasing RPMs to compensate. Thus the power will have it's maximum at the end of the torque plateau.

The problem everyone has is that you FEEL acceleration and neither torque nor power. As I said, acceleration is created by the difference between power and losses.
 

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t.henss,

You're correct, you feel the acceleration, neither the torque nor power. Humans also sense acceleration change (second derivative of velocity, usually in meters per second cubed, also called "Jerk").

Physically, the acceleration comes from the propulsion force at the tire patch (which, when vehicle speed, trans and tire gearing are known, then the engine torque producing this force is known). The Jerk is how non-linear this acceleration is.

Humans percieve a constant jerk and gradually increasing acceleration as "fast". Zero jerk and constant acceleration can, in some cases, feel slower but result in quicker overall acceleration.

If I get some time, I'll resurrect some graphs I did for a Vehicle Systems class I participated in showing the difference, but let me try to tell you in words:

If you have a motor (like and electric motor) that has a constant torque throughout the entire range of useage, say from 0-60 MPH, you have a vehicle that will accelerate at a constant acceleration. If you have the correct torque to produce the tirepatch force that results in about 0.45 g's of accleration for your mass, your electric vehicle will get to 60 MPH in about 6 seconds.

BUT, it will not FEEL very fast. This is because people acclimate to acceleration very quickly. Physiologically, the body can acclimate to a new acceleration in about a third to a half a second - the body senses increasing forces, when the forces stop changing (like in your seat back), the body acclimates and settles in - for another 5 1/2 seconds, even though you are gaining speed quite quickly. Throughout the entire period of acceleration, you experience an acceleration of a constant 0.45 g, and a jerk of zero. The average acceleration is the same, about 0.45g, and the peak was pretty much identical: 0.45g. The speed just increased at a constant and linear rate right up to 60 MPH.

The power curve for the motor is basically linear and increasing - right up to the max power rating of the motor. After which the torque drops in proportion to increasing motor speed.

If this vehicle is powered by an internal combustion engine, this torque has a function that looks a bit like a logorithmic function, with some secondary term. Some torque curves are very linear at the beginning, then "curl" over. Turbo curves tend to be linear up to max torque, then somewhat flat topped as the boost controller kicks in... Superchargers tend to be linear on the lower end - a kind of blend between N/A or Turbos (yes, they DO feel different) Bottom line: the torque is not CONSTANT with respect to vehicle speed.

This means that the tire patch thrust is not constant - and since the thrust is changing, so is the acceleration. If this the acceleration is changing and increasing, the occupants experience "jerk".

If this theoretical IC vehicle is set up to coincidentally have a constant jerk of around 1.47 m/s^3, this vehicle will get to 60 MPH in 6 seconds too. BUT it will FEEL much faster. This is theoretical, but at the end, the velocity is increasing non-linearly and acceleration is approaching 0.90 g. If you look at the curve of acceleration, it starts low, you're only pulling about 0.14 g at 1 second, matching the electric motor vehicle (0.45g) at about 3 seconds, and your body experiences this "rush" of change in acceleration we all associate with speed.

So, for internal combustion engines, BOTH power and torque are important. Peak power, since most internal combustion engines operate in similar manner, has THE SINGLE MOST CORRELATION WITH ACCELERATION. Since we associate acceleration with speed, and we all feel the need for "speed", we all really want power in an IC motor.

However, there are exceptions to all of this: low end torque is good to have, high redline is good to have, because the higher the torque, the higher the acceleration that can be maintained. Low end torque gets us moving and gives us that initial "oomph" when driving below the peak torque RPM of the engine. Hi power at hi RPM's means this torque curve is still increasing through a large range - allowing us to experience increasing tire patch thrust for a greater speed range. If you get to use all of the engine and have great range of coverage in your transmission, and you really care about ET's and 0-60's, you ALWAYS WANT HIGH POWER.

