Cat vs Powerstroke torque curves

ShinerBock wrote:

4x4ord wrote:

ShinerBock wrote:

Turtle n Peeps wrote:

Read #1 for your answer. Depending on what is important to you.

I am not sure I totally agree with that article.

I think from a technical standpoint the article is right on, however, I think sometimes people want to pay too much attention to the definitions of HP and torque and forget that when it comes to an engine, torque gives some description of power. In fact when the rpm is stated along with the torque value, such as with a drawing of a torque curve, an exact measure of power can be derived. The article does explain a bit about the importance of the engine's power curve as opposed to peak HP value. In addition it explains a bit how gear ratios come into play.

I don't think he is spot on in the article. To say that it is only horsepower that helps you up hills is false. It is a combination of both. You need torque to overcome the force pulling you back and that force working against you is constantly changing. If you don't have enough torque to do the work, then all the horsepower in the world is not going to help you.

Take a Ram 2500 6.4L Hemi and 6.7L Cummins. They both have the exact same transmission gearing with the rear end gear advantage going to the Hemi(3.73 or 4.10) versus the 3.42 of the Cummins. The Hemi has 40 hp more while the Cummins has 371 lb-ft more torque. Send both up a grade 7 hill with 14k lbs strapped to them and I guarantee the 6.7L Cummins will get to the top quicker even though the Hemi has more horsepower.

The principle is this:
Think about power as the rate of doing work. Think of a shaft with a cable wrapped around it. Attached to the other end of the cable is a weight. The cable and weight create a force on the shaft referred to as torque. As the shaft rotates the cable is wound and the weight is lifted. In order to turn the shaft fast with a light weight a certain amount of power is required. Now if the shaft is turned half as fast and the weight is twice as heavy the exact same amount of power is required.

A couple of trucks pulling identical trailers up a hill both have rear axles. If both trucks have the same size tires and are traveling the same speed they will have the exact same torque on there rear axles. If the tow trucks are travelling the same speed there axles will be turning the same speed. The rear axles on both trucks will be transmitting the exact same amount of power. Now look at what is driving those axles. If one engine is turning 5 times as fast as the trucks rear axle it will have a torque of 1/5 the axle torque on its crankshaft. That engine is delivering the exact same horsepower as the rear axle. If the other engine is turning 7 times as fast as the axle it will have a torque on its crankshaft which is 1/7 the rear axle torque and it will be transmitting the same power as the rear axle.

I think the thing that gets people arguing is that high torque engines tend to have flatter power curves. With a flat power curve the engine is actually able to put close to its rated power to the axle over a wider rpm range. If infinite variable speed transmissions were coupled to engines the higher peak horsepower engine would always be able to out pull the lower horsepower engine.