I'm no engineer... But, from what I know, torque is the ability to turn the axle, and HP is... Well, different. :lol: Sorry, I don't know what the definition of HP is as it applies to an engine, but with the definition of torque, it might help to show you how the two are different.
Here's an example of how the two can be different. You can add lower gears (numerically higher) to your car, and you will increase the torque while doing nothing to increase HP ('cause you haven't added anything that lets the ENGINE make more power -- rather, you've just allowed the car to use the car's same power more effectively).
And when it comes to the headers, there may be a slight decrease in low-end torque, but HANG ON for the HP gains in the mid-to-high range!
Another example do demonstrate the difference between torque and HP -- put in a spitball in a straw, take a big breath, and blow the spitball out. You can really launch it! Now, put the same size spitball in a papertowel tube, take the same size breath, and see how hard you blow the spitball out of there. (Not too far, I bet.) Now, try running a lap around a track while only being able to breath through that straw, and try running around a track with a 1" or larger tube in your mouth. You'll do a lot better with the larger tube. The same "principles" can apply when changing header sizes, exhaust, intakes, etc.
Ok, enough of my rambling! I'm hoping I can get my car dyno'd so everyone can know what kind of power/torque I'm making!
-Kirk