chrisv, I agree w/ you to a certain extent and I kinda see what you are saying. I agree the acceleration will be increased at max HP simply because the RPM's are higher but the rate by which those RPM's rise will drop after the torque peak because ther isn't as much force pushing the engine. Normally, the higher the power curve the less low end that you will have. You mentioned that all we care about is when the car is pulling fastest- this is true to a certain extent that the bottom of the powerband is not compromised as a result. It is more difficult to make up for the lack of low end torque on the high end. A prime example is the S2k, there is a huge portion of its power curve that is simply not useable in a race.

so I guess I'm not disagreeing with you afterall

 

No it isn't HP is not a measure of force... F=ma, force is determining your acceleration. HP is a measure of power or work over time (work being Force x distance). What are you meaning by hardest if you say it pulls hardest at peak torque but not fastest there?

For fun? 5252 is where torque and hp are equal.

I don't know what to tell you here except that you are wrong, it doesn't accelerate fastest at peak HP. It is simply more efficent to spend time in a higher gearing while the torque curve holds up.

Again, it is a matter of efficency, when the torque curve still holds up it is better to take advantage of your gearing.

 

Sorry if I was unclear. Naturally torque and power are always related by that formula. (power=torque.rpm). What I meant is that if at any moment the power a car makes is directly proportional to the accelaration it achieves. That is power is acceleration. Of course here i am neglecting to take into account things like weight of car air resistance and tire grip etc.

Of course torque also affects acceleration but, but you have to take account of both the torque and the RPM to determine acceleration, and once you start talking about torque in combination with RPM you are really talking about power.

Now the question would arise -- why would anybody care about an engine's torque when power is what determines acceleration and torque is only used to calculate power in conjunction with rpm. And the reason is that when people talk about an engine's power they usually talk about its max power which usually happens at high rpm. Thus a cruicial bit of info (the power the engine makes at low rpm) is left out. And the information about an engine's torque is mostly used to get to that piece of information. If I could get a complete power graph for every engine I would not care at all how much torque each engine was making.

Getting back to the trusty formula power=torque.rpm it follows that when the RPM are low you need a lot of torque to make good power. Thus saying that this or that engine is torqey essentially means that it makes good power at low RPM.

So when I said that torque is only significant for determining low rpm power, I did not mean this mathematically, meant it practicaly, i.e. in practice an engine's torque ratings are usually used to determine the engine's power at low rpm.

 

Torque is a measure of force, not work, and it is actually measured in a car by using a dyno. Horsepower is a measure of work and is calculated by HP = (TQ * RPM) / 5252. It takes work to move a car. Technically, you can have torque with zero work (think of tightening a bolt to the point that it is no longer turning).

That said, let's see if I can give an example. Let's say you're pushing a kid in a wagon. Your ability to push at a certain pressure is comparable to torque. But, your ability to keep a certain pressure even though the wagon is moving at a high rate of speed is horsepower. If you can push really hard when the wagon is barely moving but you can't keep pushing that hard, that's an example of high torque at low RPM. The wagon might not go as fast as the guy who can push softer but can keep pushing at a higher speed. This is why we use HP to compare engines more than torque; we care about what it can do, not the force it can put out.

Now, let's get even more confused. What about gearing? Well, gears allows the car to change the RPM of the output of the engine. But, if the RPMs are lowered, the resultant torque is increased. Look at that calculation for horsepower above. Yep, the HP of the input and output from the transmission will stay the same, regardless of gears (minus parasitic losses, of course). So, why do taller final gears allow cars to go faster? Well, they really don't. They only allow the engine to use more of its RPM band and allow the driver to keep the engine in the optimal part of the band, which will make it quicker. But, the top speed is reduced (assuming it was limited by gear and not HP/drag). All of this is assuming zero losses in shifting and driveline, which is of course not what happens in the real world.

Are you more confused now?

Curt

 

Flawed logic. Power is not directly proportional to the acceleration it receives. Quick example for this ( if you stayed at a constant 40mph, according to your assertion, you would make 0 hp, which is clearly not the case.) EDIT: Flaw on my part here, thinking about it, you technically need to apply an acceleration to overcome the drag decceleration. Power is equal to W/t not acceleration.

RPM itself does not determine acceleration, rather the torque at that certain RPM does. RPM is not the actual thing accelerating the car.


