There are two things that can be measured on a dyno- Torque, or twisting force, measured in foot pounds, and secondly, RPMs.
Horsepower is a mathematically derived value, based on how hard the engine twists at various RPMs.
Why even mention HP if its only a mathematical term? To fully understand that, you must realize the characteristics of an internal combustion piston engine. Typically, a gasoline piston engine loses its ability to twist, (or produce torque), as the RPMs rise. Historically, early engine designs only produced power in lower RPM ranges, then ran out of steam early, at what would be considered low RPMs by today's standards. Modern engines are much better, but they still lose twisting force as RPMs increase, to one degree or another. This is due to losses of efficiency in getting the fuel/oxygen charge into, and out of, the combustion chamber. Some designs are better than others. For example, the old VW beetle engine had a carburetor in the center, and long tubes running down to the intake valves on either side. This extremely long intake design was too long to achive any inertia gains, such as seen in tuned length intake runners, (Chevy L98 comes to mind, among many others), and instead, the long intake became an obstruction to air flow at higher RPMs. Therefore, that engine had very low HP as a function of the torque. Its proverbial tongue was hanging out, as the RPMs rose. The degree to which an engine can maintain its torque over a given RPM range is indicative of the efficiency of the intake and exhaust design, the head design, etc. An engine is basically an air pump, and the better the air flow, the better the engine can maintain its twisting force at higher RPMs. This is all relative, of course, and a high torque, but low HP engine can still be effectively utilised with the proper gearing, a la' the big rig's Detroit Diesel pumping out hundreds and hundreds of slow-turning foot pounds of torque, pulling a 54 foot trailer loaded with 50,000 pounds of payload.
So if torque is what we can measure, and torque is what makes the vehicle move, why then do we even mention horsepower?
Here's why- We need something, a unit of measurement of some sort, that shows how well an engine can twist as RPMs rise. The HP figure gives some indication, (although the shear numerical value alone doesn't begin to describe the width of the powerband, nor does it mention the RPM at which that maximum horsepower is made)
A little more on torque- To get a high torque/but relatively low HP engine (like a big rig, aka a "lorry" to those in the UK) to accelerate quickly, you need to change to a higher gear as soon as you reach the end of the engines' powerband. Some engines like those seen on a big rig diesel need like 13 gears, because they only produce their power in a fairly narrow RPM range. Thats fine for a big rig, that doesn't need to stomp down the 1/4 mile track in record time, but for a sports car, that would be laughable!
In a sports car, we do need and want high torque, that's fine, nothing wrong with big torque, the more the better. But we also need the engine to make that torque thoughout a wide RPM range, not just down low, like a big diesel. Why? Because we don't want to be shifting gears excessively! What we ideally need is an engine with infinite "elasticity", where you can put it in one gear, and accelerate up to top speed without ever having to shift gears at all. The best case scenario would be an engine that could produce 100% of its max torque at 0 rpm, and maintain that torque up to its maximum RPMs. Of course, internal combustion engines don't supply power in this ideal way, so we must have gears to utilize the power band of the engine. Piston engines need a little rpms just to function, since they are "inhaling" the air/fuel mixture, and the combustion makes motion. Without motion, it wouldn't be an engine, it would be a bomb! You can't have that "inhaling" function, and the resultant combustion, if the RPMs is "0". Thats why piston engines must have some RPMs just to run, because they are actually "breathing", and this breathing requires some motion, or RPMs.
Many electric motors, in contrast, can supply 100% of their maximum torque at 0 RPMs, and hold that maximum torque all the way up to their maximum RPMs. If the starting torque is high enough, (yes, there are some with many hundreds, and even thousands of foot pounds), you could forego a transmission altogether, because if there is no need to stay in a certain RPM band, the need to change gear ratios is alleviated. For an automobile, a big electric motor like this would be too heavy, so current all-electric designs use a smaller motor, with a constant, unchanging gear reduction to multiply the torque. In the case of GM's now defunct EV1, it didn't shift gears at all, because its small electric motor could spin all the way up to around 20,000 RPM. Max torque from 0 to 20,000 RPM. I test drove an EV1, and it wasn't slow at all. When you step on the "electric" pedal, it just powers its way forward, with no hesitation, no shifting of gears, just a constant rush forward, provided for by its AC motor with infinitely variable phase control. (Variable Voltage, Variable Frequency, or VVVF) There are more exciting electric sports cars now, like the Italian Volta, and certainly more to come. You might wonder at this point, if electric motors can supply a more ideal power delivery than a piston engine, why don't we just switch to electric motors, and forget about piston engines completely?
Here's why- Battery technology, at this point in time, isn't sufficient to get the range and reserve of power we need for daily transportation. Gasoline is still the most potent and plentiful power source at this time, but as nations reach their "Hubbert's peak" of oil production, electric cars may come into their heyday, possibly with fuel cells, hydrogen, or other new source of renewable power. Hybrids make use of the best characteristics of both gasoline and electric power. Ok, thats enough on electric motor characteristics!
Every gear shift costs us precious time in a drag race, so if our ideal engine can supply its torque over a broad RPM range, we can get down the drag strip with a minimum amount of shifting, which results in lower ETs, as compared to an engine with the same torque value, but lower horsepower.
So while there is only torque, (that is the only force an engine produces), horsepower is still a useful criteria by which we can determine the ability of an engine to breathe and make good twisting force over its RPM range.
Summing up, engines make torque, or twisting force, period. That is all they can do, to twist the crankshaft. Thats the only output characteristic we can measure on a dyno, other than RPMs.
Horsepower is the ability of the engine to maintain its torque over its RPM range, or at least its a measurement of max twisting force at a given RPM.
Don't forget that some engines have "peaky" horsepower, where they only produce their maximum twisting force in a high band, like a small turbo-charged engine. It may have very little twisting force (torque) at lower RPMs until the turbo(s) come on line, but starts to produce a lot of twisting force as the turbo(s) spool up. Since RPMs figures into the formula for horsepower, you may end up with a very high horsepower figure, since the engine makes some good twisting force at higher RPMs. Is this power useful? Yes, if you stay in the optimum RPM band, but this makes for some very busy shifting, and artful use of the throttle. Not to mention that every nut and bolt is shaking itself to death, and generally speaking, higher RPMs translates to faster wear. Dual stage turbos mitigate this effect. Turbos have an advantage over superchargers, in that they benefit from less parsitic losses to the engine output, such as crankshaft loading, and heat soak. Well designed turbo engines usually have sodium filled exhaust valves to help transfer heat away from the valve seat, caused by backpressure in the exhaust, as it labors to spin the turbos. Turbo technology continues to evolve.
Our beloved Ford GT is under boost from some relatively low RPMs right up to redline, giving it a broad, very useful powerband. I have heard it takes about 100HP to spin the blower at redline. But the gains it imparts, exceeds the power it draws from the engine. Purpose-built supercharged engines have specially designed forward crank snouts to handle the load of the blower.
*author's apology*
In an attempt to relay basic principles of torque vs horsepower, many complexities must be temporarily overlooked, so that the beginner can absorb the gestalt of the matter. No doubt some of our readers can point out more details, offer examples of exceptions to the basic conventions, elucidate various subtleties, but this doesn't serve to help the beginner to grasp the basic concepts. If time and space permitted, no doubt a whole volume of work could be written on the matter.
arty