[jessrayo]

It is interesting to me that so many people have so many different opinions on "concepts" like area under the curve. Since no one has explained it like I have understood it I'll throw out my 2 cents.

If you look at the formula comparing hp and torque you can see that they are mathematically related and the only constant in the equation is 5250. Anytime you look at a dyno graph with torque and HP both plotted, the 2 lines will always cross at 5250rpm. It is a mathematical certainty. Because of this mathematical relationship the torque will always be more significant left of 5250 on the graph and the HP will be more significant to the right of 5250. Because of the mathematical formula, a certain number of torque will always have an exact correlation of HP depending on what the rpms are at. When people talk about "area under the graph" they are generally talking the area under the combined graph (torque left of 5250 and HP right of 5250) because this gives a general representation of how much grunt the engine is going to have at any given point on the rpm band. If the overall line is flat the engine has grunt everywhere. If there is a lot more graph to the right then the engine is going to have a lot more pull at higher rpm.

As others have pointed out, when racing, and at wide open throttle we seldom get the engine much below 5000rpm so if all you want to do is race, you don't need the torque at lower rpms. If you have a manual transmission and want to pick up a little speed quickly without downshifting, you need the torque on the left side of the graph. A car with more torque can climb hills, or mountains in high gear without a downshift. Auto trans usually will downshift based on throttle position even if you don't need to downshift.

If you are road racing a car with a dyno graph like Liberty hill posted you can come off the corners at 3000 rpm and the car will accelerate like a rocket because of the torque, on the first procharger graph you posted, if you come off the corner at 3000 rpm and hit it, you will get a slow progressive surge as it builds rpm until it gets into the power, you will just have to work the gearbox more to go as fast.

But after all of this, if you look at the tachometer while daily driving, you have to really hit it to get over 5000 rpm. For daily driving, the area on the left side of the graph is going to be much more useable because that is where your engine spends most of its time. Depends on how you use your car.