This is always a mystifying subject to a lot of people, so let me put this is layman’s terms for you. Horsepower is measured at higher RPM’s and torque is the low-end “grunt” or low RPM pulling power of an engine. Diesel engines only make around 300-400HP in those big rigs, but they make anywhere from 1,500 - 2,000 Ft. Lbs of torque, which is what pulls the 80,000 Lbs of payload in a fully loaded big rig. You certainly won’t see any 600HP big block Chevy in a big rig, because it would simply fall on it’s face.
Let’s take another scenario, an Indy car engine. These are tiny little 1,200 HP V-8’s that spin about 12,000-14,000RPM all day. You ever see these cars stall when trying to get out of the pits? They stall all of the time. They have to rev that engine way up and dump the clutch to get rolling, which is why you always see them getting sideways and smoking the tires-off when leaving the pits, and if they don’t do that, the engine will stall. This is simply because of the short stroke of the crank, which make no torque at all at low RPM‘s. Those engines HAVE to be up in the high RPMs to make their power, so again, you certainly wouldn’t see any engines like that in a heavy vehicle.
Let’s take yet another scenario that many of you can probably relate to, motorcycles. Many of you have probably ridden dirt bikes or street bikes before, and you probably know all too well that 2 stroke dirt bikes are real fast as long as they are in the “power band”. When they are below the power band though, they are complete turds, especially if you have a rider on the back or are trying to take-off from a dead stop up a steep hill, the bike will fall on it’s face if you aren‘t winding the living hell out of it to get it going. A 4 stroke bike will make much more torque and won’t be as “peaky” or “pipey” and the power comes-on smooth and at a much lower RPM than a 2 stroker. Picture the 2 stroker as a high “horsepower” bike, and the big 4 stroker as a high “torque” bike, and that is pretty much the difference between horsepower and torque.
You certainly wouldn’t want to put a little BOSS 302 in a big, heavy Ford truck, even though the BOSS 302’s made pretty big power at high RPM’s, and again, you wouldn’t want to put a diesel engine in an Indy car.
In a typical engine, anything below about 5,200 RPM is all torque. Horsepower doesn't even come into play until above 5,200 RPM, so 95% of your driving relies on raw torque, NOT "horsepower".
Torque is like using a large breaker bar to get a stuck bolt loose. You compound the power of your arm by the length of the wrench, the longer the wrench, the more twisting power you can put on that bolt, BUT, you can’t turn that wrench very fast because it is so long. That is kind of like a diesel engine or a big block with a long stroke. Now, you can’t break-loose a stuck bolt with a short wrench because you just can’t make any “torque” to break it loose, BUT once that bolt is loose, you can turn that wrench very fast and unscrew it quicker than if you were trying to sling around a 30” breaker bar, right? That is like a small cubic inch, high “horsepower” engine. This is why high horsepower engines require lighter vehicles, lower rear-end gears, higher stall converters and higher RPM’s to make their power useful. And that’s pretty much the difference between torque and horsepower.
In racing where there's an accident, remember this; Horsepower is how fast you hit the wall. Torque is how far you move that wall. If a 300 Lb. race bike crashes into the wall at 100 MPH, Horsepower got him to hit the wall at that speed, and the wall probably won't move much. Hit the same wall with a 75,000 LB. semi truck traveling at just half that speed, and the wall will get knocked down and pushed off the race course a few hundred feet. That is the definition betwen horsepower and torque.