http://www.dieselhub.com/tech/horsepower-vs-torque.html contians the following statement:
Power: Power is a measurement of the rate in which work is performed. The units are therefore force through a distance per a unit of time. Example: lb-ft/sec (lb = force, ft = distance, sec = time).
This statement is false as it leaves out one very important fact. Power is
not limited to the rate. It quantifies
both the amount of work that is done, as well as the rate at which that work is done. That is precisely the problem with looking at only torque ratings, and not worrying about the HP ratings. Torque
only quantifies the amount of work getting done. Best example I can give is back in my younger days, I used to be able to leg press over 1000 lbs. So give me bicycle pedals and let me pedal your DP up a 6% grade. I could move it, but not at a speed that would be practical. The problem is that the torque rating of my legs is to incomplete to quantify the total power output of Dave. SO where does that leave us. Do we only care about HP ratings? No, as there are practical implications of how each specification will affect the drivability of the vehicle.
HP is best described as the amount of performance that can be achieved with the engine at maximum RPM and the pedal on the floorboard. In the real world, this applies to how fast you will go up a very steep grade, as well as how fast you can accelerate when you pull out into traffic. HP really tells you about the absolute maximum performance out of the engine.
Torque on the other hand tells describes the RPM characteristics of where the power is produced. Technically it is a needless specification, however it does make it easier to think through the power characteristics of the engine. In a practical sense, when looking at a torque specification, high torque engines make all their power at low RPMs. What this means is that one does not need to downshift as often to raise the RPM to achieve maximum performance. The engine puts out its maximum performance at low RPMs, very close to cruising speeds. That means you have nearly all the engine's performance available without downshifting. This can give the perception of a lot of power. You just roll on the throttle and it just pulls. Most people prefer engines with a lot of torque, because the engine does not need to scream to pull the vehicle up a hill. You can not just look at torque and ignore the HP rating, as without it, you don't know if you have enough total power for the weight you are pulling.
So in a practical sense, the torque gives you an indication of how an engine will "feel" and how often it will downshift. HP rating will indicate if the engine has enough power to pull grades at a decent speed, or be able to accelerate when getting onto a crowded freeway from a winding on ramp.
I mentioned something earlier when describing torque, that it really is an unneeded specification. When HP ratings are given, they are given as a quantity and an RPM. As it turns out, if you know the maximum HP and you know what RPM that HP is produced, you will know the power characteristics of the engine. If look at the mathematical relationship of how power is determined, you can see that either torque or RPM can dominate the equation. IF the HP rating is at a higher RPM, you know that engine gets its power from spinning at high RPMs, and applies power to the wheels from its gearing. (gear ratios multiply torque) On the other hand, if the HP rating is at a lower RPM, then we know this engine does not use RPM to achieve power, so it does not shift as often, and has a good "feel". It gets a little harder to compare to engines that are very similar, because one needs to quantify all the variables in their head, which can be difficult. So we'll still keep the torque rating to make it easy.
