How are HP and Torque correlated?
 

Ive always been curious about this. When you talk about cars, people always wana know the HP and torque. On some cars ive noticed that the torque is higher than the HP and on others the torque is lower. Whats the relation between the two, if any? And what factors in the car/engine control the torque? ;)

Horsepower = (Torque * RPM) / 5252


See <a href="http://vettenet.org/torquehp.html" target="_blank">http://vettenet.org/torquehp.html</a> for a
more thorough discussion.

HP = (Tq * RPM)/5252

Gross generalization: So if you make most of your torque down low, torque will be higher, but if you make most of it up high, HP will be higher.

[quote]Originally posted by Tabor Kelly:
<strong>HP = (Tq * RPM)/5252

Gross generalization: So if you make most of your torque down low, torque will be higher, but if you make most of it up high, HP will be higher.</strong><hr></blockquote>

That's why Turbo'ed cars have more HP, and Supercharged cars have more Torque.

Also in a car that has peak horsepower at 5252RPM peak torque and HP numbers are normally the same. Also you will notice that on all HP/Torque Dyno graphs the HP & Torque cross paths at 5252.

Other than the mathematical realations above it really has to do with they way a motor is desiged as to how much hp/torque they have.

A large displacment motor like most american V8's have lots of torque. This is due to the the amount of force each rotation of the crank. Lots of displacment mean lots of gas pressure in the combustion chanber after combustion to push the cylinders to create more force at the crank. This is force is rotational so it is expressed at Torque.

Horsepower is how much force you have at a given engine speed. Now here is where it gets funky. In most cases large displacment motors have difficulty running at high engine speeds. They either can't spin very fast with our breaking stuff due to the mass of the large pistons and so forth or they become in efficient at the higher speeds. This typically happens because they can't get enough air into and out of combustion chamber fast enough to be efficient and therefore produce less crank force (torque) at say 4500 rpm than 2000 rpm. In many cases even when you consider the higher rpm and what that does to the HP numbers it is still not sufficient and HP drops.

For example the Dodge Viper V10 and the Dodge Truck V10

Both 8.0L V10 motors with basicly the same block
Truck motor 300 hp & 450ft-lbs torque
Viper motor 450 hp and 450ft-lbs torque

What happend? They improved the head on the viper motor to supply sufficient Air to keep some torque and therefore gain HP.
Note also that the truck motor redlines at 4500 rpm while the viper motor at 6k? (i think)

Now contrast this to the Honda S2000 Motor
2.0L I4 - 240 hp and 153 ft-lbs torque.

Since the motor is small it can't make as much force per crank rotation. There just is not force of expanding gas in the combustion camber to do so. Therefore the Torque is quite low. However they have done a very nice job with air flow managment and mass managment can can spin the engine to 9000 RPM and get 240 hp by maintaining the torque at higer rpms thus gets lots of hp.

F1 motor can get 800 to 900 hp from their 3.0 V10 motors. Of course they probably only get 250-400 ft-lbs of torque again because they don't have that much displacement. They way they get 900 hp is from engine speed. At 18,000 RPM you don't need too much torque to get big hp. Their biggest challenge is keep the motor together at those engine speeds. Right now anyone that can get more RPM can also get more hp from these motors. Of course they don't even run at 2000 rpm.

Now for the 951.. Well it is the turbo! The turbo in effect multipies the displacment of the motor. Such that (in a very simple approach) 14.7 psi of boost (1 standard atmosphere) will provide air in a volume equvalent to 5.0L for a 2.5L motor or 6.0L for a 3.0L motor. Now in some ways you have tricked a small motor into behaving like a big large displacement motor with respect to crank force (torque). Now the actual relation to volumetric incease is not a simple as what I have stated and in reality is quite complex. But in general increasing boost will increase torque and to some degree horsepower. To really impact horsepower means to keep the torque as engine speed increases.

Of course there are things that can be done that increase low rpm torque, but at the expense of high rpm torque. The situation is the same for increasing high rpm torque by improving air flow can hurt the air flow efficiently at lower rpm points.

Thus while the relationship between hp and torque can be boiled down to a simple equation the actual relationship between the two in an engine with respect to maximum levels and levels at any particluar rpm are quite complex and very engine design specific.

Thanks for the explanations guys. M758, your explanation was very thorough and informative in terms of practicality.

I apologize for the triple posts, i was having problems with the website for some reason.. <img src="graemlins/r.gif" border="0" alt="[king]" />

Hey check this out (from memory even)...

Torque = (Lever Arm)(Force)

My college physics professor would be proud of me don't you think!!! <img src="graemlins/r.gif" border="0" alt="[king]" />

Horsepower is for SHOW and Torque is for GO :)

[quote]Originally posted by Mike B:
<strong>Horsepower is for SHOW and Torgue is for GO :) </strong><hr></blockquote>

I like that... I'm going to start using that if you don't mind Mike?

Be my guest Cory... <img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" />

Also remember that only torque can be measured. It is measured on a dynometer. Horsepower is then calculated from the torque measurements per the above formula.

Yup, at any given moment, it's torque that matters and torque-at-the-wheels is what a dyno measures. However, it really doesn't measure that, but computes it. What it really measures is torque-at-the-drum.

Given:

The dyno-jet knows the inertia of the drum based upon its mass and diameter. Then it measures angular-acceleration when you do the dyno-pull. This is done through an RPM-sensor and timing clock. With these variables known (I and a) and measured, it can then compute torque at the drum.

Using an inductive pick-up to measure the RPM of the engine, the dyno can compute a ratio of engine-RPM to drum-RPM. This ratio can then be divided out of the torque@drum to give torque@wheels. This ratio effectively cancels out torque-differences at the wheels based upon gearing. If you use 3rd-gear to run the dyno, you'll have multiplied your torque more than running in 4th-gear. But you'll also have a higher-ratio of engine-RPM vs. drum-RPM as well, so the torque@wheels figure will end up being the same.

Then simutaneously, the dyno can compute HP using HP = (Tq * RPM)/5252 from either torque@drum & drum-RPM or torque@wheels & engine-RPM.

Some more HP & Torque reading:

<a href="http://www.pumaracing.co.uk/power1.htm" target="_blank">Power and Torque 1</a><a href="http://www.pumaracing.co.uk/power2.htm" target="_blank">Power & Torque 2</a><a href="http://www.pumaracing.co.uk/power3.htm" target="_blank">Power & Torque 3</a>