Here is some more info to clear up what I am saying since I cannot claim to be an expert in this subject and probably worded some things wrong as to the extend to making a falsehood somewhere

From http://sporttruck.com/techarticles/0312st_hp/

"According to Jimmy Watt, one horse can do 33,000 foot-pounds of work every minute. That means a horse generating 1 hp can raise 330 pounds 100 feet in one minute, 33 pounds 1,000 feet in one minute, or 1,000 pounds 33 feet in one minute. It doesn't matter what combination of feet and pounds; as long as the product equals 33,000 foot-pounds in one minute you have 1 hp."

As you can see, 1 hp does not determine the acceleration since it can cover a different distance in an equal amount of time and still be 1 hp. If the hp determined acceleration, in a set time the object would be moved a set difference. In this example the 330 ft-lbs is accelerating faster than the 1000 lbs is. This applies to cars in the following manner, the torque accelerates the car and the horsepower shows how many times that torque is applied to the ground in a certain time period. So at your peak HP the car has more ability to overcome air and friction resistance (allowing higher speed) but at the torque peak, it generates a higher acceleration, but not a higher speed.

 

"No it isn't HP is not a measure of force... F=ma, force is determining your acceleration. HP is a measure of power or work over time (work being Force x distance)."

So power is equal to force times distance over time which is force times speed. So for any particular speed max power means max force which means max accelaration.

Many people are confused by saying "well F=ma, and torque is force so therefore torque is proportional to acceleration". This is not true. This is because torque is a rotational force at the engine, and to determine the acceleration of the car you need the LINEAR force at the WHEELS. And the force on the wheels is quite different it depends on your gearbox size of your wheels and a million other factors. All these things act like levers, and levers can dramaticaly increase or decrease the force being applied. I.e. put a lever on a pivot and push on its longer side with a force F you could be applying a force of 10xF on the shorter side.

Thats why everyone uses power. Kinetic power at the engine gets transferred to kinetic power at the wheels. Of course there is some loss while the power gets to the wheels but power cannot be magically multiplied ten times by gearing. Power as measured in cars is change of kinetic energy. When the engine produces this kinetic energy it is transferred to the wheels and determines the acceleration of the car. The gearing does not change the energy delivered (except to contribute to some losses) therefore power is the unit used to determine an engine's contribution to acceleration.

Anyways, I may not be giving the best explanation. But I did find a webpage that actually gets it right:

http://www.allpar.com/eek/hp-vs-torque.html

So there you go.

 

Sigh of course 1 hp determines acceleration for a certain weight. Fair enough horsepower determines acceleration for a given weight of car, and wind resistance, road incline, tire resistance, blah blah blah. I left these factors out in order not to complicate matters and now you are throwing them back at me.

The thing that allows a car to overvome air friction resistance is the same thing that allows it to accelerate.

 

The linear force at the wheels is determined by the angular force exerted on the wheels. This force is determined by the force exerted on the axle, which funnily enough is torque. Gearing allows you to amplify the torque going to the wheels. The wheel is what converts the angular acceleration to linear acceleration.

EDIT: I suggest you read the link you posted, I will put a quote from it here. "But, you ask, isn't your acceleration greatest at the torque peak? Yes, it is! But only within that gear." As I stated before, you go to the peak horsepower (Which occurs just before the torque begins to drop off rapidly, imagine that) to take advantage of your gearing which is amplifying the torque to your wheels. As long as your car keeps making torque, it is useful to keep it in that gear since the gear amplifies the torque avaliable.

 

Wow, after reading all of these posts, I thought I was back in my High School Physics class. The way I see it is that for the street, torque is what matters. If you've ever driven a low torque, relatively high HP car like the S2000 and a torque monster like the E55 AMG, you'll understand. HP is important for the track where you are likely to be spending almost all your time near the redline.

 

I did a search on the net typing in 'torque vs horsepower'. I found these 2 articels to be interesting. Let me know what you think.

http://vettenet.org/torquehp.html

http://www.yawpower.com/tqvshp.html

RJG35

 

I'm sorry I didn't include this in the previous post but it adds another twist. Here you go.


http://www.allpar.com/eek/hp-vs-torque.html

RJG35

 

This is my last post regarding this topic. After browsing the net and reading through many sites, it seems that this debate is huge. 'Experts 'among themselves seem still debate which is better. I finally found what I think is an excellent article that finally put the pieces completely together and you'll see why if you read it. There's a word in there the other sites don't talk about and it's called potential.

Once again, sorry for the multiple posts but finding the article below made my research on this topic worth the time.

http://www.mustangsandmore.com/ubb/D...orqueVsHP.html

 

Is this why so many people have the grounding kit on their G's? I thought the grounding kits were just snake oil!

 

Okay like 50 definitions are running threw my head

what does WOT really stand for

 

WOT - Wide Open Throttle