Originally Posted by swamp2
Keeping in the segueing theme here...
I not so sure if the area under the torque vs rpm curve has any particular meaning. Sure more is better but that is almost the same as saying more power at all rpms is better (or more torque...).
What does have a meaning is the integral in time of torque x rpm (well in SI units torque x ω, but the same thing if you have consistent units). This gives the total work done by the engine or equivalently how much change in kinetic energy has been given to the vehicle. From that you get exactly how fast it is going. The key thing is you need to plot torque vs. time, not torque vs. rpm. There is a key shortcut here as well though because power is just torque x ω (again basically torque x rpm but in SI units). Thus you can time integrate power instead of messing with torque and rpm! Power is fundamental!
Knowing power is absolutely more fundamental as it does not matter what the gear/final drive ratios are. But of course if you know the full torque curve vs. rpm then you can get the full power curve. You just CAN NOT use crank torque alone in many equations to determine speed or acceleration where you can use power alone! That's the power of power.
That is why torque by itself is useless........ Torque is a static measure of the amount of weight the crankshaft could lift if it had a lever 1 foot long on the end of it (I know you know this but many people don't) hence the unit Lb/Ft. Torque can't do work or accelerate an object unless it it delivered over time.
In my main area of knowledge, diesel engines, horsepower is the way all engines are rated, the torque numbers are mentioned of course, but in reality, the amount of horsepower the engine produces determines the amount of work it can do, not the amount of torque it produces. Everyone knows that diesels make huge torque numbers. For example, a Detroit Diesel Series 60 14L engine makes 1650 Lb/Ft @ 1100 RPM yet only makes 515 HP @ 1800RPM in it's most aggressive factory rating. http://www.demanddetroit.com/pdf/Eng..._S60_specs.pdf
So the moral of what I am saying is that torque is good but unless it can be maintained across a broad RPM range you can't make much horsepower with it. You can't make horsepower without torque, but torque itself can't perform work. A torquey engine is nice, but unless that torque curve is broad, the average horsepower output of the engine is many times not as good as an engine with a lower peak torque number and a broader/flatter torque curve.