Found this very informative piece of info for those who want to learn!
Originally horsepower was considered to be one unit when a horse could raise, by single pulley, one hundred fifty pounds at a rate of 2.5 mph, or 550 foot pounds per second.
This in energy terms turns out to be 746NM per second, or in electrical terms is 746Watts.
Mechanical horsepower = torque (in foot-pounds) * RPM / 5252.
The torque output of an internal combustion engine varies with RPM, being zero at very low RPM, rising to a peak level, then falling to zero again at the engines maximum speed. Some engines produce high torque only in a narrow band of output speeds, others have a wider power band. Maximum horsepower is determined by looking at the point on the speed-torque chart where torque*rpm is greatest. This gives you an idea of the maximum power the engine can produce, but to get the full picture you need to look at the power/speed curve to see how the engine will behave in reality.
***MAD POST ALERT BELOW***
First, note that there is no such thing as a "torque" engine. Any engine that produces 300 ft-lbf of torque at 3000 rpm has 171 hp at that speed. Regardless of its stroke, bore, displacement, supercharger, etc. Also, any engine that makes 300 ft-lbf of torque at 5000 rpm will produce 286 hp. This is due to the simple relationship between power and torque:
power = (torque * rpm) / 5250
where power is in horsepower, and torque is in ft-lbf. Also note - at 5250 rpm, hp is equal to torque numerically.
Racing engines are designed to produce a lot of torque at high engine speeds, because this will lead to increased horsepower. This engine can then be geared for whatever range of speeds it will normally be driven in.
The biggest problem with production cars is they spend the vast majority of their time under 4000 rpm, and larger engines (like my Mustangs 4.6 l) spend most of their time below 2500 rpm. Another related problem is that due to the direct relationship between power and rpm, the power produced at these low engine speeds is low. So to get more power at common driving engine speeds, you must therefore increase the torque at these lower engine speeds.
Engine power increases with rpm because power is a function of time. Torque, on the other hand, is relative to the force exerted on the piston by the expanding gases in the cylinder. If you had perfect, equal cylinder filling on each stroke, then this force would be proportional to the bore, or displacement of the cylinder, and this force would be equal per stroke regardless of rpm. You can think of power as being determined therefore by how often this cylinder fills and fires in a given amount of time. One of the reasons two-stroke engines produce so much more power than a comparable four-stroke engine - although it has a poorer volumetric efficiency (the efficiency of filling the cylinder with fuel-air mixture), it fires twice as many times in the same amount of time.
So to increase low-end torque, we must actually also see what increases low-end power. And vice-versa. Engine torque will derive from the pressure exerted on the top of the piston by the exhaust gases. The average pressure exerted on the piston through the length of the stroke is known as the mean effective pressure (mep), and is given in terms of psi. Another measure, imep or indicated mean effective pressure, is determined by using some fancy equipment to actually measure on a running engine what the pressure is. Engineers commonly use mep as an indication of power plant performance in internal combustion (IC) engines.
Because its pretty hard to measure the imep, another measure, brake mean effective pressure, or bmep, is often calculated in its stead. This equation is:
bmep = (hp * 33,000) / (Length * Area * N)
where hp is power in horsepower, Length is the length of the stroke in ft, Area is the piston top surface area in square inches, and N is the power strokes per minute (or rpm / 2 for a four-stroke engine).
Now, what are the things that affect this bmep? Well, the first is volumetric efficiency. This of course is the efficiency of filling the displacement of the engine with fuel and air mixture on each stroke. You could also look at it this way - if a cylinder only fills with 85% efficiency on each stroke, this is similar to reducing your engine displacement (size) by 15%. Due to frictional losses, valve overlap, etc. this will not ever be 100% in a naturally aspirated engine. In fact, this efficiency falls off steadily as engine speed increases. One way to get back these losses is to use a turbo or supercharger, which will compress air into the cylinder, and thus give a volumetric efficiency relative to a naturally aspirated engine of more than 100% (but its not actually more than 100%, because you cant look at it that way…)
What else increases volumetric efficiency? Well, tuning of the exhaust and intake runners will achieve some level of "pressure wave supercharging", to help move the intake gases in and the exhaust gases out. A lower restriction air cleaner, fuel injection system, intake manifold, larger and more valves, etc. will also increase airflow into the engine. Likewise, lower restriction exhaust, removal of the catalyst, larger and more exhaust valves, etc. will allow the exhaust gases to flow out of the cylinder more easily. Also note that for both the intake and exhaust cycles, the valve timing and lift are also key to not only achieving greater volumetric efficiency, but also to tuning where the maximum possible flow through the valves will occur.
And when youve looked at what you can do with respect to volumetric efficiency, there is now friction horsepower loss to contend with. This tends to increase as a function of the square of the rpm of the engine, and can be very substantial at high speeds. One reference I have here gives the friction horsepower of a "stock" 350 Chevy as being 12 hp at 2000 rpm, 25 hp at 3000 rpm, 47 hp at 4000 rpm, 72 hp at 5000 rpm, and 110 hp at 6000 rpm. This friction loss can be reduced by careful engine building and lower-friction bearings and components, and by synthetic oils. But the trend of increase remains the same shape.
Now lets look at bore and stroke. Increasing the stroke of an engine by increasing the crankshaft throw not only increases the displacement of the engine, but also increases the mechanical advantage on the crankshaft. So increasing the stroke increases the torque both from increased displacement and from greater mechanical advantage. Thus, even for two engines of the same displacement, the one with the greater stroke should have the greater torque. However, there are some other things to consider here:
1) As stroke length increases, thus does the piston speed at the same rpm, and therefore the frictional power loss will increase as well.
2) As this piston speed increases, the volumetric efficiency will fall off as well. A larger bore engine will also allow for larger and/or more valves, with less shrouding needed. Thus, if the two engines have the same displacement, then the shorter stroke one will have the greater volumetric efficiency.
3) You also have to consider inertia effects of the longer connecting rod, and increased side-forces on the rings and piston due to the increased rod angularity with respect to the bore centerline.
Of course, there are those that will argue that a larger bore engine may result in an increased chance of detonation at high compression ratios than a smaller bore engine, due to an increased distance that the flame front must travel across. But this will vary greatly due to combustion chamber design, and I dont know if it can be analyzed so generally. Overall, many claim that the small bore/long stroke engines are much better suited to the higher compression ratios, and if in our example of the two equal sized engines we also have a higher compression ratio than the larger bore one, then the larger stroke engine will see even more torque improvement.
Another theory related to this is the "long rod" theory, which says that the longer the connecting rod is, the more time or "dwell" the piston will have at top dead center (TDC). And therefore, there will be more time allowed for the gases to burn completely at the highest engine pressure. Thus, you will get even more force out of each firing with the same amount of fuel as the rod descends through its stroke. However, what really happens is that the long rod has a very small mechanical advantage due to its angularity at these high pressures, and thus this effect is negated. A short rod, attached to a crankshaft with a long throw, will move towards a higher angularity faster than a long rod with a short crank throw.
Another advantage of a long throw/short rod engine is that cam timing is not nearly so important as in a short throw/long rod engine. This is due to the fact that the piston spends less time at or near TDC, and thus allows for an earlier valve lift start and longer duration. Also, the exhaust valve can stay open longer as the piston ascends, increasing the amount of exhaust gas that is pushed out of the cylinder, and thus increasing the volumetric efficiency.
And so on, and so on. There are a great many things I have left out or glossed over, because I am tired of typing. Hope this helps somewhat more.