I suppose you can also sum it up like this: The torque curve tells you at any point what the acceleration is (torque translates to force at tire patch which = acceleration). The power curve for a motor is nothing more than the time derivative of the torque curve. The vehicle jerk is nothing more than the time derivative of the vehicle acceleration. And since we associate "jerk" with "percieved speed", the more jerk, the faster the car will feel (and actually be, due to the non-linear nature of the torque curve). To get more jerk, you need more power.

Bottom line - you want more power. Even though engines are really torque devices.

If you still don't buy this, look at a diesel engine. Great low end torque. But a scant few thousand RPMS above this, you reach a lackluster peak power and decreasing torque (even turbocharged diesels). This means you need a multitude of gears, or a continuous transmission, to maintain this torque (or tire thrust) over a large speed range (say from 0-60).
 

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Discussion Starter #10
Sman that was a great post. It's goog to have people like you that spell it out and know what they are talking about
 

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I have seen a lot of Tq vs Hp debates on the forum, but S-man, that is the single best post I have ever seen explaining the two, and how we perceive them in the driving experience! Kudos to you! :thumbs:
 

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Thanks, guys, but I thought it was too long.... :lol:

I just got going, walked away, came back and did a bit more.... then ended up with this monstrosity that I can hardly read :glol:
 

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What makes sporty driving fun is the gravitational forces created by acceleration; feel in the seat of your pants. High amounts of torque or high amounts of horsepower alone won't make G forces. The combination of the 2 over a longer period make G forces!
 

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solsticeman said:
Thanks, guys, but I thought it was too long.... :lol:

I just got going, walked away, came back and did a bit more.... then ended up with this monstrosity that I can hardly read :glol:
:lol: You are fond of making posts that never end! :glol: That is ok, because they are generally full of good information.
 

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Thank you solsticeman!

I think my understanding of what a good engine should look like was a bit too much based on scientific thoughts like "fastest time to reach 60 mp/h".

I've known, that there was a difference between a good engine and a good feeling one.

The thing with the "d a/dt" term convinced me. I thought about it and ... AGREE.

Now that we are through with theory I'll jump onto my motorcycle and try if I can verify that exerimentally ;) .
 

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I hurt my head, and over lunch took a search of my harddrive and some of my archived scanned notes from school...

I've got some of my stuff backwards - because the hypothesis of people feeling "jerk" back when I was looking at this as a potential PhD research project (back when I had the motivation to do so), I realized that there was a plot missing from most engine plots that IS the deravitive function of the torque curve.

I think (too hot and tired to re-read) that I stated the power curve is the deravitive curve of torque, but now realize it is the other way 'round, and the change in torque (which would be a useful measure of vehicle "jerk") is NEVER plotted, mostly because nobody knows what to call "change in engine rotational torque" or "change in thrust".

So, read the large post with caution - when I'm inclined, I'll correct the reasoning. The hypothesis still stands - and the end result is still the same, you want power, and you want a power curve that has some curvature to it (so the torque curve has some slope, so you get an increase in acceleration as you increase speed).
 

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t.henss said:
Thank you solsticeman!

I think my understanding of what a good engine should look like was a bit too much based on scientific thoughts like "fastest time to reach 60 mp/h".

I've known, that there was a difference between a good engine and a good feeling one.

The thing with the "d a/dt" term convinced me. I thought about it and ... AGREE.

Now that we are through with theory I'll jump onto my motorcycle and try if I can verify that exerimentally ;) .
A $10K motorcycle is cheapest way to experience the same thrill a super exotic car delivers. Unfortnately, my motorcycle tops out at 158 mph; need to buy that $13K motorcycle for the 180+ mph thrill, but hell, my 0-100 mph times are just as exciting for $3K less... :lol:
 

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WaitingForBoost said:
Horsepower is what they sell, torque is what you drive.
It's jsut like computers. People always assume MHz is the main selling point. And just like car manufactureres, certain companies have specificaly designed CPUs [engines] to run extremely high MHz [HP] at the sacrifice of real performance [Torque].
 

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sman: That post deserves to be magnetized to my fridge. It was grrreat! :yesnod:
 
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